Geoderma 406 (2022) 115528

Available online 13 October 20210016-7061/© 2021 Elsevier B.V. All rights reserved.

Arbuscular mycorrhizal symbiosis enhances water stable aggregate formation and organic matter stabilization in Fe ore tailings

Zhen Li a,b, Songlin Wu b,*, Yunjia Liu a,b, Qing Yi b, Fang You b, Yuanying Ma b, Lars Thomsen c, Ting-Shan Chan d,*, Ying-Rui Lu d, Merinda Hall b, Narottam Saha b, Yuanfang Huang a, Longbin Huang b

a College of Land Science and Technology, China Agricultural University, Beijing 100193, China b Centre for Mined Land Rehabilitation, Sustainable Minerals Institute, The University of Queensland, Brisbane, Queensland 4072, Australia c Australian Synchrotron, ANSTO, Melbourne, Victoria 3168, Australia d National Synchrotron Radiation Research Center, Hsinchu Science Park, Hsinchu 30076, Taiwan

A R T I C L E I N F O

Handling Editor: Yvan Capowiez

Keywords: Alkaline Fe ore tailings Arbuscular Mycorrhizal Fungi Water stable aggregate Organic matter stabilization C 1s NEXAFS Fe K edge XAFS

A B S T R A C T

Organo-mineral association and water-stable aggregation in finely textured tailings are critically important to the eco-engineered soil formation from alkaline Fe ore tailings for sustainable mine site rehabilitation. Arbuscular mycorrhizal (AM) symbiosis plays important roles in soil aggregate formation and organic matter (OM) stabi-lization. However, it is unknown if AM symbiosis could enhance aggregate formation and OM stabilization in alkaline Fe ore tailings. The present study aimed to investigate the establishment of AM symbiosis and their role in tailing aggregate formation coupled with OM stabilization, as well as the underlying mechanisms. After initial eco-engineering (OM amendment and pioneer plant cultivation) to improve physicochemical conditions for plant survival, Sorghum spp. Hybrid cv. Silk inoculated with/without AM fungi (Glomus spp.) were cultivated in the tailings under glasshouse conditions for 14 weeks. The results indicated that AM fungi formed symbiotic asso-ciation with Sorghum spp. plants, improved mineral nutrient (e.g., P) acquisition and root growth in the eco- engineered tailings. The AM symbiosis significantly improved aggregate formation. The association of organic carbon and nitrogen with tailing minerals of the aggregates was enhanced by the AM symbiosis. As revealed by synchrotron-based C 1 s near edge X-ray absorption fine structure (C 1 s NEXAFS) and Fe K edge X-ray absorption fine structure (Fe K edge XAFS) spectroscopy, the AM symbiosis favoured carboxyl and aromatic C association with secondary Fe-Si minerals, which may have been formed from AM driven mineral weathering. Overall, the study revealed that the AM symbiosis could not only improve the growth of pioneer plant species in the early eco- engineered tailings, but also advance soil formation through enhancing organic C and N sequestration and physical structure development via water-stable aggregation. These findings help to advance our understanding of the importance of AM symbiosis in the eco-engineering of tailings into functional soil (or technosols) for sustainable rehabilitation of Fe-ore tailings.

1. Introduction

Eco-engineering Fe ore tailings into soil or soil-like matrix has been demonstrated to be a promising technology to achieve sustainable rehabilitation of tailings landscape, without resorting to topsoil re-sources excavated and transported from offsites (Huang et al., 2014). Our previous studies demonstrated that soil-like physical and chemical properties could be developed in the tailings with a series of eco- engineering amendment inputs (hereafter named eco-engineered

tailings), such as exogenous organic matter, pioneer plant colonization and water supply (Robertson et al., 2020; Wu et al., 2019a; Wu et al., 2019b). However, the soil physical structure (especially aggregate sta-bility) and organic matter (OM) content remained low, compared to natural topsoil at the mine site (Wu et al., 2019a; Wu et al., 2019b; Wu et al., 2019c). Therefore, it is essential to develop strong and extensive plant root systems to stimulate aggregate development and OM stabili-zation in the early eco-engineered tailings. In a field investigation at an Fe ore mine, arbuscular mycorrhizal (AM) fungi were found to be

* Corresponding authors. E-mail addresses: songlin.wu@uq.edu.au (S. Wu), chan.ts@nsrrc.org.tw (T.-S. Chan).

Contents lists available at ScienceDirect

Geoderma

journal homepage: www.elsevier.com/locate/geoderma

https://doi.org/10.1016/j.geoderma.2021.115528 Received 5 July 2021; Received in revised form 28 September 2021; Accepted 3 October 2021

Geoderma 406 (2022) 115528

2

present in the alkaline Fe ore tailings with low nutrient availability and subject to progressive colonization of native plants under semi-arid climatic conditions (Wu et al., 2020).

AM fungi, a ubiquitous soil fungi that can form symbiotic relation-ship with > 80% terrestrial plant species (Smith and Read, 2010). AM symbiosis can benefit plants in acquiring mineral nutrients (e.g., P), and enhance plant tolerance to various environmental stresses, such as salinity (Aliasgharzad et al., 2001), drought (Ji et al., 2019), nutrient deficiency and toxic metal stresses (Chen et al., 2007; Wu et al., 2014). These environmental stresses are commonly present in mine tailings sites. In natural soil, increasing evidence has suggested that AM fungi play important roles in soil aggregate development and OM stabilization with minerals through enlarging rhizosphere surfaces, and various biochemical mechanisms of hyphae and roots (Amezketa, 1999; Bearden and Petersen, 2000; Bedini et al., 2009; Miller and Jastrow, 2000; Rillig, 2004). Specifically, the AM fungal mycelium has a diameter of only several micrometers, providing high surface area for mineral in-teractions via direct enmeshment with mineral particles (Rillig and Mummey, 2006a). In addition, it was widely reported that AM fungi can exudate organic substances such as glomalin-related soil protein (GRSP), which is rich in functional organic groups (such as aromatic and carboxyl C) that readily bind with minerals to form organo-mineral as-sociations and aggregate structure (Rillig, 2004; Singh et al., 2013; Zhang et al., 2017). Furthermore, AM symbiosis was also reported to accelerate primary mineral (e.g., biotite) weathering and secondary mineral formation (e.g., smectites and hydroxy-interlayered vermicu-lites) (Arocena et al., 2012), which are important to organic matter stabilization and aggregate formation/development (Denef and Six, 2005). As a result, it is expected that AM symbiosis with pioneer plants could enhance organo-mineral association and water-stable aggregation in the early eco-engineered tailings. However, as far as we have known, most studies on AM fungal roles were in the context of natural soils, rather than mine tailings (such as alkaline Fe ore tailings), which are physically, chemically and mineralogically different from natural soil systems (Wu et al., 2019b). It is still not clear if AM symbiosis could survive in the finely textured tailings and perform functions towards soil aggregate development and OM stabilization under tailings conditions.

The present study thus aimed to fill in this knowledge gap by investigating the development of AM fungal symbiosis with pioneer plants and their role in aggregate formation and OM stabilization in the Fe ore tailings. Sorghum spp. Hybrid cv. Silk was selected based on its tolerance of the tailings’ environment and fibrous roots, which could extensively interact with mineral particles and enhance aggregate for-mation in the tailings (Wu et al., 2019a; Wu et al., 2021). AM fungi strains of Glomus spp. were used to inoculate the plants to form the AM fungi-Sorghum spp. symbiosis, which were cultivated in the eco- engineered tailings under glasshouse conditions for 14 weeks before harvest. For unravelling mineral sequestration of organic carbon (OC), various micro-spectroscopic analyses have been employed to capture mineral phases and OC forms, including X-ray powder diffraction (XRD), synchrotron-based Fe K edge X-ray absorption fine structure spectros-copy (Fe K edge XAFS), C 1 s near-edge XAFS (C 1 s NEXAFS), and attenuated total reflectance − Fourier transform infrared (ATR − FTIR) spectroscopy analysis, as well as Field Emission Scanning Electron Mi-croscopy with Energy Dispersive X-Ray Spectroscopy (FE-SEM-EDS) analysis. We hypothesized that (1) AM fungi can survive and make well symbiotic association with pioneer plant Sorghum spp. Hybrid cv. Silk in the Fe ore tailings, which was initially eco-engineered with OM amendment and pioneer plant colonization (Wu et al., 2019a); (2) AM symbiosis can improve the formation of water stable aggregates and increase organic carbon and nitrogen stabilization within aggregates; (3) AM symbiosis can stimulate tailings mineral weathering and sec-ondary mineral formation, which associates intimately with organics for OM stabilization and aggregate development. The study would provide an important basis for integrating mycorrhizal fungal based strategies into the eco-engineering technology to develop the tailings into

functional soil (or technosols) for sustainable tailings rehabilitation.

2. Materials and methods

2.1. Materials and experimental design

2.1.1. Eco-engineered Fe ore tailings. The Fe ore tailings were from a magnetite Fe ore mine in Western

Australia. The fresh or newly deposited Fe ore tailings were mainly composed of fine mineral particles (mainly quartz and biotite like minerals), with a textural composition of 25.6%, 56.2%, and 18.2% of clay-sized (0–5 μm), silt-sized (5–53 μm) and sand-sized (53–2000 μm) minerals, respectively. In addition, the tailings were extremely alkaline (pH 9.5) and contained low levels of organic carbon and nitrogen, without the conditions to support colonization of pioneer plants, nor AM fungi, according to our previous findings (Wu et al., 2019b). In the present study, the tailings were initially eco-engineered following methods described in our previous study (Wu et al., 2019a). In brief, the fresh tailings were amended with 3% (w/w) sugarcane mulch and incubated for two months. Then pioneer plants were cultivated in the modified tailings for additional two months. After four months of OM amendment and plant colonization treatments, the tailings were improved in physicochemistry and physical structure, and thereby defined as eco-engineered Fe ore tailings used in this study. The basic properties of this early eco-engineered Fe ore tailings are shown in Table S1, Supporting Information (SI).

2.1.2. Plant and AM fungal inoculum. Sorghum spp. Hybrid cv. Silk was from Pukalus’s farm from Injune

origin, Queensland, Australia. Plant seeds were surface sterilized and germinated in a petri dish before transplantation into river sand. The AM fungal inoculum used in this study was purchased from MycoApply company (https://www.mycoapply.com.au/) and was composed of four kinds of Glomus spp., including G. intraradices, G. mosseae, G. aggregatum and G. etunicatum. The inoculum was mixture of colonized root segments and AM fungal spores (with spore number of 20 per gram).

2.1.3. Experiment design. There were totally three treatments: 1) Eco-engineered Fe ore tail-

ings without plant cultivation and AM fungal inoculation (ET); 2) Eco- engineered Fe ore tailings colonized with Sorghum spp. Hybrid cv. Silk but without AM fungal inoculation (ET + S); 3) Eco-engineered Fe ore tailings colonized with Sorghum spp. Hybrid cv. Silk with AM fungal inoculation (ET + S + A). Based on our pre-experiment, direct inocu-lation of plant roots with AM fungi was difficult due to the poor physical structure of the eco-engineered tailings (although better than original fresh tailings). Therefore, we have developed a two-step strategy in the present study (Figure S1): first, cultivation of plants with/without AM fungal colonization in a nursery medium (i.e., acid washed and sterilized river sand); then transplantation of these mycorrhizal and non- mycorrhizal plants into eco-engineered tailings to investigate AM sym-biosis development in the tailings and their role in tailings’ aggregate formation and organic matter stabilization.

In detail, after germination in the petri dish, the Sorghum spp. seedlings (about 5 cm in length) were cultured in acid washed and sterilized river sand (121 ◦C, 40 min) with/without AM fungal inocu-lation. For the treatment with AM fungal inoculation, the sterilized river sand was mixed homogeneously with 5% (w/w) AM fungal inoculum. For the treatment without AM fungal inoculation, the sterilized river sand was mixed homogeneously with the same amount of sterilized AM fungal inoculum (121 ◦C, 40 min). In addition, the river sand was also re- inoculated with filtrations of the AM fungal inoculum to re-introduce the bacterial community in the AM fungal inoculum. The plants with/ without AM fungal inoculation were cultivated in river sand with half- strength Hoagland nutrition (to facilitate AM symbiosis formation) for eight weeks, and then transplanted to the eco-engineered Fe ore tailings

Z. Li et al.

Geoderma 406 (2022) 115528

3

(the plant growth and AM fungal colonization status upon trans-plantation are shown in Table S2). The plants with/without AM fungal colonization were cultured in eco-engineered tailings for additional 14 weeks (namely treatments “ET + S” and “ET + S +A”). Besides, a control treatment without plant cultivation was also set up (i.e., treatment “ET”). Each treatment had four replicates, totalling 12 pots. Each pot was filled with one kg of eco-engineered tailings. The tailings were packed into pots based on the natural gravity without artificially compaction, in a similar manner as the tailing deposition at the mine site. Pots of all treatments were watered daily to maintain at 55% water holding capacity. The pot experiment was conducted in a temperature- controlled glasshouse with conditions of 14/10 h (day/night) and 28/ 21℃ (day/night).

After 14 weeks’ growth, plant shoots and roots were harvested separately and washed with deionized water. About 5 g of fresh roots were stored at − 20 ℃ for mycorrhizal colonization assessment. Then the remaining plant roots and shoots were oven-dried at 70 ℃ for 48 h. The dry shoots and roots samples were weighed and recorded, then ground and digested by concentrated HNO3 for elemental concentration anal-ysis using inductively coupled plasma optical emission spectrometry (ICP-OES, Varian Vista Pro II). About 50 g of fresh tailings were stored at 4 ℃ for mycelium density analysis. In addition, about 5 g of fresh tailing were freeze-dried for synchrotron-based Fe K edge XAFS analysis. The remaining tailings were air-dried at 40 ℃ for physicochemical analysis, water stable aggregate assessment, OM fractionation and OC forms characterization.

2.2. Arbuscular mycorrhizal colonization assessment

2.2.1. AM fungal colonization in plant roots Sub-samples of fresh roots were used to assess the AM fungal colo-

nization status according to Phillips and Hayman (1970). In brief, plant roots were cut into 1-cm fragments, cleared in 10% (w/v) KOH for one hour at 90 ℃, and rinsed in 2 % (v/v) HCl, and then stained with 0.05% (w/v) Trypan blue. Thirty pieces of randomly selected stained root fragments were observed under a light microscope. The intensity of mycorrhizal colonization (M%) were estimated according to Trouvelot et al. (1986).

2.2.2. AM fungal mycelium density in tailings AM fungal mycelium density was determined by the cross method

under a microscope at × 200 magnification after aqueous extraction and membrane filtering (Abbott et al., 1984; Jakobsen, 1992). In brief, 4 g fresh tailings were added into 100 mL water plus 12 mL sodium hex-ametaphosphate (37 g/L) and the mixture was shaken vigorously for 30 s. The suspension was sieved through two sieves (the above 1 mm, below 53 µm), and the mycelium retained on the 53 µm sieve. The fungal mycelium was collected and dispersed in 250 mL water and blended for 20 s, then allowed to settle down for 10 s before filtering a 5 mL sus-pension aliquot on a Millipore filtration system with 0.45 um filter paper. The mycelium on the filter member was stained with 0.05% (w/ w) Trypan blue. The mycelium density was determined by counting intersections between the grids and mycelium in the eyepiece in 25 fields of view under × 200 magnification and then calculated by the modified Newman formula (Jakobsen, 1992; Tennant, 1975).

2.2.3. Glomalin related soil protein Glomalin related soil protein (GRSP), an important exudate of AM

fungi, was extracted by 50 mM sodium citrate buffer (pH 8) and measured by Bradford protein method (Bradford, 1976b; Wright et al., 1998; Wright and Upadhyaya, 1996). Briefly, 1 g air-dried tailings was mixed with 8 mL (1:8, w/v) of 50 mM buffer and autoclaved at 121℃ for 60 min. The mixture was centrifuged (4,000 g, 30 s) and supernatant was collected. The total GRSP was extracted twice by this way (until the supernatant almost clear). The GRSP content was determined using the Bradford protein method (Bradford, 1976).

2.3. Tailings physicochemical properties

The pH and electrical conductivity (EC) of tailings (tailings and DI water ratio of 1:5, w/v) from different treatments were measured using a pH electrode (TPS 900-P) and an EC electrode (TPS 2100), respectively. Total organic carbon (TOC) and Total Nitrogen (TN) concentrations of tailings were determined by dry combustion with a LECO CNS-2000 analyzer (LECO Corporation, San Jose, MI, USA). TOC was determined after removing the inorganic carbon by HCl (5 mol L-1). Acid ammonium oxalate (AAO) extractable Fe/Si/Al were measured to estimate the amorphous Fe/Si/Al minerals (McKeague and Day, 1966; Parfitt and Childs, 1988). Briefly, the AAO extraction was carried out by shaking 0.5 g of air-dried tailings in 40 mL AAO solution (0.175 M (NH4)2C2O4 + 0.10 M H2C2O4) for 4 h at room temperature. The mixture was centrifuged at 3,000 g for 3 min, and the supernatant was collected and filtered. The elemental concentration in the supernatant was analyzed by inductively coupled plasma-optical emission spectrometry (ICP-OES, Varian Vista Pro II). Olsen P concentration was determined colorimet-rically using the molybdate blue method after extracting tailings with 0.5 mol L-1 NaHCO3 (Murphy and Riley, 1962; Olsen, 1954).

2.4. Water stable aggregates assessment and organic matter fractionation

Water stable aggregates (WSAs) were separated and collected by the wet-sieving method modified from Kemper and Rosenau (Kemper and Rosenau, 1986). In brief, 50 g air-dried tailings were placed on two sieves composed of 250 µm and 53 µm and sprinkled with DI water for three minutes. Then, three fractions including (1) > 250 µm (fraction containing macroaggregates, referred as Ma); (2) 250 – 53 µm (fraction containing microaggregates, referred as Mi); (3) < 53 µm (discrete particles) were separated through shaking (3 rounds of 20 s intervals at an amplitude of 0.6 mm) by FRITSCH Analysette 3-laboratory model of vibration sifting device (Fritsch, Germany). The fractions remained on each sieve were carefully collected. In addition, the floating particulate organic matter (fPOM, mainly found in > 250 µm fraction) were also collected. All fractions with different sizes and fPOM were then oven- dried (40℃) and weighed to record their dry weight. The proportions of aggregates with different sizes and fPOM were expressed as the ratio of the dry weight of each fraction to the total dry weight (i.e., 50 g). Furthermore, mean weight diameter (MWD) of water stable aggregates was calculated as follows:

MWD =∑n

k=0

rk + rk+1

2× mk (1)

Where k = 0, 1, and 2 represent the three aggregate size fractions; r0, r1, r2, and r3 represent 0 µm, 53 µm, 250 µm, 2,000 µm, respectively; mk is the mass proportion of different fraction; n is the number of sieves (equals to two in this study).

To separate different OM fractions within aggregates, a common dispersing chemical and shaking process were used to disrupt the Ma and Mi fractions (Six et al., 2002; You et al., 2018). Briefly, 15 g of Ma or Mi samples were submerged into 45 mL 0.5% (w/v) sodium hexame-taphosphate and shaken for 15 h using the end-over-end shaker at room temperature. The dispersed fractions were further separated into two parts through passing 53 µm sieve and the fraction on the sieve was defined as particulate organic matter (POM) while the fraction passed through 53 µm sieve was defined as mineral associated organic matter (MOM). Through this method, the OM in WSAs were distributed in four fractions: intra-Ma particulate OM fraction (Ma-POM), intra-Mi partic-ulate OM fraction (Mi-POM), intra-Ma mineral associated OM fraction (Ma-MOM), and intra-Mi mineral associated OM fraction (Mi-MOM). TOC and TN concentration in different fractions were detected by the methods described above. The OC content in different fractions—i.e., Ma-POM, Mi-POM, Ma-MOM and Mi-MOM were calculated based on the OC concentration in each fraction and their corresponding mass

Z. Li et al.

Geoderma 406 (2022) 115528

4

proportion (see equation (2)). The contribution of OC in each fraction (either POM or MOM in aggregates) to the OC in the whole tailings was defined as the percentage of the OC amount in different fractions to the total OC amount in bulk tailings (equation (3)).

OCicontent =OCi × mi

mbulk(2)

OCicontribution(%) = 100 ×OCicontent × mbulk

OCbulk × mbulk(3)

Where, OCi represents OC concentration in a certain fraction (i), including Ma-POM, Mi-POM, Ma-MOM and Mi-MOM fractions; mi rep-resents the mass of tailings in the i fraction; mbulk represents the mass of the whole bulk tailings used for wet-sieving; OCbulk is the OC concen-tration of the whole bulk tailings.

2.5. Mineralogical analysis

2.5.1. Colloids isolation. The colloid fraction, as the most active part of the tailings (Bolt et al.,

2013; De Boodt et al., 2013; Huang et al., 2020), were extracted ac-cording to Schumacher et al. (2005). Briefly, air-dried tailings were suspended in DI water at a ratio of 1:5 (w/v) and shaken for 8 h at 22 ◦C. Then the solution was centrifuged for 6 min at 2,500 g and the sus-pension were then air-dried. The mineralogical composition and organic C forms of the colloidal fraction were further analyzed.

2.5.2. XRD analysis The Ma-MOM, Mi-MOM fractions and the colloids were ground into

fine powder for XRD analysis using a Bruker D8 DISCOVER diffrac-tometer (Bruker AXS D8, Karlsruhe, Germany). The XRD spectrum data were obtained from 2θ = 5◦ to 70◦ with 0.02◦ per step. The minerals were identified from the spectrum using the Diffracplus Evaluation Package V5.1 (Bruker AXS, Germany) based on the PDF-4 mineral database (2020 release).

2.5.3. Fe K edge XAFS analysis The sub-sample of freeze-dried tailings were ground to fine powder

for Fe K edge (7,112 eV) XAFS analysis at Beamline 17C1 at the National Synchrotron Radiation Research Centre (NSRRC) of Taiwan. The XAFS spectra were processed by using the ATHENA from IFEFFIT (DEMETER) software package (Ravel and Newville, 2005). The specific process is shown in supplementary information. It is essential to point out that the linear combination fittings (LCF) of Fe K-edge X-ray absorption near edge fine structure spectroscopy (XANES) and extended X-ray absorp-tion fine structure spectroscopy (EXAFS) were different in the current study. It is recognised that XANES is mainly on the spectra features near the absorption edge (-20–30 eV), while EXAFS is more on the post edge (30–800 eV) (Koningsberger et al., 2000). Both XANES and EXAFS gave complementary information on Fe speciation.

2.6. Organic carbon forms analysis

2.6.1. C 1 s NEXAFS analysis MOM fraction tailings were ground and subject to OC forms analysis

to investigate mechanisms of OM stabilization by minerals. The finely ground subsample of Ma-MOM and Mi-MOM fractions were firmly mounted on a copper tape and fixed to a stainless ruler for synchrotron- based C 1 s NEXAFS analysis at the Soft X-ray Spectroscopy beamline (14ID)(Cowie et al., 2010) in Australian Synchrotron. The spectra were collected under the energy range of 270–330 eV with a step size of 0.1 eV and scanning spot size of 1 mm2 under partial electron yield mode at an angle of 55◦to the beam. Each sample was measured three times. The spectra was energy calibrated, baseline corrected and normalized using QANT software (Gann et al., 2016). The spectra were then subject to peak fitting with a baseline of 290 eV, an edge height of 1.3 a.u. and

width of 1.2 eV, by using QANT package (Igor Pro 8 software). Three Gaussian peaks at 285.2 eV (also 285.5 eV), 287.6 eV, and 288.4 eV (also 288.9 eV) were fitted, representing aromatic C, alkyl C, carboxyl C, respectively (Schumacher et al., 2005). In addition, a peak at 291.76 eV induced by the resonance (corresponding to C 1 s → σ* transitions of saturated single covalent bonds) was also included in the fittings for a better quantification (Ehlert et al., 2014; Schumacher et al., 2005). The percentage of different OC forms was estimated as the ratio of individual area of each Gaussian peak to the sum of the area of all peaks except the peak at 291.76 eV (Prietzel et al., 2018).

2.6.2. ATR-FTIR Organic functional groups of Ma-MOM and Mi-MOM were investi-

gated by ATR-FTIR analysis by using a Cary 630 FTIR (Agilent Tech-nologies, Palo Alto, CA, USA). Spectra were recorded between 4000 and 600 cm− 1 with a resolution of 2 cm− 1 and 64 scans per spectrum. The spectral region between 2700 and 1800 cm− 1 was excluded from anal-ysis because the information related to organic matter normally is masked by the C-O noise from CO2 (Dhillon et al., 2017). In addition, the spectral region<1250 cm− 1 was also excluded from analysis because of the major contribution from minerals (Haberhauer et al., 2000; Calderon et al., 2011). The structural assignments of several important ATR-FTIR spectral bands were interpreted based on previous literatures, mainly including: 1) a broad peak spanning at ~ 1660–1600 cm− 1

possibly representing the C = C vibration from aromatic structures, C =O from amides, quinones and H-bonded conjugated ketones, or C = O stretching from COO– (Dhillon et al., 2017); 2) a broad peak between 3650 and 2960 cm− 1, possibly attributed to the vibration of O–H pri-marily from carboxyl, phenol, and alcohol with possible minor contri-butions from N-H of amine and amide (Pedersen et al., 2011); 3) a weak absorption band at around 1423 cm− 1, representing the asymmetric C-O stretching of carboxyl groups C (Calderon et al., 2011; Cocozza et al., 2003); 4) a shoulder at ~ 1715 cm− 1 probably induced by the C = O stretching vibration from COOH, saturated or unsaturated aliphatic ketone and aldehyde, aromatic ketone and aldehyde, and esters (Ped-ersen et al., 2011).

2.7. FE-SEM-EDS analysis of aggregates

The microaggregates isolated from tailings with “ET + S + A” treatment were carefully mounted onto an aluminium stub (12 mm in diameter) with carbon adhesive tab. It is estimated that each stub would have held hundreds of microaggregates. These microaggregates on each tab were coated with carbon for examining aggregate morphology and identifying the presence of AM fungal mycelium, by FE-SEM-EDS anal-ysis (JEOL JSM 7100F, Tokyo, Japan). Around 10 microaggregates were randomly selected for FE-SEM-EDS analysis. The accelerating voltage, focus and probe current were set to 2 kV, 4 and 8, respectively, during the analysis.

2.8. Statistical analysis

The significant difference in physico-chemical properties of tailings subject to different treatments were tested by one-way analysis of variance (P < 0.05) followed by Tukey’s test (P < 0.05). Kolmogorov- Smirnov and Levene’s methods were used to test the normal distribu-tion and the homogeneity of variance before ANOVA analysis, which confirmed that the data met the requirements. The differences in plant dry weight, nutrient concentrations, and AM fungal colonization rates between “ET + S” and “ET + S + A” treatments were assessed by in-dependent t-test (P < 0.05). Data is presented as the mean of four rep-licates plus standard error (SE). All statistical tests were conducted using SPSS software (Ver 20, IBM, Armonk, NY, USA).

Z. Li et al.

Geoderma 406 (2022) 115528

5

3. Results

3.1. AM fungal colonization, plant growth and nutrition

In the amended Fe ore tailings, the intensity of mycorrhizal coloni-zation in the whole root system reached>64% in the treatment with AM fungal inoculation. However, no mycorrhizal colonization in roots was detected in the treatment without AM fungal inoculation (Figure S2). Consistently, AM fungal mycelium density in the tailings of “ET + S + A” treatment was 1.57 m g− 1, but no mycelium was detected in the control treatment without AM fungal inoculation (“ET + S”) (Table 1). AM fungal colonization did not influence plant root and shoot dry weights, but significantly decreased the shoot to root ratio (P < 0.05, Table 1). Moreover, AM fungal inoculation increased P concentration in both shoots and roots (P < 0.05, Table 1). In addition, AM symbiosis also resulted in higher concentrations of Fe and Zn in shoots and roots (P <0.05, Table 1).

3.2. Physicochemical properties of tailings

Plant colonization without AM fungal inoculation did not influence pH and EC of the tailings, as well as the TOC and TN concentration (Table 2). However, plant colonization with AM fungal inoculation significantly increased TOC and TN concentrations, but decreased TOC: TN ratio of the tailings (P < 0.05, Table 2). It is noteworthy that the concentration of Olsen-P in tailings decreased significantly after plant colonization, and AM fungal colonization further decreased it again (P < 0.05, Table 2). GRSP concentration was generally low in the tailings for all treatments. Plant colonization and AM fungal inoculation did not influence GRSP concentration in the bulk tailings, but increased GRSP concentration in the Mi-MOM faction (i.e., mineral-associated organic matter within microaggregates, P < 0.05, Table 2). In addition, plant colonization with AM fungal inoculation increased GRSP concentration in the Ma-MOM faction (i.e., mineral-associated organic matter within macroaggregates), compared to the plant only treatment (P < 0.05, Table 2). Compared with the eco-engineered Fe ore tailings at the start of the experiment (Table S1), further incubation of tailings with/without plant and mycorrhizal colonization increased the concentration of AAO- extractable Fe and Al (Table 2, Table S1). However, plant growth did not influence AAO-extractable Fe and Al concentration regardless of AM fungal inoculation (Table 2). Unexpectedly, plant growth with AM fungal inoculation significantly decreased the concentrations of AAO- extractable Si, K and Mg in the tailings (P < 0.05, Table 2).

3.3. Distribution of water stable aggregates

Incubation of the early eco-engineered Fe ore tailings for an addi-tional period increased the percentage of microaggregates (250–53 µm) from < 30% to ~ 33.2 %, compared to the amended tailings at the start

of the experiment (Table S1, Fig. 1). Plant colonization without AM fungal inoculation had no effects on the percentage of macroaggregates (>250 µm) but decreased that of microaggregates (P < 0.05, Fig. 1a). AM fungal inoculation significantly increased the percentage of micro-aggregates and increased the MWD value as well, compared with plant- colonised treatment without AM fungal inoculation (P < 0.05, Fig. 1b).

3.4. Organic carbon and nitrogen in different fractions of tailings aggregates

Generally, mineral-associated organic matter (MOM) contained greater TOC and TN than particulate organic matter (POM) in both macroaggregates and microaggregates, regardless of treatments (P <0.01, Fig. 2a and 2b). Plant colonization alone generally increased TOC and TN concentration in POM within aggregates, especially for macro-aggregates (i.e., Ma-POM), but decreased TOC and TN concentrations in MOM within macroaggregates (Ma-MOM) (P < 0.01, Fig. 2a and 2b). However, AM fungal inoculation increased TOC and TN concentrations in MOM within macroaggregates (Ma-MOM) (Fig. 2a and 2b). Moreover, plant colonization also increased the TOC and TN concentrations in MOM within microaggregates (i.e., Mi-MOM), regardless of AM fungal inoculation (P < 0.05, Fig. 2a and 2b).

The ratio of TOC to TN in all aggregate fractions of the treatments increased from the initial values after the incubation (Fig. 2c, Table S1). In addition, TOC:TN ratio in MOM was significantly smaller than that in POM for both macroaggregates and microaggregates (P < 0.01, Fig. 2c). Plant colonization generally did not influence TOC:TN ratio, except that it increased TOC:TN value in the Ma-POM fraction (P < 0.05, Fig. 2c). Interestingly, AM fungal colonization decreased the TOC: TN value in MOM fraction within microaggregates (but not macroaggregates) (P <0.05, Fig. 2c).

Plant colonization generally increased OC contents in all fractions except for the Mi-POM fraction (Fig. 2d). Particularly, AM fungal inoc-ulation further increased the OC content in Ma-MOM fraction (P < 0.05, Fig. 2d). The amount of OC sequestrated in POM accounted for 21.0% of the OC content of bulk tailings for “ET” treatment, 28.3% for “ET + S” treatment, and 30.1% for “ET + S + A” treatment. The contributions of OC in MOM fractions to the bulk tailings OC were 21.31% for “ET” treatment, 26.95% for “ET + S” treatment, and 29.38% for “ET + S + A” treatment, respectively (Table S3).

3.5. Organic carbon forms in mineral-associated OM within aggregates

The OC forms in MOM fractions were chosen for further character-isation to identify possible mechanisms underlying OM stabilization by minerals within aggregates. As revealed by synchrotron-based C 1 s NEXAFS analysis, OC in Ma-MOM and Mi-MOM fractions was domi-nated by carboxyl C (~62%), alkyl C (~25%) and aromatic C (~13%) in all treatments (Fig. 3b and 3c). Plant and/or AM fungal colonization

Table 1 Arbuscular mycorrhizal (AM) fungal colonization, plant dry weight and element concentration of plant tissue from “ET + S” and “ET + S + A” treatments. Note: “ET +S” represents eco-engineered Fe ore tailings cultivated with Sorghum spp. Hybrid cv. Silk but without AM fungal inoculation; “ET + S + A” represents eco-engineered Fe ore tailings cultivated with Sorghum spp. Hybrid cv. Silk with AM fungal inoculation; “Dw_S”: Dry weight of plant shoots; “Dw_R”: Dry weight of plant roots. Data is presented as: “mean (standard error)” of 4 replicates. The P-value below 0.05 indicates that the difference is significant.

Treatment AM colonization (%) Roots (mg g− 1)

Dw Fe K P Mg Zn Cu

ET + S 0.00(0.00) 4.87(0.14) 6.87(0.44) 11.7(0.83) 0.47(0.01) 1.34(0.02) 0.02(0.00) 0.02(0.00) ET + S + A 64.1(2.33) 5.28(0.43) 12.8(1.03) 17.7(3.32) 0.81(0.06) 2.19(0.14) 0.04(0.00) 0.02(0.00) P-value 0.00 0.16 0.04 0.15 0.02 0.02 0.02 1.00

Treatment Hyphal density (m g− 1)

Ratio of Dw_S to Dw_R Shoots (mg g− 1) Dw Fe K P Mg Zn Cu

ET + S 0 3.28(0.19) 15.9(0.45) 0.08(0.01) 26.2(1.70) 0.68(0.07) 1.15(0.11) 0.02(0.00) 0.01(0.00) ET + S + A 1.57(0.11) 2.62(0.22) 13.7(1.12) 0.14(0.01) 29.5(2.50) 1.26(0.02) 0.71(0.10) 0.03(0.01) 0.01(0.00) P-value 0.00 0.03 0.48 0.02 0.77 0.02 0.08 0.04 0.02

Z. Li et al.

Geoderma 406 (2022) 115528

6

generally did not influence OC forms in Ma-MOM but altered the OC forms in the Mi-MOM fraction. Specifically, AM fungal symbiosis increased the abundance of carboxyl and aromatic C, while decreasing that of alkyl C (Fig. 3c).

The results of ATR-FTIR analysis demonstrated that OC forms of mineral-associated OM were similar in macroaggregates (Ma-MOM fraction) and microaggregates (Mi-MOM fraction) (Figure S3). It was found that carboxyl and aromatic groups (1660–1600 cm− 1 and 1423 cm− 1) were present in MOM fractions, which was consistent with the results of C 1 s NEXAFS analysis. Interestingly, the treatment with AM fungal inoculation exhibited the highest absorbance at around 1640 and 3300 cm− 1 both in Ma-MOM and Mi-MOM fractions, indicating the enrichment of carboxyl and/or aromatic C in these mineral-associated OM. This was also consistent with the results of C 1 s NEXAFS analysis.

3.6. Mineral composition and Fe phases of tailings in response to AM symbiosis

Mineralogical composition of Ma-MOM and Mi-MOM fractions were determined by XRD to identify the minerals associated with OM. The quartz and mica (especially biotite) were the dominant mineral com-ponents, followed by magnetite and amphibole minerals (richterite), as well as small amounts of magnesioferrite and hedenbergite. Vermiculite and epidote were found to have appeared in the Ma-MOM fraction after plant colonization in the tailings. The AM fungal inoculation further stimulated the formation of these minerals in both Ma-MOM and Mi- MOM fractions (Fig. 4).

Further Fe K edge XAFS analysis showed that the Fe phases in the bulk tailings were dominant with biotite-like minerals and Fe-Si rich

short range ordered minerals (Fe-Si-SRO) (Figure S4b). The plant colo-nization and AM fungal inoculation did not alter Fe phases in the bulk tailings under current conditions (Figure S4b).

Colloidal fractions are usually the most active part of the tailings, which should be more sensitive representatives of mineral changes in response to plant and AM symbiosis colonization. The XRD spectra of colloidal fraction further indicated that AM symbiosis facilitated the generation of kaolinite and piemontite-like minerals, while decreasing primary minerals such as richterite and sodalite-like minerals (Fig. 5a).

Furthermore, plant colonization and AM fungal inoculation stimu-lated the transformation of Fe phases in the colloids of tailings, as revealed by Fe K edge XANES and EXAFS analysis (Fig. 5b, Figure S4c). Compared with the bulk tailings, the colloidal fraction generally con-tained more Fe-Si-SRO and less biotite-like minerals, based on Fe K edge EXAFS analysis (Figure S4). The AM symbiosis decreased the proportion of biotite-like minerals in the colloids (Figure S4c). Fe K edge XANES analysis revealed that plant colonization facilitated the formation of epidote-like minerals and stimulated Fe(III) complexation with carboxyl groups (Fe(III)-oxalate-like) in colloids (Fig. 5b). Particularly, the AM fungal inoculation further enhanced carboxyl-Fe(III) complexation (Fig. 5b).

4. Discussion

This study has among the first revealed that the AM symbiosis could be established in the early eco-engineered Fe ore tailings and signifi-cantly stimulate microaggregate formation and OM stabilization. Spe-cifically, AM fungal colonization facilitated the water stable aggregate formation and promoted organic C and N enrichment in aggregates via

Table 2 Physicochemical characteristics of alkaline Fe ore tailings from different treatments. Note: “ET”, Eco-engineered Fe ore tailings without plants and AM fungal inoc-ulation; “ET + S”, Eco-engineered Fe ore tailings cultivated with Sorghum spp. Hybrid cv. Silk but without AM fungal inoculation; “ET + S + A”, Eco-engineered Fe ore tailings cultivated with Sorghum spp. Hybrid cv. Silk and AM fungal inoculation. “Mi-MOM” represents mineral-associated organic matter within micro-aggregates; “Ma-MOM” represents mineral-associated organic matter within macroaggregates; Data is presented as: “mean (standard error)” of 4 replicates. Different letters indicate significant difference between different treatments based on Tukey’s test (P < 0.05).

Treatment pH EC (mS cm− 1)

TOC (g kg− 1)

TN (g kg− 1)

TOC:TN Olsen-P (mg kg− 1)

GRSP (g kg− 1)

ET 8.82(0.06)a 0.30(0.04)a 3.04(0.05)b 0.66(0.013)b 4.61(0.08)a 6.65(0.50)a 0.24(0.01)a

ET + S 8.85(0.03)a 0.27(0.02)a 3.05(0.10)b 0.65(0.08)b 4.70(0.20)a 4.56(0.18)b 0.22(0.01)a

ET + S + A 8.80(0.05)a 0.29(0.03)a 3.33(0.01)a 0.79(0.03)a 4.20(0.11)b 3.70(0.26)c 0.24(0.01)a

Treatment GRSP in Ma-MOM (g kg− 1) GRSP in Mi-MOM (g kg− 1) AAO-extractable elements (g kg− 1) Fe Si Al K Mg

ET 0.33(0.02)a 0.16(0.01)b 12.4(0.42)a 2.02(0.05)a 0.48(0.02)a 1.69(0.04)a 0.52(0.01)a

ET + S 0.10(0.01)b 0.21(0.01)a 11.3(0.25)a 1.70(0.07)ab 0.46(0.01)a 1.37(0.07)b 0.46(0.02)ab

ET + S + A 0.33(0.01)a 0.23(0.01)a 11.3(0.16)a 1.58(0.04)b 0.45(0.01)a 1.32(0.03)b 0.39(0.01)b

Fig. 1. Distribution of water stable aggregate size fractions (a) and Mean Weight Diameter (MWD) (b) of tailings from different treatments. Note: “ET”, Eco- engineered Fe ore tailings without plants and AM fungal inoculation; “ET + S”, Eco-engineered Fe ore tailings cultivated with Sorghum spp. Hybrid cv. Silk but without AM fungal inoculation; “ET + S + A”, Eco-engineered Fe ore tailings cultivated with Sorghum spp. Hybrid cv. Silk and AM fungal inoculation. Data is presented as mean of 4 replicates, with error bars representing standard error. Different letters indicate significant difference between different treatments based on Tukey’s test (P < 0.05).

Z. Li et al.

Geoderma 406 (2022) 115528

7

Fig. 2. Concentration of total organic carbon (TOC) (a), total nitrogen (TN) (b), TOC: TN ratios (c) of fractionated tailings and OC content (d) in the Ma-POM, Ma- MOM, Mi-POM, and Mi-MOM per kg tailings of different treatments. Note: “ET”, “ET + S” and “ET + S + A” are treatments defined in Fig. 1; Ma-POM and Mi-POM indicate the particulate organic matter distributed in Ma and Mi fractions, respectively; Ma-MOM and Mi-MOM indicate the mineral associated organic matter distributed in Ma and Mi fractions, respectively. Data is presented as mean of 4 replicates, with error bars representing standard error. The letters above the columns indicate the significance of variance analysis by Tukey’s test (P < 0.05); The “***” and “**” indicates the significance of difference of TOC, TN and TOC:TN between POM and MOM by Tukey’s test with P < 0.001 and P < 0.01, respectively.

Fig. 3. Representative original normal-ized (solid black line) and fitted (dashed red line) C 1 s NEXAFS spectra (a); and the constitution of different C species of OM distributed in the Ma-MOM (b) and Mi-MOM fractions (c), including aro-matic C (285.2 eV, 285.5 eV), alkyl C (287.5 eV), carboxyl C (288.4 eV, 288.8 eV), as quantified by the peakfit pro-cedure in QANT. Note: “ET”, “ET + S”, and “ET + S + A” are treatments defined in Fig. 1. Each value in subfigures (b) and (c) is the mean of 3 measurements, with error bars representing standard error. The letters above the columns indicate the significance of variance analysis by Tukey’s test (P < 0.05).

Z. Li et al.

Geoderma 406 (2022) 115528

8

mineral association. The AM symbiosis enriched functional organic groups (especially carboxyl and aromatic C) in the tailings and accel-erated mineral weathering and formation of secondary Fe-Si minerals (e. g., vermiculite like minerals, and/or amorphous Fe-Si-SRO like min-erals), contributing to the OM sequestration and aggregate stability in the eco-engineered tailings. This successful demonstration of AM fungal- plant symbiosis in the Fe ore tailings provides a promising strategy to assist rhizosphere modification of tailings properties for eco-engineering of Fe-ore tailings into soil for sustainable mine site rehabilitation.

4.1. Establishment of arbuscular mycorrhizal symbiosis in eco-engineered Fe ore tailings

It is well established that AM fungi could survive in harsh environ-ments such as saline (Aliasgharzad et al., 2001), alkaline (Abd-Alla et al., 2014), and heavy metal contaminated soils (Wu et al., 2014), as well as mine tailings (Chen et al., 2007; Solis-Dominguez et al., 2011). How-ever, our pre-experiments found that plant and AM fungi survivability in fresh Fe-ore tailings was much less likely. This may indicate that the AM fungi are not tolerant of the extremely alkaline conditions, compacted physical structure, and extremely low organic matter and nutrition of fresh Fe-ore tailings (Wu et al., 2019b). Besides, AM fungi rely on host plants for carbohydrates through symbiotic association (Smith and Read, 2010). Without the survival of plants, it is difficult for AM fungi to survive. Consistent with the first assumption, AM symbiosis with Sor-ghum spp. Hybrid cv. Silk successfully developed in the eco-engineered Fe ore tailings with initial OM amendment and pioneer plant coloniza-tion. This was demonstrated by the high colonization intensity and mycorrhizal structure in the roots with AM fungal inoculation (Figure S2 and Table 1). The initial OM amendments and plant colonization resulted in weathering of biotite-like minerals, neutralization of alkaline pH, and improvement of physical structure, collectively contributing to successfully pioneer plants colonization. This plant growth triggered

further improvement of tailings physicochemical properties, as well as aggregate formation (Robertson et al., 2020; Wu et al., 2019a). There-fore, the circumneutral pH, initial aggregate structure, and small in-crease in tailings organic matter after initial eco-engineering inputs probably supported the subsequent development of AM symbiosis with pioneer plants.

The establishment of AM symbiosis in the tailings would in return facilitate host plant performance, which was demonstrated by the enhanced nutrient (P, Fe, Zn etc.) uptake by the plants with AM fungal colonization. Consistent with previous studies (Liu et al., 2021; Mackay et al., 2017), AM fungal colonization improved the phosphorous (P) uptake by plants, which coincided with decreased Olsen-P concentration in the tailings (Table 1 and Table 2). In addition, significant elevated Fe, Mg, and Zn were also found in plant shoots and roots after AM fungal colonization (Table 1). The enhancement of plant nutrient uptake by AM symbiosis may be due to the direct uptake and transport of these nutritional elements by extraradical mycelium (Parniske, 2008), or by increasing the elemental bioavailability in the tailings (Arocena et al., 2012; Hart and Forsythe, 2012). Although the AM symbiosis increased plant nutrition status (especially P), it did not increase shoot and root growth. This may be because the AM fungal acquisition of carbohydrates from host plants may suppress plant growth (Kiers et al., 2011). How-ever, the AM symbiosis decreased the dry weight shoot to root ratio, indicating the enhancement of root development by AM symbiosis in the tailings.

The rhizosphere interactions with tailings minerals would have greatly benefited from the growth of large amounts of extraradical mycelium after AM fungal inoculation (Table 1). These mycelia could not only assist plant nutrient acquisition from the tailings, but also played an effective role in enhancing aggregate formation and organic matter stabilization for pedogenesis of the tailings.

Fig. 4. XRD spectra of the tailings in Ma-MOM (a) and Mi-MOM fractions (b) from different treatments. Note: Mi: mica, such as biotite (K(Mg, Fe)3(AlSi3O10)(F,OH)2), phlogopite (KMg3 (AlSi3 O10)(F,OH)2), annite (K(Fe2+)3AlSi3O10(OH)2); E: epidote, Al2.21Ca2Fe0.58HMn0.21O13Si3; H: heden bergite (Ca0.503FeNa0.497Si2O6); M: magnetite; Ma: magnesioferrite, Mg(Fe3+)2O4; V: vermiculite (Mg0.7(Mg,Fe,Al)6(Si,Al)8O20(OH)4⋅8H2O); Q: qu artz; R: richterite, (Ca1.05Mg5Na2.85O24Si8), be-longs to amphibole groups; Note: “ET”, “ET + S”, “ET + S + A” are treatments labelled in Fig. 1.

Z. Li et al.

Geoderma 406 (2022) 115528

9

Fig. 5. Mineral composition (a) and Fe species (b) of colloids of tailings from different treat-ments shown by XRD spectra and linear combi-nation fitting (LCF) of Fe K edge X-ray absorption near edge fine structure (XANES) spectroscopy, respectively. Note: Mi: mica, such as biotite (K (Mg,Fe)3(AlSi3O10)(F,OH)2); R: richterite, (Ca1.05Mg5Na2.85O24Si8), belongs to amphiboles groups; Ka: Kaolinite (Al2Si2O5(OH)4); E: epidote, Al2.21Ca2Fe0.58HMn0.21O13Si3; Sd: sodalite (Na8Mg3Si9O24(OH)2); P: piemontite, Al2.15Ca2-

Fe0.12H Mn0.73O13Si3; Q: quartz; Sy: sylvite (KCl); H: hedenbergite (Ca0.503FeNa0.497Si2O6); Ma: magnesioferrite, Mg(Fe3+)2O4; M: magnetite. Fe K edge XANES spectra of some Fe reference standards are shown in Fig. 5b. The line in the bottom left of sub-Fig. 5b showed the Fe K edge XANES spectra (solid line) and the fitted spectra (dash line) of colloids sample. R-factor for the LCF-EXAFS fitting is a measure of mean square sum of the misfit at each data point, i.e., R-factor = Sum (data-fit)2 / Sum (data)2 and used for estimation of the goodness of fit. The “ET”, “ET +S”, “ET + S + A” are treatments labelled in Fig. 1.

Fig. 6. Field Emission Scanning Electron Micro-scopy coupled with Energy Dispersive X-Ray Mi-croscopy (FE-SEM-EDS) showing minerals entangled by arbuscular mycorrhizal fungal mycelium in microaggregates of eco-engineered Fe-ore tailings cultivated with Sorghum spp. Hybrid cv. Silk and AM fungal inoculation (treatment “ET + S + A”). a) Overview of a randomly selected microaggregate; b) magnified view of circled area of picture “a)”, showing intertwined minerals and mycelia; c) magnified view of circled area of picture “b)”, showing Fe and Si rich minerals interacting with mycelia; d) magnified view of circled area of picture “c)”, showing the shrivelled hyphae coated with small mineral particles.

Z. Li et al.

Geoderma 406 (2022) 115528

10

4.2. AM symbiosis improved water stable aggregate development and organic matter stabilization

It was found that AM symbiosis significantly increased the percent-age of water stable aggregates (especially microaggregates) in the tail-ings, which supported our second hypothesis, confirming the positive role of AM fungi in aggregate formation. Plant colonization without concurrent AM fungi inoculation decreased the percentage of micro-aggregates in the tailings (Fig. 1). This might be due to the functions of root exudates, which could disperse mineral particles within aggregates, leading to the breakdown of tailings aggregates (Naveed et al., 2017). However, AM symbiosis increased the percentage of microaggregates and macroaggregates in the tailings (Fig. 1). Generally, AM symbiosis could improve aggregate development through enhancing fine root development and their role in aggregate formation, or through the direct role of AM fungal mycelium (Ji et al., 2019; Vezzani et al., 2018; Zhang et al., 2019). It is reported that plant roots (especially fine roots) could entangle mineral particles to form aggregates (Tisdall and Oades, 1982; Vezzani et al., 2018). In addition, AM fungal mycelium and their exu-dates could also contribute greatly to aggregate formation (Ji et al., 2019; Zhang et al., 2019). Compared with roots, AM fungal mycelium have a diameter of only a few micrometres, but are present in huge volume, therefore providing high surface areas for interfacing and interacting with minerals via direct physical bonds (i.e., direct enmeshment of the mineral particles) and/or fungal metabolite - min-eral complexation (Rillig and Mummey, 2006b; Tisdall, 1991). In the present study, abundant mycelia were produced in the AM fungal inoculated tailings, as revealed by the high mycelium density in the mycorrhizosphere tailings (1.57 m g− 1) (Table 1). The direct role of mycelium (and/or their exudates) in aggregate formation was further revealed by FE-SEM-EDS analysis, in which mycelia-entangled mineral particles in micro-aggregates were showed (Fig. 6). AM fungal mycelium can exudate abundant glomalin-related soil protein (GRSP), which is widely reported to contribute to soil aggregation (Rillig, 2004; Singh et al., 2013). As a kind of hydrophobic glycoprotein rich in functional organic groups (e.g., aromatic and carboxyl), GRSP acts as strong “glue” binding mineral particles together, forming an agglomerate structure, and leading to the development of water-stable aggregates (Rillig, 2004; Zhang et al., 2017).

In addition, plant colonization decreased the percentage of fPOM from 0.98% (w/w) to 0.66% (w/w) (Table S4) in the tailings. The fPOM mainly came from exogenous OM, such as sugarcane mulch initially added in the first stage of the eco-engineering process for improving the physico-chemical properties of the original Fe ore tailings. The decreased fPOM by plant colonization may be due to “rhizosphere priming effects”, which stimulated fPOM decomposition by live roots and associated rhizosphere organisms (Cheng et al., 2014). However, fPOM content in the treatment with AM fungal inoculation increased to 0.79% (Table S4), perhaps, because the AM fungal colonization reduced the “rhizosphere priming effect” for fPOM decomposition in the tailings (Leifheit et al., 2015; Zhou et al., 2020).

The formation of stable aggregates also contributed to OM stabili-zation. In the present findings, about 59% and 55% TOC were stabilized in water stable aggregates in “ET + S + A” and “ET + S” treatments, respectively (Table S3). The stabilized OM was mainly in the form of occluded POM and MOM within aggregates. The POM should originate from partially decomposed exogenous OM initially used to amend and improve physicochemical conditions of the tailings during early eco- engineering processes (Wu et al., 2019a), as well as possible biomass from the dead roots. More importantly, a large proportion of OC was stabilized in the MOM fraction via association with minerals, which was considered to be more stable than POM due to its resistance to microbial attack (Sokol et al., 2019). Typically, the TOC:TN ratio in MOM was much smaller than that in POM, indicating that POM may be mainly enriched by plant derived OM (such as polysaccharides with high TOC: TN ratio), while MOM was enriched in microbially derived OM (such as

protein with low TOC:TN ratio). The present study found that plant colonization slightly decreased TOC concentrations of MOM within macroaggregates, which may be due to OM decomposition driven by “rhizosphere priming effect” during aggregate turnover (Blanco-Canqui and Lal, 2004; Wang et al., 2020). In contrast, AM fungal colonization generally increased the concentration of TOC and TN of MOM within macroaggregates, which may originate from AM fungal mycelia and their exudates readily binding to minerals (Rosling et al., 2004; Zhou et al., 2020). AM symbiosis also increased TOC and TN concentration of MOM within microaggregates, and further decreased the TOC:TN ratio in this fraction, indicating the enrichment of microbially derived OM (probably from microbial biomass or exudates) (Schindler et al., 2007; Singh et al., 2013). This microbial-derived OM may be at least, partially from the AM fungal mycelium or its exudates, as FE-SEM-EDS analysis (Fig. 6) revealed the presence of AM fungal mycelium in micro-aggregates. These findings were consistent with our second assumption, supporting the role of AM symbiosis in OM stabilization within the ag-gregates formed in the tailings. The stabilized intra-aggregate OM probably also contributed greatly to the improvement in aggregate stability in the mycorrhizosphere tailings.

4.3. Organo-mineral association in tailings driven by AM symbiosis

The above findings revealed that plant colonization with AM fungal inoculation significantly stimulated aggregate development and OM stabilization. The AM symbiosis enriched mineral-associated OM within aggregates. To further uncover the critical mechanisms of organo- mineral association, the OC forms and mineralogical composition of MOM fractions were identified by spectroscopic analysis. As revealed by synchrotron-based C1s NEXAFS analysis, the carboxyl C, aromatic C, and alkyl C were dominant OC forms in both Ma-MOM and Mi-MOM fractions. Compared with eco-engineered Fe ore tailings at the start of the experiment (Table S1), the further incubation and plant/AM fungal colonization decreased the proportion of aromatic C, while increased that of carboxyl C in MOM, indicating that aromatic OM-mineral asso-ciations were gradually replaced by carboxyl OM-mineral associations (Fig. 3). This may be because during further incubation, microbial and plant root functions may have stimulated the further oxidation of plant derived OM (aromatic rich lignin like OM) into carboxyl groups through enzymatic oxidative depolymerization (Chauhan, 2020; Higuchi, 2004; Lehmann and Kleber, 2015). Besides, plant exudates or organic com-pounds derived from microbial processes may have also contributed to the shift of organic groups in MOM (Angst et al., 2021).

It is to be noted that AM symbiosis particularly increased the per-centage of carboxyl and aromatic C while decreased that of alkyl C in Mi- MOM (Fig. 3). These OC phase changes may be partially due to the contribution of the AM fungal biomass or exudates, such as GRSP, which is known to be rich in aromatic C (~30%) and carboxyl C (~40%) (Schindler et al., 2007; Zhang et al., 2017). This was consistent with the finding that plant colonization and AM symbiosis increased GRSP con-tent in Mi-MOM fraction, although the GRSP content in the tailings were at a low level (Table 2). At the same time, AM symbiosis was found to have increased N concentration of the MOM fractions, indicating the possible involvement of N rich glomalin-like compounds, or other microbially derived N-rich compounds in MOM (Wright et al., 1998), which could directly interact with minerals or protect the existing organo-mineral associations from water interruption (Kleber et al., 2015b). The GRSP detected in tailings (both bulk tailings and MOM fractions within aggregates) without AM fungal inoculation may origi-nate from other microbes instead of AM fungi. This is because that other organic components not of AMF origin may be co-extracted and cross- reactive in the detection methods of the Bradford protein assay (Purin and Rillig, 2007; Rosier et al., 2006). The slightly decreased GRSP concentrations in tailings after plant colonization may possibly result from the rhizosphere induced microbial community and activity changes (Cheng et al., 2014).

Z. Li et al.

Geoderma 406 (2022) 115528

11

Organic substances rich in functional groups are essential to the initiation and progress of stable aggregation (Six et al., 2000; Tisdall and Oades, 1982), which are closely linked with the molecular forms and functional polymers of organic substances involved at mineral surfaces. Organic compounds rich in oxygen (e.g., carboxylic, aromatic, and phenolic compounds) readily interact with phyllosilicates and/or Fe/Al oxyhydroxides (Coward et al., 2018; Lv et al., 2016). This could explain why aromatic and carboxyl groups were rich in MOM fraction. The carboxyl and aromatic rich organic compounds may interact with min-erals through ligand exchange or hydrophobic interactions (Carmo et al., 2000; Kleber et al., 2015b).

Organo-mineral associations are also regulated by mineral compo-sition, such as different phases of Fe-bearing minerals, which vary greatly in their capacity to bind organic matter (Chen et al., 2014a; Kleber et al., 2015a; Steffens et al., 2017). Compared with those crys-talline Fe minerals, amorphous Fe-minerals (e.g. short-range-order (SRO) minerals, such as ferrihydrite) have greater capacity for organic matter adsorption/binding, because of their high specific surface area (SSA) and large numbers of hydroxyl groups present on mineral surfaces (Cao et al., 2011; Chen et al., 2014b; Fang et al., 2012; Torn et al., 1997; Xiao et al., 2016). As revealed by synchrotron-based Fe K edge XAFS analysis of the bulk tailings, most of Fe was in the form of biotite like minerals, as well as Fe-Si-SRO like minerals (Figure S4b). The amor-phous mineral — Fe-Si-SRO minerals may play an important role in organic matter stabilization and aggregate formation. However, AM fungal inoculation did not influence Fe phases in the tailings, possibly due to their slow bio-weathering rate. Nevertheless, XRD analysis of MOM samples (Fig. 4) revealed that AM symbiosis facilitated the for-mation of vermiculites, probably driven by solid-alteration or progres-sive decomposition of primary minerals like biotite induced by mycorrhizosphere activities (Arocena et al., 2012).

The AM symbiosis driven mineral weathering and secondary mineral formation (as per the third assumption) was further confirmed by the changes of mineral composition and Fe phase in the colloidal fractions, which is the most active part of the eco-engineered tailings (Bolt et al., 2013). It was found that AM symbiosis increased the formation of kaolinite and piemontite-like minerals in the colloidal fraction (Fig. 5a), which was formed probably by hydrothermal alteration of other phyl-losilicate minerals (Nagashima, 2006). Further Fe K edge EXAFS anal-ysis revealed that AM symbiosis tended to decrease the proportion of biotite-like minerals in colloids (Figure S4c). Nearly 50% of Fe in the colloids existed in the form of Fe-Si-SRO, which was greater than that in the bulk tailings (Figure S4). The formation and enrichment of these amorphous Fe-Si minerals would have favoured organic C sequestration. This was further confirmed by the increase of Fe(III)-oxalate complexes in couple with the decrease of amorphous secondary Fe(III) minerals such as Fe-Si-SRO like minerals in the colloids, in response to AM symbiosis colonization, based on Fe K edge XANES (Fig. 5b). The functional organic groups (like carboxyl groups) interactions with Fe minerals would stimulate the mineral weathering (ligand exchange pathway (Kiczka et al., 2010)), organo-mineral association, and organic carbon sequestration in the tailings (Kleber et al., 2007). Overall, the formation of amorphous Fe-Si-SRO like minerals or other secondary minerals (like vermiculite) and the accumulation of N-rich OM induced by AM fungal colonization probably facilitated the organo-mineral as-sociation and aggregate formation in the tailings, which supported the third hypothesis.

5. Environmental implications

The present study has among the first revealed that AM symbiosis could be established in the early eco-engineered Fe ore tailings (primed by initial OM amendment and pioneer plant colonization). We confirmed our assumptions that AM symbiosis could improve aggregate development and enhance OM stabilization by minerals through organo- mineral association. The enhanced organo-mineral association would be

due to the enrichment of functional organic groups (rich in carboxyl/ aromatic C with low TOC:TN ratio) and their association with key sec-ondary Fe-Si minerals formed during mycorrhizosphere driven mineral weathering. The AM symbiosis with pioneer plants could be thus exploited to advance water-stable aggregation and soil formation in the tailings. These findings thus provide important information for inte-grating AM fungi into the process of eco-engineering alkaline Fe ore tailings into soil, or functional technosols for sustainable plant growth and revegetation outcomes. It is important to point out that the current study was based on glasshouse conditions where external factors could be well controlled. The long-term functions of AM fungi in the tailings may be influenced by both biotic (plants and microbial communities) and abiotic (P nutrition, climate conditions such as drought) factors. Therefore, future study is required to evaluate the establishment and functionality of AM fungal-plant symbiosis in eco-engineered soil for-mation of tailings under field conditions at mine sites.

CRediT authorship contribution statement

Zhen Li: Writing – original draft. Songlin Wu: Conceptualization, Supervision, Methodology, Writing – review & editing. Yunjia Liu: Methodology, Writing – review & editing. Qing Yi: Methodology. Fang You: Methodology. Yuanying Ma: Methodology. Lars Thomsen: Methodology. Ting-Shan Chan: Supervision, Methodology. Ying-Rui Lu: Methodology. Merinda Hall: Methodology, Writing – review & editing. Narottam Saha: Methodology. Yuanfang Huang: Writing – review & editing. Longbin Huang: Supervision, Funding acquisition, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was funded by Australian Research Council Linkage Project (LP160100598), Karara Mining Limited, and The Botanic Gar-dens and Parks Authority (BGPA), as well as CSC-UQ PhD scholarship (File No. 201906350122), Taiwan Ministry of Science and Technology (contract no. MOST110-2113-M-213-002). Dr. Jyh-Fu Lee in 17C within NSRRC, Taiwan is acknowledged for the beamtime support. The C 1 s NEXAFS was conducted on the SXR beamline at the Australian Syn-chrotron, part of ANSTO (Project Reference No: AS2/SXR/16207). Bruce C. C. Cowie at the SXR beamline of the Australian synchrotron is acknowledged for his assistance in the data collection of C 1 s NEXAFS analysis. The authors acknowledge Rick Webb and Ying Yu, in the Centre for Microscopy and Microanalysis, The University of Queensland, as well as the Queensland Node of the Australian National Fabrication Facility (ANFF-Q) for technical assistance in FE-SEM-EDS, XRD, and ATR-FTIR analysis. The authors thank Steven Mason in SCMB, UQ, for technical assistance with the experiment.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.geoderma.2021.115528.

References

Abbott, L., Robson, A., DE BOER, G., 1984. The effect of phosphorus on the formation of hyphae in soil by thevesicular-arbuscular mycorrhizal fungus. Glomus fasciculatum. New Phytologist 97 (3), 437–446.

Abd-Alla, M.H., El-Enany, A.W., Nafady, N.A., Khalaf, D.M., Morsy, F.M., 2014. Synergistic interaction of Rhizobium leguminosarum bv. viciae and arbuscular

Z. Li et al.

Geoderma 406 (2022) 115528

12

mycorrhizal fungi as a plant growth promoting biofertilizers for faba bean (Vicia faba L.) in alkaline soil. Microbiol Res 169 (1), 49–58.

Aliasgharzad, N., Rastin, S.N., Towfighi, H., Alizadeh, A., 2001. Occurrence of arbuscular mycorrhizal fungi in saline soils of the Tabriz Plain of Iran in relation to some physical and chemical properties of soil. Mycorrhiza 11 (3), 119–122.

Amezketa, E., 1999. Soil Aggregate Stability: A Review. J. Sustainable Agric. 14 (2–3), 83–151.

Angst, G., Mueller, K.E., Nierop, K.G.J., Simpson, M.J., 2021. Plant- or microbial- derived? A review on the molecular composition of stabilized soil organic matter. Soil Biology and Biochemistry 156.

Arocena, J.M., Velde, B., Robertson, S.J., 2012. Weathering of biotite in the presence of arbuscular mycorrhizae in selected agricultural crops. Applied Clay Science 64, 12–17.

Bearden, B.N., Petersen, L., 2000. Influence of arbuscular mycorrhizal fungi on soil structure and aggregate stability of a vertisol. Plant Soil 218 (1), 173–183.

Bedini, S., Pellegrino, E., Avio, L., Pellegrini, S., Bazzoffi, P., Argese, E., Giovannetti, M., 2009. Changes in soil aggregation and glomalin-related soil protein content as affected by the arbuscular mycorrhizal fungal species Glomus mosseae and Glomus intraradices. Soil Biol. Biochem. 41 (7), 1491–1496.

Blanco-Canqui, H., Lal, R., 2004. Mechanisms of Carbon Sequestration in Soil Aggregates. Critical Reviews in Plant Sciences 23 (6), 481–504.

Bolt, G.H., De Boodt, M., Hayes, M.H., McBride, M., De Strooper, E., 2013. Interactions at the Soil Colloid: Soil Solution Interface, 190. Springer Science & Business Media.

Bradford, M.M., 1976a. A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding. Analytical biochemistry 72 (1–2), 248–254.

Calderon, F.J., Reeves, J.B., Collins, H.P., Paul, E.A., 2011. Chemical Differences in Soil Organic Matter Fractions Determined by Diffuse-Reflectance Mid-Infrared Spectroscopy. Soil Science Society of America Journal 75 (2), 568–579.

Cao, Y., Wei, X., Cai, P., Huang, Q., Rong, X., Liang, W., 2011. Preferential adsorption of extracellular polymeric substances from bacteria on clay minerals and iron oxide. Colloids Surf. B. Biointerfaces 83 (1), 122–127.

Carmo, A.M., Hundal, L.S., Thompson, M.L., 2000. Sorption of hydrophobic organic compounds by soil materials: application of unit equivalent Freundlich coefficients. Environ. Sci. Technol. 34 (20), 4363–4369.

Chauhan, P.S., 2020. Role of various bacterial enzymes in complete depolymerization of lignin: A review. Biocatalysis and agricultural biotechnology 23, 101498.

Chen, B.D., Zhu, Y.G., Duan, J., Xiao, X.Y., Smith, S.E., 2007. Effects of the arbuscular mycorrhizal fungus Glomus mosseae on growth and metal uptake by four plant species in copper mine tailings. Environ Pollut 147 (2), 374–380.

Chen, C., Dynes, J.J., Wang, J., Karunakaran, C., Sparks, D.L., 2014a. Soft X-ray spectromicroscopy study of mineral-organic matter associations in pasture soil clay fractions. Environ. Sci. Technol. 48 (12), 6678–6686.

Chen, C., Dynes, J.J., Wang, J., Sparks, D.L., 2014b. Properties of Fe-organic matter associations via coprecipitation versus adsorption. Environmental science & technology 48 (23), 13751–13759.

Cheng, W., Parton, W.J., Gonzalez-Meler, M.A., Phillips, R., Asao, S., McNickle, G.G., Brzostek, E., Jastrow, J.D., 2014. Synthesis and modeling perspectives of rhizosphere priming. New Phytol 201 (1), 31–44.

Cocozza, C., D’orazio, V., Miano, T., Shotyk, W., 2003. Characterization of solid and aqueous phases of a peat bog profile using molecular fluorescence spectroscopy, ESR and FT-IR, and comparison with physical properties. Organic Geochemistry 34 (1), 49–60.

Coward, E.K., Ohno, T., Plante, A.F., 2018. Adsorption and molecular fractionation of dissolved organic matter on iron-bearing mineral matrices of varying crystallinity. Environ. Sci, Technol.

Cowie, B., Tadich, A., Thomsen, L., 2010. The current performance of the wide range (90–2500 eV) soft X-ray beamline at the australian synchrotron, AIP Conf. Proc. American Institute of Physics 307–310.

De Boodt, M.F., Hayes, M.H., Herbillon, A., 2013. Soil colloids and their associations in aggregates. Springer Science & Business Media, p. 214.

Denef, K., Six, J., 2005. Clay mineralogy determines the importance of biological versus abiotic processes for macroaggregate formation and stabilization. European Journal of Soil Science 56 (4), 469–479.

Dhillon, G.S., Gillespie, A., Peak, D., Van Rees, K.C.J., 2017. Spectroscopic investigation of soil organic matter composition for shelterbelt agroforestry systems. Geoderma 298, 1–13.

Ehlert, C., Unger, W.E., Saalfrank, P., 2014. C K-edge NEXAFS spectra of graphene with physical and chemical defects: a study based on density functional theory. Phys Chem Chem Phys 16 (27), 14083–14095.

Fang, L., Cao, Y., Huang, Q., Walker, S.L., Cai, P., 2012. Reactions between bacterial exopolymers and goethite: A combined macroscopic and spectroscopic investigation. Water Res. 46 (17), 5613–5620.

Gann, E., McNeill, C.R., Tadich, A., Cowie, B.C., Thomsen, L., 2016. Quick AS NEXAFS Tool (QANT): a program for NEXAFS loading and analysis developed at the Australian Synchrotron. J Synchrotron Radiat 23 (1), 374–380.

Hart, M.M., Forsythe, J.A., 2012. Using arbuscular mycorrhizal fungi to improve the nutrient quality of crops; nutritional benefits in addition to phosphorus. Scientia Horticulturae 148, 206–214.

Higuchi, T.J.P.o.t.J.A., Series B, 2004. Microbial degradation of lignin: Role of lignin peroxidase, manganese peroxidase, and laccase. 80(5), 204-214.

Huang, L., Baumgartl, T., Zhou, L., Mulligan, R., 2014. The new paradigm for phytostabilising mine wastes–ecologically engineered pedogenesis and functional root zones. Life-of-Mine Conference 16–18.

Huang, X., Kang, W., Guo, J., Wang, L., Tang, H., Li, T., Yu, G., Ran, W., Hong, J., Shen, Q., 2020. Highly reactive nanomineral assembly in soil colloids: Implications for paddy soil carbon storage. Sci. Total Environ. 703, 134728.

Jakobsen, L., 1992. External hyphae of vesicular-arbuscular mycorrhizal fungi associated with Trifolium subterraneum L. New Phytol 120.

Ji, L., Tan, W., Chen, X., 2019. Arbuscular mycorrhizal mycelial networks and glomalin- related soil protein increase soil aggregation in Calcaric Regosol under well-watered and drought stress conditions. Soil and Tillage Research 185, 1–8.

Kemper, W., Rosenau, R., 1986. Aggregate stability and size distribution. Methods of soil analysis: Part 1 Physical and mineralogical methods 5, 425–442.

Kiczka, M., Wiederhold, J.G., Frommer, J., Kraemer, S.M., Bourdon, B., Kretzschmar, R., 2010. Iron isotope fractionation during proton-and ligand-promoted dissolution of primary phyllosilicates. Geochim. Cosmochim. Acta 74 (11), 3112–3128.

Kiers, E.T., Duhamel, M., Beesetty, Y., Mensah, J.A., Franken, O., Verbruggen, E., Fellbaum, C.R., Kowalchuk, G.A., Hart, M.M., Bago, A., 2011. Reciprocal rewards stabilize cooperation in the mycorrhizal symbiosis. Science 333 (6044), 880–882.

Kleber, M., Eusterhues, K., Keiluweit, M., Mikutta, C., Mikutta, R., Nico, P.S., 2015a. Mineral-Organic Associations: Formation, Properties, and Relevance in Soil Environments. Adv Agron 130, 1–140.

Kleber, M., Eusterhues, K., Keiluweit, M., Mikutta, C., Mikutta, R., Nico, P.S., 2015b. Mineral-Organic Associations: Formation, Properties, and Relevance in Soil Environments. Advances in Agronomy 1–140.

Kleber, M., Sollins, P., Sutton, R., 2007. A conceptual model of organo-mineral interactions in soils: self-assembly of organic molecular fragments into zonal structures on mineral surfaces. Biogeochemistry 85 (1), 9–24.

Koningsberger, D., Mojet, B., Van Dorssen, G., Ramaker, D., 2000. XAFS spectroscopy; fundamental principles and data analysis. Top. Catal. 10 (3–4), 143–155.

Lehmann, J., Kleber, M., 2015. The contentious nature of soil organic matter. Nature 528 (7580), 60–68.

Leifheit, E.F., Verbruggen, E., Rillig, M.C., 2015. Arbuscular mycorrhizal fungi reduce decomposition of woody plant litter while increasing soil aggregation. Soil Biol. Biochem. 81, 323–328.

Liu, Y., Zhang, G., Luo, X., Hou, E., Zheng, M., Zhang, L., He, X., Shen, W., Wen, D., 2021. Mycorrhizal fungi and phosphatase involvement in rhizosphere phosphorus transformations improves plant nutrition during subtropical forest succession. Soil Biology and Biochemistry 153.

Lv, J., Zhang, S., Wang, S., Luo, L., Cao, D., Christie, P., 2016. Molecular-Scale Investigation with ESI-FT-ICR-MS on Fractionation of Dissolved Organic Matter Induced by Adsorption on Iron Oxyhydroxides. Environ. Sci. Technol. 50 (5), 2328–2336.

Bradford, M.M., 1976b. A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding. Analytical biochemistry 72 (1–2), 248–254.

Mackay, J.E., Cavagnaro, T.R., Müller Stover, D.S., Macdonald, L.M., Grønlund, M., Jakobsen, I., 2017. A key role for arbuscular mycorrhiza in plant acquisition of P from sewage sludge recycled to soil. Soil Biology and Biochemistry 115, 11–20.

McKeague, J., Day, J., 1966. Dithionite-and oxalate-extractable Fe and Al as aids in differentiating various classes of soils. Canadian journal of soil science 46 (1), 13–22.

Miller, R., Jastrow, J., 2000. Mycorrhizal fungi influence soil structure. Arbuscular mycorrhizas: physiology and function. Springer 3–18.

Murphy, J., Riley, J.P., 1962. A modified single solution method for the determination of phosphate in natural waters. Analytica chimica acta 27, 31–36.

Nagashima, M., 2006. Hydrothermal syntheses of epidote and piemontites on the join Ca2Al2Fe3+ Si3O12 (OH)-Ca2Al2Mn3+ Si3O12 (OH) at relatively low pressures of 200–400 MPa. Journal of mineralogical and petrological sciences 101 (1), 1–9.

Naveed, M., Brown, L., Raffan, A., George, T.S., Bengough, A.G., Roose, T., Sinclair, I., Koebernick, N., Cooper, L., Hackett, C.A., 2017. Plant exudates may stabilize or weaken soil depending on species, origin and time. Eur. J. Soil Sci. 68 (6), 806–816.

Olsen, S.R., 1954. Estimation of available phosphorus in soils by extraction with sodium bicarbonate. US Department of Agriculture.

Parfitt, R., Childs, C., 1988. Estimation of forms of Fe and Al-a review, and analysis of contrasting soils by dissolution and Mossbauer methods. Soil Research 26 (1), 121–144.

Parniske, M., 2008. Arbuscular mycorrhiza: the mother of plant root endosymbioses. Nat Rev Microbiol 6 (10), 763–775.

Pedersen, J.A., Simpson, M.A., Bockheim, J.G., Kumar, K., 2011. Characterization of soil organic carbon in drained thaw-lake basins of Arctic Alaska using NMR and FTIR photoacoustic spectroscopy. Organic Geochemistry 42 (8), 947–954.

Phillips, J.M., Hayman, D., 1970. Improved procedures for clearing roots and staining parasitic and vesicular-arbuscular mycorrhizal fungi for rapid assessment of infection. Transactions of the British mycological Society 55 (1), 158–161.

Prietzel, J., Muller, S., Kogel-Knabner, I., Thieme, J., Jaye, C., Fischer, D., 2018. Comparison of soil organic carbon speciation using C NEXAFS and CPMAS (13)C NMR spectroscopy. Sci Total Environ 628–629, 906–918.

Purin, S., Rillig, M.C.J.P., 2007. The arbuscular mycorrhizal fungal protein glomalin: limitations, progress, and a new hypothesis for its function. 51 (2), 123–130.

Ravel, B., Newville, M., 2005. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT. Journal of synchrotron radiation 12 (4), 537–541.

Rillig, M.C., 2004. Arbuscular mycorrhizae, glomalin, and soil aggregation. Can. J. Soil Sci. 84 (4), 355–363.

Rillig, M.C., Mummey, D.L., 2006. Mycorrhizas and soil structure. New Phytol 171 (1), 41–53.

Robertson, L.M., Wu, S., You, F., Huang, L., Southam, G., Chan, T.S., Lu, Y.R., Bond, P.L., 2020. Geochemical and mineralogical changes in magnetite Fe-ore tailings induced by biomass organic matter amendment. Sci Total Environ 724, 138196.

Z. Li et al.

Geoderma 406 (2022) 115528

13

Rosier, C.L., Hoye, A.T., Rillig, M.C., 2006. Glomalin-related soil protein: Assessment of current detection and quantification tools. Soil Biology and Biochemistry 38 (8), 2205–2211.

Rosling, A., Lindahl, B.D., Finlay, R.D., 2004. Carbon allocation to ectomycorrhizal roots and mycelium colonising different mineral substrates. New Phytologist 162 (3), 795–802.

Schindler, F.V., Mercer, E.J., Rice, J.A., 2007. Chemical characteristics of glomalin- related soil protein (GRSP) extracted from soils of varying organic matter content. Soil Biology and Biochemistry 39 (1), 320–329.

Schumacher, M., Christl, I., Scheinost, A.C., Jacobsen, C., Kretzschmar, R., 2005. Chemical heterogeneity of organic soil colloids investigated by scanning transmission X-ray microscopy and C-1s NEXAFS microspectroscopy. Environ. Sci. Technol. 39 (23), 9094–9100.

Singh, P.K., Singh, M., Tripathi, B.N., 2013. Glomalin: an arbuscular mycorrhizal fungal soil protein. Protoplasma 250 (3), 663–669.

Six, J., Conant, R.T., Paul, E.A., Paustian, K., 2002. Stabilization mechanisms of soil organic matter: implications for C-saturation of soils. Plant and soil 241 (2), 155–176.

Six, J., Paustian, K., Elliott, E., Combrink, C., 2000. Soil structure and organic matter I. Distribution of aggregate-size classes and aggregate-associated carbon. Soil Science Society of America Journal 64 (2), 681–689.

Smith, S.E., Read, D.J., 2010. Mycorrhizal symbiosis. Academic press. Sokol, N.W., Sanderman, J., Bradford, M.A., 2019. Pathways of mineral-associated soil

organic matter formation: Integrating the role of plant carbon source, chemistry, and point of entry. Glob Chang Biol 25 (1), 12–24.

Solis-Dominguez, F.A., Valentin-Vargas, A., Chorover, J., Maier, R.M., 2011. Effect of arbuscular mycorrhizal fungi on plant biomass and the rhizosphere microbial community structure of mesquite grown in acidic lead/zinc mine tailings. Sci. Total Environ. 409 (6), 1009–1016.

Steffens, M., Rogge, D.M., Mueller, C.W., Hoschen, C., Lugmeier, J., Kolbl, A., Kogel- Knabner, I., 2017. Identification of distinct functional microstructural domains controlling C storage in soil. Environmental science & technology 51 (21), 12182–12189.

Tennant, D., 1975. A test of a modified line intersect method of estimating root length. The Journal of Ecology 995–1001.

Tisdall, J., 1991. Fungal hyphae and structural stability of soil. Soil Research 29 (6), 729–743.

Tisdall, J.M., Oades, J.M., 1982. Organic matter and water-stable aggregates in soils. Eur. J. Soil Sci. 33 (2), 141–163.

Torn, M.S., Trumbore, S.E., Chadwick, O.A., Vitousek, P.M., Hendricks, D.M., 1997. Mineral control of soil organic carbon storage and turnover. Nature 389 (6647), 170–173.

Trouvelot, A., Kough, J., Gianinazzi-Pearson, V., 1986. Mesure du taux de mycorhization VA d’un systeme radiculaire. Recherche de methode d’estimation ayant une signification fonctionnelle, Physiological and genetical aspects of mycorrhizae: proceedings of the 1st european symposium on mycorrhizae, Dijon, 1-5 July 1985, pp. 217-221.

Vezzani, F.M., Anderson, C., Meenken, E., Gillespie, R., Peterson, M., Beare, M.H., 2018. The importance of plants to development and maintenance of soil structure,

microbial communities and ecosystem functions. Soil and Tillage Research 175, 139–149.

Wang, X., Yin, L., Dijkstra, F.A., Lu, J., Wang, P., Cheng, W., 2020. Rhizosphere priming is tightly associated with root-driven aggregate turnover. Soil Biol. Biochem. 149.

Wright, S.F., Upadhyaya, A.J.P., soil, 1998. A survey of soils for aggregate stability and glomalin, a glycoprotein produced by hyphae of arbuscular mycorrhizal fungi. 198 (1), 97-107.

Wright, S.F., Upadhyaya, A.J.S.s., 1996. Extraction of an abundant and unusual protein from soil and comparison with hyphal protein of arbuscular mycorrhizal fungi. 161 (9), 575-586.

Wu, S., Liu, Y., Bougoure, J.J., Southam, G., Chan, T.S., Lu, Y.R., Haw, S.C., Nguyen, T.A. H., You, F., Huang, L., 2019a. Organic Matter Amendment and Plant Colonization Drive Mineral Weathering, Organic Carbon Sequestration, and Water-Stable Aggregation in Magnetite Fe Ore Tailings. Environ Sci Technol 53 (23), 13720–13731.

Wu, S., Liu, Y., Southam, G., Robertson, L., Chiu, T.H., Cross, A.T., Dixon, K.W., Stevens, J.C., Zhong, H., Chan, T.S., Lu, Y.J., Huang, L., 2019b. Geochemical and mineralogical constraints in iron ore tailings limit soil formation for direct phytostabilization. Sci Total Environ 651 (Pt 1), 192–202.

Wu, S., Liu, Y., Southam, G., Robertson, L.M., Wykes, J., Yi, Q., Hall, M., Li, Z., Sun, Q., Saha, N., Chan, T.-S., Lu, Y.-R., Huang, L., 2021. Rhizosphere Drives Biotite-Like Mineral Weathering and Secondary Fe–Si Mineral Formation in Fe Ore Tailings. ACS Earth and Space. Chemistry.

Wu, S., Nguyen, T.A.H., Liu, Y., Southam, G., Wang, S., Chan, T.-S., Lu, Y.-R., Huang, L., 2019c. Deficiencies of secondary Fe (oxy)hydroxides associated with phyllosilicates and organic carbon limit the formation of water-stable aggregates in Fe-ore tailings. Chem. Geol. 523, 73–87.

Wu, S., You, F., Wu, Z., Bond, P., Hall, M., Huang, L., 2020. Molecular diversity of arbuscular mycorrhizal fungal communities across the gradient of alkaline Fe ore tailings, revegetated waste rock to natural soil sites. Environ Sci Pollut Res Int 27 (11), 11968–11979.

Wu, Z., McGrouther, K., Huang, J., Wu, P., Wu, W., Wang, H., 2014. Decomposition and the contribution of glomalin-related soil protein (GRSP) in heavy metal sequestration: Field experiment. Soil Biology and Biochemistry 68, 283–290.

Xiao, J., He, X., Hao, J., Zhou, Y., Zheng, L., Ran, W., Shen, Q., Yu, G., 2016. New strategies for submicron characterization the carbon binding of reactive minerals in long-term contrasting fertilized soils: implications for soil carbon storage. Biogeosciences 13 (12), 3607–3618.

You, F., Dalal, R., Huang, L., 2018. Initiation of soil formation in weathered sulfidic Cu- Pb-Zn tailings under subtropical and semi-arid climatic conditions. Chemosphere 204, 318–326.

Zhang, J., Tang, X., Zhong, S., Yin, G., Gao, Y., He, X., 2017. Recalcitrant carbon components in glomalin-related soil protein facilitate soil organic carbon preservation in tropical forests. Sci. Rep. 7 (1), 2391.

Zhang, Z., Mallik, A., Zhang, J., Huang, Y., Zhou, L., 2019. Effects of arbuscular mycorrhizal fungi on inoculated seedling growth and rhizosphere soil aggregates. Soil and Tillage Research 194.

Zhou, J., Zang, H., Loeppmann, S., Gube, M., Kuzyakov, Y., Pausch, J., 2020. Arbuscular mycorrhiza enhances rhizodeposition and reduces the rhizosphere priming effect on the decomposition of soil organic matter. Soil Biology and Biochemistry 140.

Z. Li et al.