effects of landscape position and temperate alley … · agricultural practices can stimulate soil...

AFTA 2005 Conference Proceedings

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EFFECTS OF LANDSCAPE POSITION AND TEMPERATE ALLEY CROPPING PRACTICES ON SOIL CARBON DIOXIDE AND NITROUS OXIDE FLUX IN AN

AGRICULTURAL WATERSHED

Neal J. Bailey and Peter P. Motavalli, Department of Soil, Environmental & Atmospheric Sciences, University of Missouri, Columbia, MO

Ranjith P. Udawatta, Department of Forestry, University of Missouri, Columbia, MO and

Kelly A. Nelson, Department of Agronomy, University of Missouri Greenley Center, Novelty, MO

ABSTRACT

Agricultural practices can stimulate soil nitrous oxide (N2O) and carbon dioxide (CO2) efflux which may contribute to global warming. However, the effects of temperate agroforestry practices and landscape position in agricultural watersheds on efflux of these two gases have not been extensively studied. The objective of this study was to assess the effects of different vegetative conservation practices, including alley cropping and grass contour strips, and landscape position on soil total organic C and total N distribution and N2O and CO2 efflux in three adjacent agricultural watersheds in northeast Missouri. The three watersheds were in a corn-soybean rotation, and contained one of three management systems: (1) cropped only, (2) cropped, interspersed with grass contour strips, or (3) cropped, interspersed with grass-tree contour strips. The three corresponding landscape positions within each watershed were: upper, middle, and lower backslope. Soil N2O efflux was highest in the cropped area and at the upper two landscape positions within each watershed. Soil CO2 efflux was lowest in the cropped area and at the upper two landscape positions across all three watersheds. Additional research was also conducted under controlled conditions to determine the effects of soil water content and N source on soil N2O and CO2 efflux from soil collected under each management system. Keywords: carbon dioxide, nitrous oxide, alley cropping, watershed, soil water content, soil temperature

INTRODUCTION Concerns that tropospheric gases, such as N2O and CO2, are responsible for the current upward trend in global temperatures have resulted in extensive research regarding the sources and sinks of these two gases in agroecosystems (Lal et al. 1995; Dalal et al. 2003). Several agricultural management practices influence CO2 and N2O efflux. For example, conservation tillage practices, such as no-till, may reduce CO2 emissions relative to conventional tillage from agricultural soils by sequestering carbon in the form of soil organic matter (Rochette and Angers 1999; Sauerbeck 2001). However, the increase in soil water content generally associated with no-till may stimulate the production of CO2 and N2O under certain conditions (Linn and Doran 1984).


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Nitrous oxide production from agricultural soils may be reduced by limiting superfluous application of N based fertilizers, selecting appropriate fertilizer application methods and timing of application, using nitrification inhibitors, avoiding soil compaction, and improving soil structure (Freney 1997; Ball et al. 1999; Pathak 1999; Dalal et al. 2003). In addition, N2O production may also be mitigated by the implementation of vegetative contour strips. Vegetative contour strips have been shown to increase water infiltration rates and retain nutrients (Udawatta et al. 2002; Seobi et al. 2005). The presence of the vegetative contour strips allows for runoff to be slowed and nitrate contained in the runoff to be subsequently taken up by the plants in the contour strip (Freney 1997; Tufekcioglu et al. 2003). This type of vegetative barrier reduces the transport of N to lower landscape positions or depressions where it has a greater potential to be transformed into N2O (Parkin 1993). Agroforestry practices, such as alley cropping, have also been examined for their effects on soil C sequestration. In the US, alley cropping practices have been estimated to have the potential to sequester up to 73.8 Tg C yr-1 (Montagnini and Nair 2004). Several factors may influence soil C accumulation in alley cropping practices including increased below- and aboveground biomass accumulation, changes in soil water content and soil temperature, and physical reduction in soil runoff and erosion losses. Changes in soil morphology across a watershed may also influence soil CO2 and N2O efflux. For example in the central claypan region with an area of approximately 4 million ha within Missouri, Illinois, and Kansas, the presence of a claypan subsoil often results in relatively high soil water content of the overlying horizon and subsequent large runoff during precipitation events (Udawatta et al. 2002). Claypan soils are soils with an argillic horizon 13 to 46 cm below the soil surface, which strongly influences its hydrological properties (Blanco-Canqui et al. 2002). Soils with claypans, therefore, may have higher N2O efflux due to denitrification compared to more well-drained soils. Although changes in soil microbial processes and the production of greenhouse gases at different landscape positions has previously been examined (Pennock et al. 1992; Corre et al. 1996; Florinsky et al. 2004), few studies are available which have investigated changes in soil CO2 and N2O efflux at different landscape positions in a watershed with different vegetative conservation practices, including temperate alley cropping and grass contour strips. The objective of this study was to assess the effects of vegetative conservation practices and landscape position on soil total organic C and total N distribution and N2O and CO2 efflux in three adjacent agricultural watersheds with claypan soils in northeast Missouri.

MATERIALS AND METHODS Experimental Watershed

The study was conducted at the University of Missouri Greenley Memorial Research Center in Knox County, Missouri, USA (400 01' N, 920 11' W) (Fig. 1). Udawatta et al. (2002) give specific details on the research watersheds, soils, weather, and management practices. In brief, three adjacent watersheds were established in 1991 and no-till cropped in a corn-soybean


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rotation. In 1997, each watershed was randomly assigned one of three treatments. The 1.65 ha watershed was cropped-only (CR) with a corn-soybean rotation, the 4.44 ha watershed was cropped with grass-tree buffer strips (AF), and the 3.16 ha watershed was cropped with grass-only buffer strips (GR). The buffer strips for both AF and GR watersheds were 4.5 m wide and 36.5 m apart (22.8 m at lower slope positions) and were planted to redtop (Agrostis gigantea Roth), brome grass (Bromus spp.), and birdsfoot trefoil (Lotus corniculatus L.) in 1997. Pin oak (Quercus palustris Muenchh.), swamp white oak (Q. bicolor Willd.), and bur oak (Q. macrocarpa Michx.) were planted 3 m apart in the center of the buffers for the agroforestry watershed in 1997. In 2004, corn (Garst 8484Bt) was planted on May 22nd in the cropped areas of all three watersheds and harvested on November 18. Nitrogen fertilizer was broadcast applied on May 7 at a rate of 180 kg N ha-1 (as ammonium nitrate).

GR AF

CR

Figure 1. Watershed layout with contour lines at the 0.5 m intervals; shaded area represents permanent vegetation with grass-only (GR) in the west watershed, grass with trees (AF) in the central watershed, and grass drainage waterways in all three watersheds, including the cropped (CR) watershed.


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The soils in the study area were mapped as Putnam silt loam (fine, smectitic, mesic Vertic Albaqaulfs) and Kwilwinning silt loam (fine, smectitic, mesic Vertic Epiaqaulfs). The watershed has a drainage restrictive B horizon with a claypan at a variable depth between 4 and 37 cm (Udawatta et al. 2002). The restrictive claypan produces surface runoff during high rainfall periods in combination with periods of low evapotranspiration during winter, spring, and early summer.

Soil Sampling

After the soybeans were harvested, two soil sampling transects were established on October 30, 2003, extending from the upper backslope to the lower backslope landscape positions. Soil samples to a 10 cm depth were collected using a Uhland core sampler at three positions (upper, middle, and lower backslope) along the transects. The soil cores were taken in the middle of the vegetative strips in the GR and AF watersheds, and at corresponding locations in the CR watershed. Soil bulk density was determined at each sampling position using the core method and expressed on an oven-dry soil basis (Blake and Hartge 1986). Half the soil sample was air dried for chemical analysis and the other half was oven dried at 105 °C for determination of soil gravimetric water content.

Determination of Soil CO2 and N2O Efflux

Gas sampling occurred regularly from April through November 2004 before and after the N fertilizer application. The locations for the sampling collars were in corresponding landscape positions within each watershed. The three positions were the upper, middle, and lower backslope. Three replications were established with three transects for the N2O and two replications were established with two transects for the CO2. The collars were installed on April 16, 2004, three days prior to the first gas collection event to allow for soil disturbance effects to subside (Hutchinson and Livingston 1993). Thirty-six PVC collars (12.5 cm high and 20 cm inside dia.) were installed in the field to a depth of 5 cm for collection of N2O gas. Another set of collars to measure CO2 (5 cm H. and 7.62 cm inside dia.) were installed to a depth of 2.5 cm. The collars acted as the base for the closed chambers (Smith et al. 1995). The N2O collection chamber was completed after a flexible PVC cap was placed over the top of the collar onto stop screws and a hose clamp tightened. The chamber was then covered with a white 20 L bucket to minimize the effect of solar radiation (Hutchinson and Mosier 1993). It was assumed the inside of the chamber remained at ambient temperature and pressure. The chambers were then left alone to accumulate the gases evolved from the soil for a period of approximately one hour (Smith et al. 1995; Ball et al. 1999). Head space of the chamber was mixed with a 60 mL syringe fitted with a 21 gauge needle. The 60 mL syringe was pumped seven times to ensure adequate mixing. After the headspace was thoroughly mixed, a 10 ml syringe (25 ga. needle, 3.8 cm L.) was used to withdraw an 8 mL sample from the headspace. The gas samples were then injected into pre-evacuated 5 mL serum bottles (Rolston 2002). Soil temperature was measured in triplicate around each chamber during the gas collection process with a digital thermometer at the 5 cm depth. Soil samples to a 5 cm depth were also taken in triplicate within 50 cm from the center of the chamber for determination of gravimetric soil water content and inorganic nitrogen (NH4

+-N + NO3--N) levels.


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Nitrous oxide samples were transported from the field to the laboratory in a cooler and stored at room temperature until analysis on a gas chromatograph (GC) (Buck Scientific, Inc., East Norwalk, CT) within 3 days. The GC was fitted with an electron capture detector (ECD) and a 1.8 m Porapak Q column. Subsamples of 1.5mL were withdrawn from the serum bottles and injected into the GC. The concentration of the subsampled gas was determined using a simple linear regression equation created from a standard curve using incremental aliquots of a 10 µL L-1 N2O standard gas (Scott Specialty Gases, Plumsteadville, PA). Carbon dioxide in the field was measured with a LI-COR 6200 portable photosynthesis detector fitted with a closed chamber (LI-COR Inc., Lincoln, NE). The CO2 concentration inside the chamber was logged every 10 sec. Carbon dioxide flux rates at ambient CO2 concentration were then calculated by simple linear regression. Incubation CO2 and N2O efflux rates were determined in a similar manner as the field N2O efflux. The exception was that the gas removed from the chamber was not stored in a pre-evacuated bottle but was injected directly into the GC. A 3 mL aliquot of the incubation chamber headspace gas was analyzed for CO2 on the GC with a thermal conductivity detector (TCD) and a 0.9 m silica gel column. The GC procedure for the incubation N2O was the same as the field N2O procedure.

Incubation Study

Bulk soils from each watershed were collected in November 2003 from the upper backslope position to depth of 10 cm for the incubation study. Each of the three bulk samples consisted of three subsamples within each watershed within the strips for the GR and AF watersheds, and at the corresponding position within the CR watershed. The soils were air dried, ground, and passed through a sieve with 2 mm openings. Treatments for the incubation study consisted of 2 N fertilizer rates (0 and 1.39 g KNO3 kg-1 soil) and 4 water-filled pore space (WFPS) (40, 60, 80, and 100%) with 3 replicates. The N fertilizer rate was approximately equivalent to the field application rate (180 kg N ha-1). The cores (7.7 cm H. and 7.7 cm inside dia.) were packed to a bulk density of 1.2 g cm-3 and incubated at 25 ºC in the dark. Carbon dioxide and N2O gas efflux measurements were taken 1, 3, 6, 10, 15, 23, 43, and 72 days after initiation of the incubation.

Soil Analysis

Triplicate samples of the transect and bulk soils were analyzed for pH (1:1 water), total N, and total organic C by combustion using a LECO Truspec C/N analyzer (LECO Corp., St. Joseph, MI) (Nelson and Sommers 1996), total inorganic N (NH4

+-N (Zellweger Analytics, 1993) + NO3

--N (Zellweger Analytics, 1997)), dissolved total organic C and total dissolved N (Jandl and Sollins 1997) by combustion using a Shimadzu TOC/TN analyzer (dissolved organic N was the difference between total dissolved N and total inorganic N), and particulate organic matter C and N (Cambardella and Elliot 1992).


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Statistical Analysis Soil properties and gas efflux rate data were analyzed using the Proc Mixed statistical procedure in SAS (SAS Institute 2000). All statistical analyses for the incubation were tested for significance at á ≤ 0.05. All field analyses were tested for significance at á ≤ 0.10.

RESULTS AND DISCUSSION Soil Properties

Soil total organic carbon (TOC) and total nitrogen (TN) levels were higher at the upper backslope position in the AF as compared to the GR and CR treatments (Table 1). Soil bulk density (Db) at the upper backslope position was 1.16 Mg m-3 for GR and 1.22 Mg m-3 for CR, compared to 1.07 Mg m-3 for the AF treatment (Table 1). The Db in the GR at the upper backslope position was not statistically lower than the CR as may be expected due to the potential effects of long-term cropping on soil structure. This lack of a statistical difference may be due to vehicle compaction in that particular area of the GR watershed since this grass strip is accessible from the main road and has been driven on for the purpose of sampling the central watershed. The higher Db in the grass strips may have suppressed root development of the grass resulting in lower soil TOC and TN at the upper backslope position. The CR treatment at the lower backslope position had lower TOC and TN than did the AF and GR treatments (Table 1). The reason for the CR treatment having the lowest TOC and TN at the lower backslope position may be attributable to having the highest Db and possibly higher surface erosion and translocation loss of nutrients in this lower position. However, at the middle position, TOC was not significantly different among treatments, and only the AF TN and Db were significantly different than the CR TN and Db. The soil POM C and N exhibited the same spatial distribution pattern as soil TOC and TN across the watersheds except that there was no statistical difference at the middle backslope position across the watersheds (Table 1). The CR watershed consistently had the lowest levels of dissolved organic C (DOC), and the AF the highest, except for the lower backslope position, where there was no significant difference between the AF and GR. The distribution pattern of dissolved organic N (DON) was similar to DOC except that there was no significant difference between the CR and GR watersheds at the upper backslope position.

Field CO2 and N2O Efflux Rates The effects of landscape position and management practice on cumulative soil CO2-C and N2O-N evolved over the cropping season are shown in Figure 2 A-D. No significant interactions between landscape position and management practice were observed for cumulative CO2-C and N2O-N evolved and, therefore, the main effects of these treatments are shown separately. Cumulative CO2-C evolved over the cropping season ranged from 0.929 kg CO2–C m-2 to 1.517 kg CO2–C m-2 (Fig. 2A & B) and was significantly lower in the CR as compared to the AF and GR which were not significantly different from one another (Fig. 2A). Cumulative CO2-C


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Table 1. Watershed soil TOC, TN, POM – C, POM – N, DOC,and DON at upper, middle, and lower backslope positions for grass-only (GR), agroforestry (AF), and cropped (CR) treatments at Greenley Memorial Research Center, Novelty, MO in 2003.

Landscape Total Organic C Total N Particulate Organic

Matter -C Particulate Organic

Matter -N Dissolved

Organic C Dissolved

Organic N Bulk Density position GR AF CR GR AF CR GR AF CR GR AF CR GR AF CR GR AF CR GR AF CR ------------------------------------------- g kg soil-1 ------------------------------------------- ------------------- mg kg soil-1 ----------------- ---------Mg m-3-------

Upper backslope 19.5 24.5 19.5 1.87 2.46 1.64 3.54 6.60 3.03 0.19 0.40 0.18 106.8 123.3 80.1 7.1 15.5 5.8 1.16 1.07 1.22 Middle backslope 22.5 23.5 22.0 2.20 2.45 1.87 5.42 5.76 4.52 0.34 0.38 0.29 105.7 147.2 75.0 9.2 15.5 2.1 1.11 1.02 1.16

Lower backslope 25.5 24.0 18.5 2.33 2.21 1.45 7.12 7.38 3.28 0.43 0.47 0.06 133.5 124.6 72.4 11.6 11.0 6.4 1.12 1.09 1.23

LSD(0.10) ---------0.24--------- ---------0.04-------- --------2.68--------- ---------0.19-------- -----------22.2---------- ---------2.5--------- ----------0.08---------


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Figure 2. Effects of management system on (A) cumulative CO2 efflux and (C) cumulative N2O efflux and landscape position on (B) cumulative CO2 efflux and (D) cumulative N2O efflux1A and 1C) for the 2004 sampling season at the Greenley Research Center, Novelty, MO. Management systems were agroforestry (AF), cropped (CR), and grass-only (GR) watersheds and landscape positions were middle backslope (MBS), upper backslope (UBS), and lower backslope (LBS). Vertical bars show LSD at á ≤ 0.10. evolved was significantly highest in the lower backslope position across all watersheds as compared to the middle and upper landscape positions which were not significantly different from each other (Fig. 2B). The measured N2O production was highest on day 6 after N application except for the middle backslope position in the CR and AF practices where the highest measured production occurred on day 25 (data not shown). Cumulative N2O-N evolved over the cropping season ranged from 0.497 g N2O-N m-2 to 1.896 g N2O-N m-2 (Fig. 2 C & D) and was significantly higher in the CR watersheds as compared to the AF and GR, which were not significantly different from one another (Fig. 2C). The highest rate of N2O-N evolved represents approximately 10% of the fertilizer N applied to the watershed. Cumulative N2O for all three watersheds was significantly lower in the lower backslope position than the upper backslope position (Fig. 2D). The middle backslope position was not significantly different than either the upper or lower backslope positions. In claypan soils, the flatter topography <1% slope on the upper landscape positions may tend to perch water (Blanco-Canqui et al. 2002), resulting in higher WFPS during the sampling period and less lateral transportation of nitrate compared to up to 9% slope in the lower backslope position. The resulting difference in soil water content may result in higher N2O efflux rates in the upper landscape position (Ball et al. 1997).

A.

D. C.

B.


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The CO2 and N2O efflux rates observed among the management practices and landscape were generally higher than observed by others (Pathak 1999). However, a similar N2O-N efflux rate was observed by Clayton et al. (1994) who found peak efflux rates approaching 1000g N2O-N ha-1 day-1 for poorly drained heavy clay fertilized grassland soils. Another consideration that may have affected CO2 and N2O efflux rates was the effect of installing permanent PVC gas sampling collars on soil water content, a commonly used field method of determining soil gas efflux. The lower landscape position collars may have had better drainage than the upper landscape position collars because of the importance of surface flow in removing water from the upper landscape position in the claypan soil. The collars throughout the season were left uncovered except for the sampling period. Because the collars remained open and the watersheds are claypan soils, the collars would retain water, eliminating surface flow within the collar. This resulted in very wet conditions within the collar which may have promoted higher denitrification rates. Therefore, higher N2O efflux rates (especially the CR watershed) and lower CO2 efflux rates were observed in the upper landscape positions. Seobi et al. (2005) in these research watersheds found the AF had three times the saturated hydraulic conductivity (Ksat) as that of the GR, and 14 times the Ksat as that of the CR.

Incubation Results Changing soil WFPS had a significant effect on both cumulative CO2-C and N2O-N evolved over a 72-day incubation from bulk soils collected from the different management practices and amended with and without fertilizer N (Fig. 3 A & B). For the N2O there were no significant differences among the management soils at the 40 and 60% WFPS (Fig. 3B). At the 80% WFPS, the AF soil with KNO3

- (N) added was significantly higher for N2O production than the GR soil with N and the GR soil with N was significantly higher than the CR soil with N. At the 100% WFPS, the AF soil with N had significantly higher N2O production than all other treatments. Across the range of WFPS established, no significant differences were detected within the no N treatments. With N added, no significant difference was present between the 40 and 60% WFPS. All management soils with N added had significantly higher cumulative N2O evolved at the 80%WFPS than at the 40 and 60% WFPS. The GR soil and CR soil with N were significantly lower at the 100% WFPS than at 80% WFPS. There was no significant difference between the AF soil with N at 80% WFPS and the AF soil with N at 100% WFPS. Only the AF soil with N showed a significant increase in N2O from 40 and 60% WFPS compared to the 100% WFPS. The significantly higher cumulative N2O evolved at 80% WFPS for the GR soil over the CR soil (Fig. 3B) may be due to the higher levels of DOC in the GR soil (Bremner 1997). The high levels of DOC in the GR soil may also explain why the GR soil’s flux rates approach the AF soil’s flux rates in the 80% WFPS even though the GR soil had very low initial NO3

--N (0.01 mg NO3

--N kg soil-1) as compared to the AF soil (42.9 mg NO3--N kg soil-1) (Table 2). The drop-off

in N2O production at the 100% WFPS (Fig 3B) is likely due to a more complete reduction of nitrate to the dinitrogen (N2) species (Davidson et al., 2000) and very low gas diffusivity values (Ball et al. 1997). However the effect is more pronounced in the the GR soil and CR soil than the AF soil. The initial levels of NO3

--N in the GR soil and CR soil may explain this rapid drop-off since the GR soil and CR soil (13.5 mg NO3

--N kg soil-1) had the lowest levels of initial


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NO3--N for the transformation to N2O as compared to the AF soil. The relative high levels in the

AF soil may have sustained the efflux rate.

Figure 3. Effects of water-filled pore space (WFPS) on (A) cumulative CO2 efflux and (B) cumulative N2O efflux in the incubation study. Management systems were agroforestry (AF), cropped (CR), and grass-only (GR) watersheds. Designation of NON indicates no fertilizer N was added and N indicates fertilizer N was added. Vertical bars show LSD at á ≤ 0.05. Adding N did not have a consistent influence on the production of CO2-C (Fig. 3A). This may be explained by the labile carbon fraction (POM) having a C:N ratio of less than 25. Regardless of N rate, the GR soil was always significantly higher than all other soils within the same %WFPS. Possible reasons for the GR soil being consistently higher were likely due to the higher amounts of TC, DOC, and POM-C in the GR soil (1.67%, 121.51 mg DOC kg soil-1, and 8.06 g POM-C kg soil-1) compared to the CR soil (1.31%, 62.13 mg DOC kg soil-1, and 2.84 g POM-C kg soil-1) and AF soil (1.56%, 69.41 mg DOC kg soil-1, and 5.29 g POM-C kg soil-1) (Table 2). The GR soil with no N added was significantly higher than the GR soil with N added at the 40 and 60% WFPS. The total CO2 evolved was not significantly different between the GR soil with N added and the GR soil with no N added at the 80% WFPS, but the GR soil with N added was higher than GR soil with no N at the 100% WFPS.

CONCLUSIONS The results of this study indicate that both vegetative conservation practices, including establishment of grass buffer strips and alley cropping, and landscape position can have effects on changing the distribution of total soil organic C, total N and soil C and N fractions across agricultural watersheds. The permanent grass and agroforestry buffer strips had lower amounts of soil N2O production relative to the cropped areas even though the strips have been shown to capture excess N that would otherwise be lost out of the system as a component in runoff. The buffer strips had higher levels of cumulative soil CO2 production, suggesting that the strips had increased microbial activity or plant root respiration relative to the cropped areas. The grass and agroforestry buffer strips also generally had higher levels of soil total organic C and C fractions,


Table 2. Bulk soil properties collected from each watershed in the upper backslope position at Greenley Memorial Research Center, Novelty, MO in 2003 used for the laboratory incubation study.

Management

Total Organic

C Total N Dissolved Organic C

Dissolved Organic

N NH4+-N NO3

--N

Total Dissolved

N

Particulate Organic Matter-C

Particulate Organic

Matter-N pH

(water) ------ g kg soil-1 ---- -------------------------- mg kg soil-1 ----------------------------- --------- g kg soil-1 --------

Grass-only 16.71 1.87 121.5 6.54 10.34 0.01 16.89 8.06 0.42 7.44

Cropped 13.08 1.52 62.1 0.00 8.92 13.5 17.28 2.84 0.17 7.67

Agroforestry 15.60 1.60 69.4 0.00 3.63 42.9 44.02 5.29 0.31 7.31 LSD(0.05) 0.70 0.58 10.9 0.85 1.10 1.0 2.22 2.40 0.17 0.56


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which is an indicator of soil carbon sequestration and improved soil quality. The spatial and temporal distribution of soil water and the changes in soil C and N fractions as influenced by management practices and landscape position across the watersheds can impact soil CO2 and N2O efflux rates. We speculate that differences in cumulative N2O and CO2 efflux at different landscape positions may have been partially influenced by the variable depth of the restrictive claypan soil layer and slope gradient which affected soil water content and possibly altered our measurement of soil CO2 and N2O efflux using permanent PVC rings installed in the field. Further research may be needed to compare other methods of measuring soil CO2 and N2O efflux in soils on slopes with restrictive layers in order to avoid impeding surface runoff.

ACKNOWLEDGEMENTS This research was funded through the University of Missouri Center for Agroforestry under cooperative agreements AG-02100251 with the USDA ARS Dale Bumpers Small Farm Research Center, Booneville, AR, and CR 826704-01-0 with the US EPA. The results presented are the sole responsibility of the authors and/or the University of Missouri and may not represent the policies or positions of the funding agencies. We gratefully acknowledge the technical assistance of the University of Missouri Greenley Research Station and University of Missouri soil fertility staff, including Eduardo Navarro, Kelly Sammons, Matt Jones, and Randy Smoot. Special thanks also to Dr. Mark Ellersieck for statistical analytical assistance.

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