Total Soil Organic C Research Articles

The impact of long-term liming on soil organic carbon and aggregate stability in low-input acid soils

This study used two field trials with 5 and 34 years of liming histories, respectively, and aimed to elucidate the long-term effect of liming on soil organic C (SOC) in acid soils. It was hypothesized that long-term liming would increase SOC concentration, macro-aggregate stability and SOC concentration within aggregates. Surface soils (0–10 cm) were sampled and separated into four aggregate-size classes: large macro-aggregates (>2 mm), small macro-aggregates (0.25–2 mm), micro-aggregates (0.053–0.25 mm) and silt and clay fraction (<0.053 mm) by wet sieving, and the SOC concentration of each aggregate-size was quantified. Liming decreased SOC in the bulk soil and in aggregates as well as macro-aggregate stability in the low-input and cultivated 34-year-old trial. In contrast, liming did not significantly change the concentration of SOC in the bulk soil or in aggregates but improved macro-aggregate stability in the 5-year-old trial under undisturbed unimproved pastures. Furthermore, the single application of lime to the surface soil increased pH in both topsoil (0–10 cm) and subsurface soil (10–20 cm) and increased K2SO4-extractable C, microbial biomass C (Cmic) and basal respiration (CO2) in both soil layers of both lime trials. Liming increased the percentage of SOC present as microbial biomass C (Cmic/Corg) and decreased the respiration rate per unit biomass (qCO2). The study concludes that despite long-term liming decreased total SOC in the low-input systems, it increased labile C pools and the percentage of SOC present as microbial biomass C.

Conversion from forests to pastures in the Colombian Amazon leads to contrasting soil carbon dynamics depending on land management practices.

Strategies to mitigate climate change by reducing deforestation and forest degradation (e.g. REDD+) require country- or region-specific information on temporal changes in forest carbon (C) pools to develop accurate emission factors. The soil C pool is one of the most important C reservoirs, but is rarely included in national forest reference emission levels due to a lack of data. Here, we present the soil organic C (SOC) dynamics along 20years of forest-to-pasture conversion in two subregions with different management practices during pasture establishment in the Colombian Amazon: high-grazing intensity (HG) and low-grazing intensity (LG) subregions. We determined the pattern of SOC change resulting from the conversion from forest (C3 plants) to pasture (C4 plants) by analysing total SOC stocks and the natural abundance of the stable isotopes (13) C along two 20-year chronosequences identified in each subregion. We also analysed soil N stocks and the natural abundance of (15) N during pasture establishment. In general, total SOC stocks at 30cm depth in the forest were similar for both subregions, with an average of 47.1±1.8MgCha(-1) in HG and 48.7±3.1MgCha(-1) in LG. However, 20years after forest-to-pasture conversion SOC in HG decreased by 20%, whereas in LG SOC increased by 41%. This net SOC decrease in HG was due to a larger reduction in C3-derived input and to a comparatively smaller increase in C4-derived C input. In LG both C3- and C4-derived C input increased along the chronosequence. N stocks were generally similar in both subregions and soil N stock changes during pasture establishment were correlated with SOC changes. These results emphasize the importance of management practices involving low-grazing intensity in cattle activities to preserve SOC stocks and to reduce C emissions after land-cover change from forest to pasture in the Colombian Amazon.

Eucalyptus and Pinus stand density effects on soil carbon sequestration

Although afforestation of pasture and agricultural systems can increase C sequestration in soils, studies have shown wide variation in the magnitude, timing and direction of soil organic C (SOC) dynamics, depending on site conditions, management practices, and previous land use. Effects of stocking density on SOC have yet to be elucidated, however. The objectives proposed for this work were to quantify: (i) SOC content and distribution for the top 30cm of the A horizon under native pasture compared with that following 8years of afforestation (Eucalyptus grandis and Pinus taeda planted at different densities); (ii) SOC accumulation in the AB and the top of Bt horizons in afforested soils; and (iii) the contribution of the new vegetation (Eucalyptus and Pinus) to SOC. A field trial with three stand-densities of E. grandis and P. taeda and corresponding native pasture was established and soil samples were collected and assayed for five layers of the A horizon (0–5, 5–10, 10–15, 15–20, 20–30cm), and the AB and Bt1 horizons. Soil organic C concentration and δ13C were determined, and the total SOC stock and C derived from the Eucalyptus and Pinus vegetation (young C) were calculated. Our results suggest that there was likely no significant change in SOC stocks in response to 8years of afforestation with either Eucalyptus or Pinus. No significant differences in SOC stocks in the upper 30cm soil layer as a whole were found among pasture, Eucalyptus, and Pinus treatments. By contrast, SOC in the AB and Bt1 horizons was significantly higher under afforested sites than native pasture (P<0.001 and P<0.039, respectively). Soil δ13C in the afforested treatments (0–30cm) reflects the contribution of the new vegetation (Eucalyptus or Pinus) to SOC. Net accumulation of new SOC from the planted trees in the top 15cm soil layer was equivalent to 0.20MgCha−1yr−1 for Eucalyptus and 0.30MgCha−1yr−1 for Pinus. Assuming initial declines in SOC following afforestation have largely ended and young C sequestration rates continue at the same rate, SOC stocks in the top 15cm layer would reach those found under pasture after 15 and 11years for Eucalyptus and Pinus plantations, respectively. Measurements of additional sites afforested with Eucalyptus and Pinus of different ages are needed to draw firm conclusions about the net SOC balance in these afforested pasture soils.

Effects of land-use change and management on soil carbon and nitrogen in the Brigalow Belt, Australia: I. Overview and inventory

Soil and land-management interactions in Australian native-forest regrowth remain a major source of uncertainty in the context of the global carbon economy. We sampled soil total organic C (TOC) and soil total N (TN) stocks at 45 sites within the Brigalow ecological community of the Brigalow Belt bioregion, Queensland, Australia. The sites were matched as triplets representing three land uses, specifically: uncleared native brigalow forest (‘Remnant’); grassland pasture (‘Pasture’), derived by clearing native vegetation and maintained as pasture for a minimum of 10 years, and; regrowing native brigalow forest (‘Regrowth’, stand ages ranging from 10 to 58 years) that had developed spontaneously after past vegetation clearing for pasture establishment. Soil TOC fractions and natural abundance of soil C and N isotopes were examined to obtain insight into C and N dynamics. An updated above- and belowground carbon budget for the bioregions was generated. Average soil TOC stocks at 0–0.3-m depth ranged from 19 to 79 Mg ha–1 and soil TN stocks from 1.8 to 7.1 Mg ha–1 (2.5th and 97.5th percentiles, respectively). A trend in stocks was apparent with land use: Remnant &gt; Regrowth ≅ Pasture sites. Soil δ13C ranged from –14 to –27‰, and soil δ15N ranged from 4‰ to 17‰, in general reflecting the difference between Pasture (C4-dominated) land use and N2-fixing (C3-dominated) Remnant and Regrowth. Mid-infrared spectroscopy predicted C fractions as a percentage of soil TOC stock, which ranged from 5% to 60% (particulate), 20–80% (humus) and 9–30% (resistant/inert). The geo-referenced soil and management information we collected is important for the calibration of C models, for the estimation of national C accounts, and to inform policy developments in relation to land-resource management undertaken within the Brigalow Belt bioregions of Australia.

Carbon transfer from maize roots and litter into bacteria and fungi depends on soil depth and time

Plant-derived carbon (C) transfer to soil is one of the important factors controlling the size and structure of the belowground microbial community. The present study quantifies this plant-derived C incorporation into abiotic and biotic C pools in top- and subsoil in an arable field over five years. Stable isotope analysis was used to determine the incorporation of maize root and shoot litter C into soil organic C (SOC), extractable organic C (EOC), total microbial biomass (Cmic), ergosterol and phospholipid fatty acids (PLFAs). The following treatments were investigated: corn maize (CM), providing root- and shoot-derived C (without corncobs), fodder maize (FM), providing only root-derived C, and wheat plus maize shoot litter amendment (WL), providing only shoot-derived maize C. Wheat plants (W) without maize litter amendment served as control. Soil samples were taken each September directly before harvest from 2009 to 2013. During the experiment, the maize-derived C signal increased in SOC, EOC, Cmic, ergosterol, bacterial and fungal PLFAs in the topsoil (0–10 cm). Although total maize shoot C input was threefold lower than maize root C input, similar relative amounts of maize C derived from shoots and roots were incorporated into the different C pools in the WL and the FM treatments, indicating the importance of shoot-derived C sources for microorganisms in the topsoil. An additive effect of both C sources was found in the CM treatment with almost twice as much maize-derived C in the respective pools. Furthermore, the proportion of maize-derived C varied between the different pools with lower incorporation into the total SOC (17%) and total EOC (24%) pools and higher incorporation ratios of maize C into PLFAs of different microbial groups (29% in Gram-positive (Gr+) bacterial PLFA-C, 44% in Gram-negative (Gr−) bacterial PLFA-C, 69% in fungal PLFA-C and 78% in ergosterol) in the CM treatment in topsoil after five years. After the third and fifth vegetation periods, we also detected maize-derived C in the rooted zone (40–50 cm depth) and the root-free zone (60–70 cm depth). The maize-derived C incorporation was lower in subsoil C pools in comparison to topsoil C pools. In the root-free zone, the maize-derived C was found to be 2% in total SOC, 28% in total EOC, 9% in Gr+ bacterial PLFA-C, 20% in Gr− bacterial PLFA-C and 53% in fungal PLFA-C. Saprotrophic fungi incorporated maize-derived C in all soil depths to a greater degree than Gr+ and Gr− bacteria, indicating the importance of saprotrophic fungi in this agro-ecosystem.

Short-term responses of soil organic carbon and carbon pool management index to different annual straw return rates in a rice–wheat cropping system

Crop straw return is recommended as an effective management practice to alter soil organic carbon (C) content. Understanding the effects of straw return rate on changes of soil organic C (SOC) is helpful for improving soil quality and sustaining crop productivity. To determine the short-term responses of SOC content and grain yield to different annual straw return rates, a 2-year field experiment with a random block design was carried out in a rice–wheat rotation system. Seven gradient rates of straw return, including 0%, 25%, 50%, 75% and 100% of total straw from previous crop returned in rice and wheat seasons, and 100% of straw returned only in rice or wheat season were set. Both rice and wheat yields were investigated and SOC pools and C pool management index (CPMI) were measured. The soil total organic C (TOC) and labile organic C fraction contents were higher under the straw return treatments compared to the no straw return treatment (0%S) at a 0–21 soil depth. The 50% annual straw return rate (50%S) had significantly higher soil TOC, dissolved organic C (DOC), easily oxidizable C (EOC), and microbial biomass C (MBC) contents than the 0%S treatment at a 0–21cm depth. All of the straw return treatments had a significantly higher DOC content than the 0%S treatment at a 0–21cm depth, except for the 100% only rice straw return treatment (100%RS). Compared to the 0%S, the 25%S and 50%S treatments had a significantly higher SOC lability (L), lability index (LI), C pool index (CPI) and CPMI at a 0–21cm soil depth. With the increase of straw return rate, the crop yields first increased and then decreased, and the 50%S treatment had the highest crop yields. There were significant and positive correlations between the soil TOC and the annual crop yield. We concluded that the 50% annual straw return rate is the best option for improving SOC sequestration and crop productivity in the rice–wheat rotation system of China.

Long-term variations in soil organic matter under different tillage intensities

Tillage practices have a major effect on soil C storage and cropping sustainability, due to their impact on soil aggregation, organic residue decomposition rate, OC dynamics, microbial abundance and diversity, N mineralization and nutrient availability.Our research was aimed at assessing the long term effects of different tillage treatments on soil organic matter (SOM) quantity and quality and its evolution with time, in a loam textured-soil from central Italy cultivated with continuous maize. The tillage treatments included a conventional tillage (DP) by mouldboard ploughing to 40cm depth, a ripper subsoiling (RS) to 40–45cm, a shallow tillage by mouldboard ploughing to 20cm depth (SP) and minimum tillage to 10cm by disk harrowing (DH). The soil was sampled in 1999 and 2011 (after 5 and 17 years from the beginning of the trial, respectively), at depth increments of 0–10, 10–20, 20–30 and 30–40cm and analysed for total organic C (TOC), OC recalcitrant and labile fractions by chemical hydrolysis, total N, bulk density, aggregate stability and hydraulic conductivity.After 17 year of treatments, the different tillage systems did not affect the overall amount of OC stored in a 0–40cm equivalent soil mass; nevertheless, they produced significant differences in soil OC vertical distribution along the soil profile and OC recalcitrance. Both DH and RS increased soil TOC in the surface layer, with predominance of labile OC under DH and recalcitrant OC under RS. Differently, DP caused a net loss of recalcitrant OC, probably due to a detrimental impact on soil aggregate stability and, subsequently, on SOM physical protection. RS showed the largest potential for OC sequestration in stable form in the considered agroecosystem.

Conservation agriculture effects on soil organic carbon accumulation and crop productivity under a rice–wheat cropping system in the western Indo-Gangetic Plains

Retention of carbon (C) in arable soils has been considered as a potential mechanism to mitigate soil degradation and to sustain crop productivity. Hence, we appraised the 3-year effect of different conservation agriculture (CA) practices on grain yield, biomass productivity and soil organic C (SOC) accumulation rates under a tropical rice (Oryza sativa L.)-wheat (Triticum aestivum L.) and rice–wheat–green gram (mungbean in Hindi; Vigna radiata) cropping systems. Results indicate mean (of three years) rice grain yield under mungbean residue+direct seeded rice (DSR) followed by zero tilled wheat (ZTW) with rice residue (RR) retention and zero tilled relay summer mungbean (MBR+DSR-ZTW+RR-ZTMB) plots was similar to farmers’ practice [transplanted rice (TPR)- conventionally tilled wheat (CTW)], despite TPR-CTW plots had ∼18% higher rice yield than MBR+DSR-ZTW+RR-ZTMB plots in the first year. The MBR+DSR-ZTW+RR-ZTMB treated plots had about 15 and 10% higher mean wheat grain yield and mean system productivity (sum of grain yields of all crops) than TPR-CTW plots, respectively. The plots under DSR+brown manuring (BM)-ZTW+RR plots had comparable mean rice and wheat yields to MBR+DSR- ZTW+RR-ZTMB plots. Harvestable aboveground biomass productivity of MBR+DSR-ZTW+RR-ZTMB treated plots was ∼2.89Mgha−1yr−1 higher than TPR-CTW. Total estimated C input (∼12.1MgCha−1 in three years) under MBR +DSR-ZTW+RR-ZTMB treated plots was ∼117 and 127% higher than DSR-ZTW and TPR-CTW treatments, respectively. All CA plots had significantly higher gain (over initial value) in total SOC than that in TPR-CTW and TPR-ZTW treatments in the 0–15cm layer and the gain in total SOC in the plots under MBR+DSR- ZTW+RR-ZTMB was significantly higher than all CA plots, despite having similar total SOC stocks. Again, plots under MBR+DSR-ZTW+RR-ZTMB had ∼24% larger labile C pools than that of TPR-CTW (3.1gkg−1) treated plots in the topsoil. Soil bulk density under MBR+DSR- ZTW+RR-ZTMB and DSR+BM-ZTW+RR treated plots significantly decreased in the 5–15cm layer compared to TPR-CTW plots. Thus, the MBR+DSR-ZTW+RR-ZTMB treatment (a novel CA practice), has considerable potential to retain C in surface soil, decrease soil compaction and increase system (rice–wheat–green gram) productivity and hence its adoption is recommended.

Effects of long-term litter manipulation on soil carbon, nitrogen, and phosphorus in a temperate deciduous forest

Changes in above-ground litterfall can influence below-ground biogeochemical processes in forests. In order to examine how above-ground litter inputs affect soil carbon (C), nitrogen (N) and phosphorus (P) in a temperate deciduous forest, we studied a 14-year-old small-scale litter manipulation experiment that included control, litter exclusion, and doubled litter addition at a mature Fagus sylvatica L. site. Total organic C (TOC), total N (TN) and total P (TP), total organic P (TOP), bioavailable inorganic P (Pi), microbial C, N and P, soil respiration and fine root biomass were analyzed in the A and in two B horizons. Our results showed that litter manipulation had no significant effect on TOC in the mineral soil. Litter addition increased the bioavailable Pi in the A horizon but had no significant effect on N in the mineral soil. Litter exclusion decreased TN and TP in the B horizon to a depth of 10 cm. In the A horizon of the litter exclusion treatment, TP, TOP and bioavailable Pi were increased, which is most likely due to the higher root biomass in this treatment. The high fine root biomass seems to have counteracted the effects of the excluded aboveground litter. In conclusion, our study indicates that aboveground litter is not an important source for C in the mineral soil and that P recycling from root litter might be more important than from above-ground litter.

Great Plains Climate and Land-Use Effects on Soil Organic Carbon

Soil organic C (SOC) is essential to agricultural productivity and sustainability in response to climate and land-use change. Here, we examine 14 sites across the U.S. Great Plains to determine the sensitivity of important SOC fractions to climatic gradients (temperature and precipitation) and land-use change (native, conservation reserve program [CRP], and cropped). We measured particulate organic matter C (POM-C), soil mineral associated C (Cmin-C), soil microbial biomass C (SMB-C), and soil aggregate stability (%AggStab). All fractions (POM-C, SMB-C, and %AggStab) except Cmin-C were affected by land-use and decreased from native > CRP > cropped sites. Total SOC and all the fractions decreased with increasing temperature and increased with greater clay content. Surprisingly, only the Cmin-C as well as the %AggStab increased with increasing mean annual precipitation (MAP)/evapotranspiration (ET) ratio; the more labile fractions POM-C and SMB-C did not. Despite greater %AggStab, there was not POM-C protection, suggesting that aggregate turnover and subsequent C loss was greater in sites with more moisture. The Cmin-C fraction comprised the majority of total SOC (70%) and also comprised most of the whole soil response to temperature and moisture gradients. The POM-C fraction could be used as an easily measured indicator of land-use impacts in soil, since POM-C and SMB-C were highly correlated. Conservation practices that promote soil aggregation and reduce disturbance and erosion will be critical in maintaining mineral soil C and ameliorating soil C loss though increased temperatures. These practices have additional benefit of building the POM-C fraction and SMB-C.

Tillage and Residue Management Effects on Soil Carbon and Nitrogen Under Irrigated Continuous Corn

Demand for corn (Zea mays L.) stover as forage or as a cellulosic biofuel has increased the importance of determining the effects of stover removal on biomass production and the soil resource. Our objectives were to evaluate grain yield, soil organic C (SOC), and total soil N (0–150 cm) in a 10‐yr, irrigated, continuous corn study under conventional disk tillage (CT) and no‐till (NT) with variable corn stover removal rates (none, medium, and high). Natural abundance C isotope compositions (δ13C) were used to determine C additions by corn (C4–C) to the soil profile and to evaluate the retention of residual C3–C. After 10 yr of management treatments, mean grain yields were 7.5 to 8.6% higher for NT when stover was removed compared with no stover removal, while grain yields were similar for CT in all stover removal treatments. Turnover of SOC occurred as C3–C stocks were replaced by C4–C in the 0‐ to 120‐cm soil profile. Total SOC and N stocks changed mainly in surface soils (0–30 cm), with no detectable cumulative changes at 0 to 150 cm. Specifically, SOC declined after 10 yr under CT at 0 to 15 cm and was affected by residue management at 15 to 30 cm. Total soil N was greater when no stover was removed (P = 0.0073) compared with high stover removal at 0 to 15 cm. Long‐term NT ameliorated medium stover removal effects by maintaining near‐surface SOC levels. Results support the need to evaluate SOC cycling processes below near‐surface soil layers.

Film-Mulched Ridge-Furrow Management Increases Maize Productivity and Sustains Soil Organic Carbon in a Dryland Cropping System

Alleviating the hydrothermal limitations to growth, clear film fully mulched ridge–furrow (FMRF) cropping significantly improves maize (Zea mays L.) grain yield on the Loess Plateau of China. Major concerns for FMRF cropping are the stability of maize productivity and whether the system is detrimental to the soil organic C (SOC) balance under changed hydrothermal conditions. We investigated the effects of maize production with FMRF and its effect on SOC concentration for five consecutive years from 2008 to 2012. Three treatments were imposed: no mulch (narrow ridges with the crop sown beside the ridges), half mulch (the same as no mulch, except the narrow ridges were mulched), and full mulch (i.e., FMRF; alternate narrow and wide ridges, all mulched, with maize in furrows). The 5-yr average of the grain yield was 3.8 Mg ha−1 under no mulch. Half and full mulch increased the grain yield by 68 and 102%, respectively, relative to no mulch. Root biomass was 69 and 104% greater under half and full mulch, respectively, than no mulch. The maize yield and biomass differed among years depending on the growing-season precipitation and its distribution, but the increased yield and biomass from mulching was consistent in all years. The mulch stimulated SOC mineralization and enzymatic activity but had no effect on light (density <1.8 g cm–3) and total SOC concentrations compared with no mulch. We conclude that increased SOC mineralization under FMRF was offset by increased SOC addition; FMRF cropping increased maize productivity without detriment to the SOC balance.

Trees increase soil carbon and its stability in three agroforestry systems in central Alberta, Canada

Agroforestry land-use systems have significant potential for increasing soil carbon (C) storage and mitigating increases in atmospheric greenhouse gas (GHG) concentrations. We studied the impact of three agroforestry systems (hedgerow, shelterbelt, and silvopasture) on soil organic C (SOC) and nitrogen (N) in the 0–10cm mineral layer, by comparing SOC and N distributions in whole soils and three particle-size fractions (<53, 53–250, 250–2000μm) to assess the potential role of physical protection on soil C and N storage. We assessed thirty-five sites (12 hedgerows, 11 shelterbelts and 12 silvopastures), each comprised of 2 paired plots (forest and adjacent agricultural herbland), that were distributed along a 270km long north–south soil/climate gradient in central Alberta, Canada. Across all sites, 48.4%, 28.5%, and 23.1% of SOC was found in the fine (<53μm), medium (53–250μm) and coarse fractions (250–2000μm), respectively. Mean SOC in the whole soil was 62.5, 47.7 and 81.3gkg−1 in hedgerow, shelterbelt and silvopasture systems, respectively. Soil C in the more stable fine fraction was 34.3, 28.8 and 29.3gkg−1 in the hedgerow, shelterbelt and silvopasture systems, respectively. Within each agroforestry system, the forested land-use consistently had greater total SOC and SOC in all size fractions than the agricultural component. Our results demonstrate the potential for trees to increase soil C sequestration in agroforestry systems within the agricultural landscape.

Land‐Use Type Effects on Soil Organic Carbon and Microbial Properties in a Semi‐arid Region of Northeast Brazil

ABSTRACTLand‐use change is one of the most important anthropogenic environmental change drivers affecting the biodiversity and functioning of ecosystems. However, there is limited knowledge of the consequences for soil processes in many regions around the globe. The Brazilian semi‐arid ecosystem known as Caatinga has experienced the transformation from native forest into agricultural land, with heretofore unknown effects on soil processes and microbial properties. The aim of this study was to evaluate the impact of five land‐use changes (to maize and cowpea cropland, grape orchard, and cut and grazed pasture) on total organic C (TOC) and total N (TN) stocks and soil microbial properties of Ultisol from Caatinga. Soil samples (0–10 and 10–20 cm depth) were collected during the wet and dry periods. Split–split plot analysis of variance was used to test the effects of land use, soil depth, season and the interaction between land‐use and soil depth on soil microbial properties, TOC and TN stocks. Land‐use effects were more pronounced in the top soil layer than in the lower layer, while the pattern was less consistent in soil microbial properties. Land conversion from native forest to cropland may cause C losses from the soil, but conversion to pastures may even increase the potential of soils to function as C sinks. Grazed pastures showed not only high C and N stocks but also the highest soil microbial biomass and lowest respiratory quotients, all indications for elevated soil C sequestration. Thus, grazed pastures may represent a land‐use form with high ecosystem multifunctionality in Caatinga. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Effects of Rice Straw and Its Biochar Addition on Soil Labile Carbon and Soil Organic Carbon

Whether the biochar amendment could affect soil organic matter (SOM) turnover and hence soil carbon (C) stock remains poorly understood. Effects of the addition of 13C-labelled rice straw or its pyrolysed biochar at 250 or 350°C to a sugarcane soil (Ferrosol) on soil labile C (dissolved organic C, DOC; microbial biomass C, MBC; and mineralizable C, MC) and soil organic C (SOC) were investigated after 112 d of laboratory incubation at 25°C. Four treatments were examined as (1) the control soil without amendment (Soil); (2) soil plus 13C-labelled rice straw (Soil+Straw); (3) soil plus 250°C biochar (Soil+B250) and (4) soil plus 350°C biochar (Soil+B350). Compared to un-pyrolysed straw, biochars generally had an increased aryl C, carboxyl C, C and nitrogen concentrations, a decreased O-alkyl C and C:N ratio, but similar alkyl C and δ13C (1 742–1 877 ‰). Among treatments, significant higher DOC, MBC and MC derived from the new C (straw or biochar) ranked as Soil+Straw>Soil+B250>Soil+B350, whilst significant higher SOC from the new C as Soil+B250>Soil+Straw≈Soil+B350. Compared to Soil, DOC and MBC derived from the native soil were decreased under straw or biochar addition, whilst MC from the native soil was increased under straw addition but decreased under biochar addition. Meanwhile, native SOC was similar among the treatments, irrespective of the straw or biochar addition. Compared to Soil, significant higher total DOC and total MBC were under Soil+Straw, but not under Soil+B250 and Soil+B350, whilst significant higher total MC and total SOC were under straw or biochar addition, except for MC under Soil+B350. Our results demonstrated that the application of biochar to soil may be an appropriate management practice for increasing soil C storage.

Effects of Different Tillage and Straw Return on Soil Organic Carbon in a Rice-Wheat Rotation System

Soil management practices, such as tillage method or straw return, could alter soil organic carbon (C) contents. However, the effects of tillage method or straw return on soil organic C (SOC) have showed inconsistent results in different soil/climate/cropping systems. The Yangtze River Delta of China is the main production region of rice and wheat, and rice-wheat rotation is the most important cropping system in this region. However, few studies in this region have been conducted to assess the effects of different tillage methods combined with straw return on soil labile C fractions in the rice-wheat rotation system. In this study, a field experiment was used to evaluate the effects of different tillage methods, straw return and their interaction on soil total organic C (TOC) and labile organic C fractions at three soil depths (0–7, 7–14 and 14–21 cm) for a rice-wheat rotation in Yangzhong of the Yangtze River Delta of China. Soil TOC, easily oxidizable C (EOC), dissolved organic C (DOC) and microbial biomass C (MBC) contents were measured in this study. Soil TOC and labile organic C fractions contents were significantly affected by straw returns, and were higher under straw return treatments than non-straw return at three depths. At 0–7 cm depth, soil MBC was significantly higher under plowing tillage than rotary tillage, but EOC was just opposite. Rotary tillage had significantly higher soil TOC than plowing tillage at 7–14 cm depth. However, at 14–21 cm depth, TOC, DOC and MBC were significantly higher under plowing tillage than rotary tillage except for EOC. Consequently, under short-term condition, rice and wheat straw both return in rice-wheat rotation system could increase SOC content and improve soil quality in the Yangtze River Delta.

Influence of Soil Geochemical and Physical Properties on Chromium(VI) Sorption and Bioaccessibility

The Department of Defense (DoD) is faced with the daunting task of possible remediation of numerous soil-Cr(VI) contaminated sites throughout the continental U.S. The primary risk driver at these sites is hand-to-mouth ingestion of contaminated soil by children. In the following study we investigate the impact of soil geochemical and physical properties on the sorption and bioaccessibility of Cr(VI) in a vast array of soils relevant to neighboring DoD sites. For the 35 soils used in this study, A-horizon soils typically sorbed significantly more Cr(VI) relative to B-horizon soils. Multiple linear regression analysis suggested that Cr(VI) sorption increased with increasing soil total organic C (TOC) and decreasing soil pH. The bioaccessibility of total Cr (CrT) and Cr(VI) on the soils decreased with increasing soil TOC content. As the soil TOC content approached 0.4%, the bioaccessibility of soil bound Cr systematically decreased from approximately 65 to 10%. As the soil TOC content increased from 0.4 to 4%, the bioaccessibility of Cr(VI) and CrT remained relatively constant at approximately 4% and 10%, respectively. X-ray absorption near edge structure (XANES) spectroscopy suggested that Cr(VI) reduction to Cr(III) was prevalent and that the redox transformation of Cr(VI) increased with increasing soil TOC. XANES confirmed that nearly all bioaccessible soil Cr was the Cr(VI) moiety. Multiple linear regression analysis suggested that the bioaccessibility of Cr(VI) and its reduced counterpart Cr(III), decreased with increasing soil TOC and increasing soil pH. This is consistent with the observation that the reduction reaction and formation of Cr(III) increased with increasing soil TOC and that Cr(III) was significantly less bioaccessible relative to Cr(VI). The model was found to adequately describe CrT bioaccessibility in soils from DoD facilities where Cr(VI) contaminated sites were present. The results of this study illustrate the importance of soil properties on Cr(VI) sorption and bioassessability and help define what soil types have the greatest risk associated with Cr(VI) exposure.

Impacts of conservation agriculture on total soil organic carbon retention potential under an irrigated agro-ecosystem of the western Indo-Gangetic Plains

Sequestration of C in arable soils has been considered as a potential mechanism to mitigate the elevated levels of atmospheric greenhouse gases. We evaluated impacts of conservation agriculture on change in total soil organic C (SOC) and relationship between C addition and storage in a sandy loam soil of the Indo-Gangetic Plains. Cotton (Gossypium hirsutum L.) and wheat (Triticum aestivum L.) crops were grown during the first three years (2008–2011) and in the last year, maize (Zea mays L.), wheat and green gram (Vigna radiate L.) were cultivated. Results indicate the plots under zero tillage with bed planting (ZT-B) and zero tillage with flat planting (ZT-F) had nearly 28 and 26% higher total SOC stock compared with conventional tillage and bed planting (CT-B) (∼5.5Mgha−1) in the 0–5cm soil layer. Plots under ZT-B and ZT-F contained higher total SOC stocks in the 0–5 and 5–15cm soil layers than CT-B plots. Although there were significant variations in total SOC stocks in the surface layers, SOC stocks were similar under all treatments in the 0–30cm soil layer. Residue management had no impact on SOC stocks in all layers, despite plots under cotton/maize+wheat residue (C/M+W RES) contained ∼13% higher total SOC concentration than no residue treated plots (N RES; ∼7.6gkg−1) in the 0–5cm layer. Hence, tillage and residue management interaction effects were not significant. Although CT-B and ZT-F had similar maize aboveground biomass yields, CT-F treated plots yielded 16% less maize biomass than CT-B plots. However, both wheat and green gram (2012) yields were not affected by tillage. Plots under C/M+W RES had ∼17, 13, 13 and 32% higher mean cotton, maize, wheat and green gram aboveground biomass yields than N RES plots, yielding ∼16% higher estimated root (and rhizodeposition) C input in the 0–30cm soil layer than N RES plots. About 9.3% of the gross C input contributed towards the increase in SOC content under the residue treated plots. However, ∼7.6 and 10.2% of the gross C input contributed towards the increase in SOC content under CT and ZT, respectively. Thus, both ZT and partial or full residue retention is recommended for higher soil C retention and sustained crop productivity.

Responses of labile soil organic carbon and enzyme activity in mineral soils to forest conversion in the subtropics

& Aims Globally, extensive areas of native forest have been almost replaced by plantations to meet the demands for timber, fuel material and other forest products. This study aimed to evaluate the effects of forest conversion on labile soil organic C (SOC), soil respiration, and enzyme activity, and to quantify their relationship in subtropical forest ecosystems. & Methods Surface mineral soil (0–20 cm) was collected from a Cunninghamia lanceolata Hook. plantation, Pinus massoniana Lamb. plantation, Michelia macclurei Dandy plantation, and an undisturbed native broadleaf forest. Soil microbial biomass C, dissolved organic C, permanganateoxidizable C, basal respiration, and six enzyme activities were investigated. & Results Soil microbial biomass C was higher by 45.9 % in native broadleaf forest than that in M. macclurei Dandy plantation. The ratio of soil microbial biomass C to total SOC was 27.6 % higher in the M. macclurei Dandy plantation than in the native broadleaf forest. The soil respiration increased by 25.2 %a nd 21.7 %a fter conversion from native broadleaf forest to P. massoniana Lamb. and M. macclurei Dandy plantations respectively. The effects of forest conversion on the soil enzyme activities differed among the tree species. Soil microbial biomass C had higher correlation with soil respiration than with the other SOC fractions. Moreover, soil microbial biomass C was positively correlated with urease and negatively correlated with cellulase activity. Soil respiration had higher correlation with soil microbial biomass C, dissolved organic C and permanganate-oxidizable C. & Conclusion Forest conversion affected the soil microbial biomass C, soil respiration, invertase, cellulase, urease, catalase, acid phosphatase, and polyphenol oxidase activities, but their response depended on tree species. Soil respiration was mainly controlled by labile SOC, not by total SOC.

Soil Organic Carbon Accretion vs. Sequestration Using Physicochemical Fractionation and CQESTR Simulation

Accurate estimates of soil organic C (SOC) stocks are required to determine changes in SOC resulting from agricultural management practices. Our objectives were to: (i) determine total SOC; (ii) estimate the contribution of light fraction C (LF-C) to total SOC; and (iii) simulate SOC dynamics using CQESTR to examine the effect of climate change for three cropping systems in the Pacific Northwest. The LF-C masked small gains or losses in measured SOC for all cropping systems. Simulated data indicated no significant changes in SOC in the top 30 cm of the sweep-tillage winter wheat (Triticum aestivum L.)–tillage fallow rotation (WW–TF) and no-till (NT) spring wheat–chemical fallow rotation (SW–CF/NT), whereas SOC increased in the NT spring barley (Hordeum vulgare L.)–spring wheat rotation (SB–SW/NT). The apparent increase in measured SOC with continuous NT spring cropping was the result of accumulated undecomposed crop residues that contributed to the labile C pool and was confirmed via LF-C analysis. The contributions of the LF-C to total SOC across cropping systems ranged from 13.4 to 18.4% (fall soil samples) and 14.4 to 18.9% (spring soil samples). Modeling predicted no significant change in SOC stocks for the WW–TF and SW–CF/NT rotations, even with a 30% crop biomass increase based on potential climate change scenarios. Differences between the observed and predicted SOC were due to artifacts associated with protocols used to determine SOC that did not completely remove accrued crop residue and could be explained by LF-C, which provided a first approximation of organic C accretion.