Soil organic matter dynamics under soybean exposed ...

Plant Soil (2008) 303:69–81 DOI 10.1007/s11104-007-9474-3

RESEARCH ARTICLE

Soil organic matter dynamics under soybean exposed to elevated [CO2] Ariane L. Peralta & Michelle M. Wander

Received: 15 August 2007 / Accepted: 1 November 2007 / Published online: 28 December 2007 # Springer Science + Business Media B.V. 2007

Abstract It is unclear how changing atmospheric composition will influence the plant–soil interactions that determine soil organic matter (SOM) levels in fertile agricultural soils. Positive effects of CO2 fertilization on plant productivity and residue returns should increase SOM stocks unless mineralization or biomass removal rates increase in proportion to offset gains. Our objectives were to quantify changes in SOM stocks and labile fractions in prime farmland supporting a conventionally managed corn–soybean system and the seasonal dynamics of labile C and N in soybean in plots exposed to elevated [CO2] (550 ppm) under free-air concentration enrichment (FACE) conditions. Changes in SOM stocks including reduced C/N ratios and labile N stocks suggest that SOM declined slightly and became more decomposed

Responsible Editor: Barbara Wick. A. L. Peralta : M. M. Wander Program in Ecology and Evolutionary Biology, University of Illinois at Urbana–Champaign, 505 S. Goodwin Ave., Urbana, IL 61801, USA M. M. Wander (*) Natural Resources and Environmental Sciences, University of Illinois at Urbana–Champaign, 1102 S. Goodwin Ave., Urbana, IL 61801, USA e-mail: [email protected]

in all plots after 3 years. Plant available N (>273 mg N kg−1) and other nutrients (Bray P, 22–50 ppm; extractable K, 157–237 ppm; Ca, 2,378–2,730 ppm; Mg, 245–317 ppm) were in the high to medium range. Exposure to elevated [CO2] failed to increase particulate organic matter C (POM-C) and increased POM-N concentrations slightly in the surface depth despite known increases (≈30%) in root biomass. This, and elevated CO2 efflux rates indicate accelerated decay rates in fumigated plots (2001: elevated [CO2]: 10.5±1.2 μmol CO2 m−2 s−1 vs. ambient: 8.9± 1.0 μmol CO2 m−2 s−1). There were no treatmentbased differences in the within-season dynamics of SOM. Soil POM-C and POM-N contents were slightly greater in the surface depth of elevated than ambient plots. Most studies attribute limited ability of fumigated soils to accumulate SOM to N limitation and/or limited plant response to CO2 fertilization. In this study, SOM turnover appears to be accelerated under elevated [CO2] even though soil moisture and nutrients are non-limiting and plant productivity is consistently increased. Accelerated SOM turnover rates may have long-term implications for soil’s productive potential and calls for deeper investigation into C and N dynamics in highly-productive row crop systems. Keywords Carbon sequestration . Climate change . Priming . Soil organic matter

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Abbreviations Db bulk density H-CF coarse heavy fraction IL-N Illinois-N test LF light fraction POM particulate organic matter SOM soil organic matter TOC total organic carbon TN total nitrogen

Introduction Increasing soil organic matter (SOM) stocks in agroecosystems could potentially help mitigate atmospheric change by increasing soil C sequestration, while simultaneously improving soil productivity (Nissen and Wander 2003). Increased sequestration in an annual crop system would be mainly achieved through stabilization of increased plant inputs in SOM (Lal 2004; Six et al. 2002). There is gathering evidence that increased inputs will not always result in SOM accrual, and that the influence of elevated [CO2] on soil organic C (SOC) stocks varies as a consequence of plant type and soil nutrient status. According to van Groenigen et al. (2006), increases in SOM levels are restricted when soil N supply is limiting; or when a legume is considered, if non-N nutrients restrict plant response to CO2 fertilization. When nutrients are limiting, plant-microbe competition for N can accelerate SOM decay and ultimately degrade soils even as plant growth increases under elevated [CO2] (Barron-Gafford et al. 2005). Several studies, mostly focusing on woody and herbaceous species under CO2 fertilization, suggest that accelerated SOM turnover rates prevent SOM accumulation in systems where nutrient supply is low (de Graaff et al. 2006; van Groenigen et al. 2006). Thus, the implications for SOM status in fertilized annual row crop systems are not clear. The positive growth response of C3 crops exposed to elevated [CO2] has been observed in past studies, but the resulting alterations in SOM dynamics are largely unexplored (Kimball et al. 2002; Sage and Kubien 2003). The meta-analysis of de Graaff et al. (2006) suggests that SOM accrues in terrestrial systems when fertility levels are high. This is contradicted, however, by the

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work of Dijkstra et al. (2006) who found SOM decay rates under FACE conditions to be accelerated by N fertilization and that the extent of this effect varied with plant species composition. This example may be important for intensive agriculture where N fertility is maintained at high levels. Positive N priming can boost microbial respiration and population growth, resulting in accelerated N turnover and net mineralization from decomposition of microbial residues (Amelung et al. 2001; Ebersberger et al. 2003; Mengel 1996). In agricultural settings, high background levels of readily available N may also support root accelerated mineralization of SOM which can be driven by rhizodeposits (positive C priming) and facilitated by increased soil wetting/ drying and associated physical disturbance (Kuzyakov 2002, 2006). Effects of elevated [CO2] on SOM stocks and dynamics have not been tested on a tilled, cornsoybean cropping system under free-air concentration enrichment (FACE) conditions. The soybean FACE (SoyFACE) facility at the University of Illinois provides a unique opportunity to study effects of elevated [CO2] on a corn-soybean agroecosystem under field conditions. This crop rotation dominates the Midwest’s agricultural landscape and provides a model for annual growth habit that will permit investigation of legume-grass interactions and temporal aspects of soil C and N dynamics. Corn, which is a powerful integrator of soil condition is far less responsive to CO2 fertilization than soybean (Drake et al. 1997); however, corn plants do respond to CO2 fumigation when water-stressed (Leakey et al. 2004, 2006; Sage and Kubien 2003). Work completed at SoyFACE has shown that elevated [CO2] increases soybean aboveground net primary productivity (ANPP = sum of shoot mass and litter fall per square meter) by 15–16% (Morgan et al. 2005) and increases belowground biomass by ≈30% (Rodriguez 2004). The inclusion of soybean, a grain legume, in the rotation makes the influence of elevated [CO2] on SOM and soil N dynamics somewhat difficult to predict. Yield increases might be accompanied by increased soil N uptake or N2-fixation, depending upon background soil N levels (Schubert 1995). Whether or not soybean are active N2-fixers, their production will not increase overall soil N stocks as bean harvest extracts most

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plant-acquired N. Even still, the return of easily decomposable soybean residues is expected to provide short-term N availability (Bergerou et al. 2004; Gentry et al. 2001) and increase subsequent corn yield (Bullock 1992; Maloney et al. 1999). Subtle changes in total SOM and soil productivity are difficult to quantify in the short-term because organic matter contains materials of mixed age with the largest reservoirs being thousands of years old. Fortunately, labile SOM fractions, which consist largely of newly incorporated residues that are closely associated with root dynamics and microbial biomass, can identify changes in SOM turnover and renewal before net changes in stock size are revealed (Cambardella and Elliott 1992; Rasse et al. 2005; Wander 2004). The particulate organic matter (POM) fraction and the light fraction (LF) are measurable, labile fractions of SOM that are closely associated with partially decomposed plant matter, including fresh root inputs, which serve as early indicators of SOM accrual or loss (Biederbeck et al. 1994; Cambardella and Elliott 1992; Janzen et al. 1992). Separation of the LF from the heavier or aggregate protected POM fractions can provide insight into the dynamics of root inputs and residue decay (Marriott and Wander 2006a; Personeni et al. 2005). Labile N that is considered plant-available and biologically active can be estimated using the POM-N fraction and hydrolysisbased methods (Boone 1994; Khan et al. 2001). The Illinois Soil N Test (IL-N), is a simple soil nitrogen test that relies on hydrolysis and titration to estimate amino N and hydrolysable NH4 (Khan et al. 2001). This measure has been shown to predict maizeresponse to N additions (Mulvaney et al. 2006). The objectives of this work were: (1) to quantify total and labile SOC and N stocks in soils supporting a corn–soybean agroecosystem after 3 years of exposure to elevated [CO2], and (2) to track withinseason dynamics of labile SOC and N fractions and soil CO2 efflux rates in surface soils under soybean exposed to elevated [CO2]. Changes in SOM, which may be too subtle to detect in total SOC or N fractions, are expected to become more pronounced over time, where both corn and soybean have been fumigated each season. Changes should be most apparent in labile fractions where root inputs and N extraction are greatest.

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Materials and methods Open-field experiment A soybean free-air concentration enrichment (SoyFACE) facility (http://www.soyface.uiuc.edu) was created in 2001 on 32 ha of the University of Illinois at Urbana–Champaign South Farms (40°03′ 21.3″N, 88°12′3.4″W, 230-m elevation). The soils are Drummer–Flanagan (fine-silty, mixed, mesic Typic Endoaquoll) that were formed from loess over glacial till and outwash. The site has been conventionally cultivated for over 100 years, and corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] continue to be rotated annually. Fertilizer was applied to corn fields at a rate of 202 kg N ha−1 (157 kg ha−1 as 28% 1:1 urea: ammonium nitrate liquid preplant and 45 kg ha−1 credit from soybean N2-fixation) (Leakey et al. 2004). All SoyFACE plots were encompassed by ring piping (20 m diameter) through which CO2 and/or O3 gas was released into the crop canopy based on the FACE design of Miglietta et al. (2001). These plots were spaced 100 m from each other to avoid cross-contamination of treatment gases. In 2001, eight octagonal plots were established within a 16 ha area of soybean on the west half of the site. Each block contained an ambient plot at 370 ppm [CO2] and a fumigated plot that was elevated to 550 ppm [CO2] during daylight hours. There was no application of elevated [O3] during the first year of SoyFACE, therefore, the longer-term (3 year) assessment of C sequestration did not include that treatment. In 2002, 12 SoyFACE plots were established on the other 16 ha east half of the field (east field site), where each block included an ambient plot (370 ppm [CO2]), an elevated [CO2] plot (550 ppm) and an elevated [O3] plot (1.2 ×ambient [O3]). For this study, only analysis from elevated [CO2] plots were completed. Both soybean and corn were exposed to elevated [CO2] on the west half of the field even though the influence of CO2 exposure on C4 crops was expected to be negligible. In this study, half the SoyFACE plot was planted with a soybean indeterminate Pioneer 93B15 cultivar (Pioneer Hi-Bred, Des Moines, IA, USA) except for 2001, where an indeterminate PANA cultivar (IL Foundation Seeds, Champaign, IL, USA) was planted; and the other half

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of the plot was planted with various soybean cultivars. The west side of the SoyFACE site during the 2002 season was planted with corn (cultivar Pioneer 34B43, Pioneer Hi-Bred International). More detailed SoyFACE site descriptions can be found in Morgan et al. (2005) and Rogers et al. (2004). Sequestration study Carbon sequestration was investigated in the west field of the SoyFACE site. Baseline (2001) and 2004 samples were taken in Spring before planting soybean using a 4.8 cm diameter splittable corer. Four, 1 m deep cores were taken in 2001, and eight cores were taken in 2004. Sampling intensity was increased to enhance our ability to detect treatment differences, which becomes increasingly difficult as one increases the depth of consideration (VandenBygaart 2006). Cores, which were randomly distributed in the main cultivar area of each of the 8 SoyFACE plots (four ambient [CO2] and four elevated [CO2]), were separated into the following depths: 0–12.5, 12.5– 25, 25–50 and 50–100 cm. All the soil samples were air-dried and ground to pass through a 2 mm sieve before C and N analyses. Bulk density of each sample was determined from dried soil divided by the volume of soil sampled. Elemental analyses of C and N were completed on whole soil subsamples from each depth using combustion methods (ECS 4010, Costech Analytical Instruments, Valencia, CA, USA). Additionally, soil was physically fractionated to retrieve the POM fraction (>53 μm) by size using the “Turbo POM” method developed for routine analysis (Marriott and Wander 2006b). For every field sample and designated depth, a 20-g subsample was measured and transferred into a 30-ml Nalgene, screw-cap bottle. To the sample, 150 ml of 5% sodium hexametaphosphate were added as a dispersant. The opening of the bottle was capped with an opening exposing a 53-μm mesh fabric that served as a sieve. The capped bottle was placed into a larger 250 ml Nalgene bottle to which soil solution was added. This was sealed and shaken for an hour before fine particles and sodium hexametaphosphate were discarded. Repeated 20-min washes with deionized water were performed until all fine particles less than 53 μm were removed. The POM fraction retained within the smaller bottle was transferred to

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a larger 53-μm mesh fabric for drying at 50°C for 24 h. Dried POM was ground to a fine powder using a disc mill and was analyzed for C and N as previously described. The Illinois Soil N Test (IL-N) was used to estimate soil N supply potential. A 1-g sample of air-dried and ground soil was measured and placed into a wide-mouth Mason jar before a 60-mm diameter Petri dish was attached to a modified jar lid and then filled with 5 ml of 4% H3BO3 (boric acid) solution. Ten milliliter of 2 M NaOH was added to the soil, and the jar was swirled to mix. The lid with the attached Petri dish was carefully placed onto the jar and tightened within 15–30 s. The jar was placed onto an electric hotplate and maintained at a temperature between 48 to 50°C for 5 h. The Petri dish containing H3BO3 was removed and 5 ml of deionized water was added before this solution was titrated with 0.01 M H2SO4 to its endpoint using a manual titrator. Refer to Khan et al. (2001) for more details on set up and methods. Carbon sequestration within the profile (1 m) was calculated, where both corn and soybean were exposed to CO2 fumigation. To account for variability in soil masses of each core, equivalent mass for each treatment was calculated following the procedure by Ellert and Bettany (1995). Soil masses for an entire soil core to 1 m were summed and individual cores were dropped if any data was missing. Soil masses for each profile were compared, and the heaviest sample was designated the equivalent mass. All other soil profiles were adjusted to match this soil mass. Total organic C of the deepest depth increment sampled (50–100 cm) was adjusted after incorporating the added soil mass. Organic matter dynamics The dynamics of labile carbon and nitrogen were investigated in a couple of ways. Soil CO2 efflux was measured during the 2001–2003 field seasons. Samples were collected approximately 2–3 weeks starting at emergence and ending at senescence. Soil CO2 efflux measurements were collected using the LICOR 6400 portable photosynthesis system in combination with the 6400–09 soil chamber (6400–09 soil chamber, Licor, Lincoln, NE, USA) every two weeks in ambient and elevated [CO2] plots. Soil temperature was monitored during the CO2 measurements using the LI-COR temperature probe. The target CO2

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concentration was set to match the atmosphere surface concentration present in each plot. Two PVC chamber collars were randomly placed into the ground adjacent to soybean plants within a 400 m2 area within each main plot. After each measurement, chamber collars were moved and relocated at random positions within the designated area to include the influence of spatial variability on results observed. Emission rates for each subsample were averaged by treatment over the entire growing season. The seasonal dynamics of SOM, POM-C and POM-N and IL-N were assessed in the east field of the SoyFACE. In this case, eight SoyFACE plots (four plots elevated with CO2 and four ambient plots) were sampled prior to soybean planting in 2002 and on four additional dates in April (pre-planting), July, (maximum biomass), September (senescence), and November (post-harvest). From each plot, four soil cores (4.8 cm diameter) from the main cultivar section were collected from the 0–12.5 and 12.5–25 cm depths. The surface depths were emphasized because this is where changes in root biomass and nutrient availability are most pronounced (Nicolardot et al. 2007; Pritchard and Rogers 2000). Note, the corn crop grown the previous year on east field site was not fumigated to keep costs down. Soils were processed as previously described. Subsamples from each SoyFACE plot by depth combination collected on an individual date were composited before analysis. A portion of the composite was ground to a fine powder in preparation for TOC and TN analyses as previously described. Particulate organic matter was collected using the “Turbo POM” method before it was further subdivided. The dried POM sample was weighed and transferred to a 50-ml Oakridge tube, where 15 ml of sodium polytungstate (1.6 g cm−3, Geoliquids, Chicago, IL, USA) was added to separate the sample by density into the light (LF, 1.6 g cm−3) fractions. After separation, the samples were processed and analyzed for C and N as before. Soil N supply was measured using the POM-N and the Illinois Soil N Test (IL-N) determined on whole soil composites as previously described.

study, total N, POM-C, POM-N, IL-N, and whole soil- and POM-C/N ratios were log-transformed while TOC concentration was square-root transformed before analysis. Analysis of variance (ANOVA) was performed using the MIXED procedure of SAS to compare treatment differences based on least-squared means for the variables bulk density, TOC, TN, POMC, POM-N, IL-N, and the C/N ratios for whole soil and POM (PROC MIXED, SAS v8.01, SAS Institute, Cary, NC, USA). Pair-wise comparisons were based on leastsquared means and were adjusted using the Tukey adjustment to minimize type I error (α=0.05). Treatment, depth and year were fixed factors, while block, block × treatment and block × treatment × year were treated as random factors. Depth was treated as a repeated measure, where block × treatment × year × replication was the subject. Adjusted profile masses were averaged by treatment and compared to track C sequestration. A similar MIXED procedure of SAS was used to compare treatment differences of soil CO2 efflux measured for all growing seasons. Post hoc linear contrasts were completed between treatments. For the seasonal dynamics study, POM-C, POM-N and POM C/N ratio were log-transformed prior to running variance analyses. Baseline data were analyzed separately from post-CO2 fumigation data to examine if initial variability in SOM characteristics was present at SoyFACE. Since multiple dates were sampled, treatment, date, and depth were considered fixed factors in the model, and ring was a random factor. The MIXED procedure of SAS was used on baseline data, where depth was treated as a repeated measure since a single time point (baseline) was considered. For the post-CO2 treatment data, date was used as a repeated measure. All means reported reflect least-squared means estimates generated from the PROC MIXED procedure (PROC MIXED, SAS v8.01, SAS Institute).

Statistical analyses

Soil pH ranged from 5.73–6.14 in the west side of the field and soil test levels were in the high to medium range for all properties considered. Basic soil nutrient mean concentrations (and standard deviations) were

Variables that did not satisfy normality tests were transformed prior to analysis. In the sequestration

Results Soil characteristics and organic matter in the sequestration study

0.2475 3.90 0.1466 0.1710 0.18 0.6703 0.8414 0.02 0.8927