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Soil Biology & Biochemistry 72 (2014) 97e99

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Soil Biology & Biochemistry

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Short communication

Increased microbial activity under elevated [CO2] does not enhanceresidue decomposition in a semi-arid cropping system in Australia

Shu Kee Lam a, Rob Norton a,b, Roger Armstrong c,d, Deli Chen a,*

aMelbourne School of Land and Environment, The University of Melbourne, Victoria 3010, Australiab International Plant Nutrition Institute, 54 Florence Street, Horsham, Victoria 3400, AustraliacDepartment of Environment and Primary Industries, Private Bag 260, Victoria 3401, AustraliadDepartment of Agricultural Sciences, La Trobe University, Bundoora, Victoria 3086, Australia

a r t i c l e i n f o

Article history:Received 11 September 2013Received in revised form13 January 2014Accepted 28 January 2014Available online 7 February 2014

Keywords:Free-air CO2 enrichmentSoil respirationResidue decompositionMicrobial activitySemi-arid cropping systemCarbon sequestration

* Corresponding author. Tel.: þ61 3 83448148; fax:E-mail address: delichen@unimelb.edu.au (D. Che

0038-0717/$ e see front matter � 2014 Elsevier Ltd.http://dx.doi.org/10.1016/j.soilbio.2014.01.028

a b s t r a c t

The association between the responses of microbial activity and residue decomposition to elevated at-mospheric [CO2] under field conditions in Australian cropping systems is unknown. We measured soilCO2 emission and decomposition of wheat and field pea residues in a wheat cropping system in the fieldusing the Australian Grains Free-Air CO2 Enrichment (AGFACE) facility in Horsham, Victoria. Elevated[CO2] (550 mmol mol�1) increased soil CO2 emission by 41%, but did not affect the percentage of theoriginal mass or C remaining for either type of residue throughout the experimental period. Our findingssuggest that the rates of residue decomposition and residue C mineralization in this semi-arid wheatcropping system were not affected by elevated [CO2] despite higher microbial activity. This has majorimplication for the C sequestration potential of semi-arid cropping systems under future CO2 climates.

� 2014 Elsevier Ltd. All rights reserved.

If carbon dioxide (CO2) emissions continue to increase at thepresent rate, atmospheric CO2 concentration (currently400 mmol mol�1) is estimated to reach about 550 mmol mol�1 by2050 (IPCC, 2007). Concerns about rising atmospheric [CO2] haveraised global interest in managing agricultural soils as carbon (C)sinks (Stockmann et al., 2013). Increases in [CO2] generally stimu-late C3 plant photosynthesis and crop growth, thereby increasingthe production of crop residues and root exudates (Drake et al.,1997; Ainsworth and Long, 2005). This increase in C input to soilhas beenwidely reported to enhancemicrobial activity and soil CO2emission (Zak et al., 2000; de Graaff et al., 2006; Kou et al., 2007).Evidence of enhanced microbial activity increasing the rate ofresidue decomposition is inconclusive (Torbert et al., 2000; Norbyet al., 2001) but has major implication for the potential of crop-ping systems to sequester C. No such information is available forfield conditions in Australian cropping systems.

To address this knowledge gap, we conducted field experimentsin the Australian Grains Free-Air Carbon dioxide Enrichment(AGFACE) facility at Horsham (36�450S,142�070E), Victoria, Australiato investigate the effect of elevated [CO2] on soil respiration andresidue decomposition. We hypothesized that elevated [CO2]

þ61 3 83445037.n).

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increases soil respiration, and that the enhanced microbial activitystimulates residue decomposition under elevated [CO2]. We usedfour ambient and four elevated [CO2] (550 mmol mol�1) experi-mental areas in this study. The design and technology of the systemwere described in Mollah et al. (2009). The soil is an alkalinecracking clay Vertosol (Isbell, 1996) with 37% clay in the soil surfaceincreasing with depth. The soil (0e0.20m) has a dry bulk density of1.30 g cm�3, pH (1:5 soil:water) of 8.33, and contains 1.10% organicC and 0.14% total nitrogen (N). The study region receives an averageof 316 mm of rainfall between May and November, which is thegrowing season for winter crops, and an average maximum tem-perature of 17.5 �C over the same period.

Wheat (Triticum aestivum L. cv. Yitpi) was sown at row spacingof 22 cm on June 3, 2008. Residues of wheat and field pea (Pisumsativum L.) grown under ambient [CO2] were collected from a farmnear the study site. The C:N ratio of wheat (non-legume) and fieldpea (legume) residue was 59.4 (45.1% C; 0.76%N) and 26.7 (44.0%C;1.65%N), respectively, and was significantly different (Table 1). Weused the same residues for decomposition under ambient andelevated [CO2]. The crop residues were washed with water, oven-dried at 60 �C and cut into pieces of around 3 cm long. Usinglitter-bagmethod (Knacker et al., 2003), we put 3 g of eachmaterialinto polyester bag of 10 cm by 15 cm, with 1 mm mesh size. Fivebags of each residue type were buried to 5 cm depth at a random

Table 1Initial C:N ratio and final mass recovery (%) of wheat and field pea residues (n ¼ 4,mean � SE).

Residue C:N Mass recovery (%)

Ambient [CO2] Elevated [CO2] l.s.d.(p ¼ 0.05)

Wheat 59.4 � 2.1 *** 73.1 � 2.0 72.5 � 3.4 6.6Field pea 26.7 � 1.5 50.0 � 2.4 49.3 � 1.4

***p < 0.001 by t-test.l.s.d.: least significant difference.

Fig. 1. Effect of elevated [CO2] on (a) soil CO2 emission and (b) gravimetric soilmoisture content (n ¼ 3). Vertical bars indicate standard errors.

S.K. Lam et al. / Soil Biology & Biochemistry 72 (2014) 97e9998

location in each treatment area on July 29. One each of these fivebags was collected 28, 56, 84, 112 and 134 days after burial of res-idue bags. The residue remaining in the bag was washed withwater, dried at 60 �C for 48 h, weighed, ground to a fine powder ofw100 mm, and analysed for total C by elemental analyzer (NA 1500NCS, Fisons).

Gas samples for CO2 analysis were taken from closed staticchamber (15 cm height by 16 cm diameter) (Lam et al., 2011) onSeptember 24, October 7 & 30, November 18 and December 10between 1300 and 1500 h of the day. In the same treatment areawhere the litter bags were placed, one chamber was inserted be-tween the rows of wheat so that wheat plants were excluded fromthe chamber. Chambers were inserted a day before the first gassampling to a soil depth of 7 cm, and then remained in situthroughout the experimental period. Five gas samples (15mL)werecollected from the chambers at 7 min intervals (chambersremained closed) using a gas-tight syringe, transferred into vacu-tainer (Exetainer�, Labco) and analysed by gas chromatography (HP6890, Hewlett Packard). The fluxes of CO2 were calculated asdescribed by Ruser et al. (1998). Two samples of top soil (0e10 cm)collected by auger near each chamber were bulked. Gravimetric soilmoisture content was determined by oven-drying the subsamples(10 g) of the soil at 105 �C for 48 h. Data were analysed withMINITAB 16 statistical package using Pearson correlation, t-test andGeneral Linear Model analysis of variance.

The fluxes of CO2 ranged from 5.5 to 16.1 mg C m�2 h�1 underambient [CO2] and 5.4e24.7 mg C m�2 h�1 under elevated [CO2],decreasing as the growing season progressed and the soil dried out(Fig. 1a, b). The mild but positive correlation between soil CO2 fluxand soil moisture (r ¼ 0.48, p < 0.001) indicates that soil moisturepartially regulated soil respiration. Elevated [CO2] had no signifi-cant effect on soil moisture (Fig. 1b), although the water use effi-ciency of crops has been reported to increase under elevated [CO2](Leakey et al., 2009). Elevated [CO2] increased CO2 flux by 41%(p< 0.05) when averaged across the growth period. As therewas nowheat growing inside the gas chambers, above-ground plantrespiration was excluded. The [CO2]-induced increase in CO2emission was therefore attributed to higher soil microbial activityand root activity under elevated [CO2] in this wheat cropping sys-tem. The contribution of microbial respiration to the enhanced soilrespiration under elevated [CO2] was reported to be greater thanthat of root respiration in a wheat field (Kou et al., 2007).

Our finding is consistent with increases (15e39%) in CO2 emis-sion recorded in other FACE sites growing wheat (Pendall et al.,2001; Kou et al., 2007; Lam et al., 2011), and is widely attributedto the greater input of root exudates into the soil under elevated[CO2] (Cheng and Johnson, 1998; Zak et al., 2000; Kou et al., 2007).But will this observed increased microbial activity stimulate res-idue decomposition?

We found that the rate of decomposition of added residuesinitially decreased with time after burial in the soil before levellingoff at around 60 days later, regardless of residue type and CO2 level

(Fig. 2a). At the end of the experiment (crop maturity), the per-centage mass remaining for field pea residue was 50% and 49%under ambient and elevated [CO2], respectively, and that for wheatresidue was 73% for both CO2 treatments (Table 1). The higherpercentage wheat residue mass remaining than that of field pearesiduewas attributed to the higher C:N ratio of wheat residue thanfor field pea residue (Table 1). This suggests that a higher C:N ratioof C3 non-legume (less profound for legume) crop residues underelevated [CO2] (Kimball et al., 2002; Lam et al., 2012) may likelyaffect the C and N cycles in cropping systems in a higher CO2 world.

Elevated [CO2] had no significant effect on the mass recovery ofeither residue type (Fig. 2a). The response of C recovery to elevated[CO2] followed similar pattern as observed with mass recovery(Fig. 2b). This indicates that C content of labile (decomposed)fraction of the residues is same as that in the remaining residues,and that the ratio of C decomposed to total mass decomposed wasnot affected by elevated [CO2]. These results suggest that elevated[CO2] did not affect the rates of residue decomposition and residueC mineralization in this wheat cropping system despite higher soilmicrobial activity, at least in the short term.

Our finding of non-significant effect of atmospheric [CO2] onresidue decomposition contrasts with the study of Lin et al. (1999),who observed increased residue decomposition (using commonsubstrates) and higher soil CO2 emission under elevated [CO2]. Theauthors attributed the increased residue decomposition to theextremely low inorganic N availability in the Douglas-fir systemwhere microbial communities rely on residue decomposition toacquire N (Lin et al., 1999). In contrast, inorganic N availability ofcropping systems is usually higher. Soil mineral N in the top 10 cmof our study site was equivalent to 42.5 kg N ha�1 at sowing, whichreflects the history of the paddock. Higher soil respiration but un-affected residue decomposition rate under elevated [CO2] was also

Fig. 2. Percentage recovery of (a) mass and (b) carbon from wheat and field pea res-idues decomposed under ambient and elevated [CO2] (n ¼ 4). Vertical bars indicatestandard errors.

S.K. Lam et al. / Soil Biology & Biochemistry 72 (2014) 97e99 99

observed in a perennial ryegrass pasture under N application of140 kg N ha�1 (de Graaff et al., 2004).

These results have major implications for potential C seques-tration in semi-arid cropping systems under future CO2 climates.An increase in crop (residue) biomass under elevated [CO2](Kimball et al., 2002) with no concurrent changes in residuedecomposition rate would likely counteract the enhanced C lossfrom soil respiration, resulting in a relative gain in soil C. None-theless, to sustain soil C accumulation under elevated [CO2] othernutrients such as N, phosphorus and sulphur will be required (deGraaff et al., 2006; Hungate et al., 2009; Kirkby et al., 2011).Long-term study is required to explore any possible interactionsbetween [CO2] and water on residue decomposition and soilrespiration across growing seasons.

Acknowledgements

This work was supported by the Grains Research and Develop-ment Corporation, the Federal Department of Agriculture, Fisheriesand Forestry, the Victorian Department of Environment and Pri-mary Industries, the University of Melbourne, and the AustralianResearch Council. The authors wish to thank Mr. Peter Howie, Mr.Russel Argall and Mr. Barry Bellman for field assistance, Dr. HelenSuter for assistance with gas analysis, and Mr. Ron Teo for chemicalanalysis.

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