Modeling of Soil Organic Carbon Concentration and Stability Variation in Top and Deep Soils with varied Aggregate Size under Climate Change of Sub-tropical India: A Review
Abstract
The effects of tillage on soil organic carbon (SOC) and nutrient content of soil aggregates can vary spatially and temporally, and for different soil types and cropping systems. Surface soil (0–15 cm) was fractionated into aggregate sizes (>4.76 mm, 4.76–2.00 mm, 2.00–1.00 mm, 1.00–0.25 mm, 0.25–0.053 mm, <0.053 mm) under two tillage regimes. The percentage of soil OCmineralized (SOC , % SOC) was in general higher in larger aggregates than in smaller aggregates. min Tillage significantly reduced the proportion of macro-aggregate fractions (>2.00 mm) and thus aggregate stability was reduced by 35% compared with RNT, indicating that tillage practices led to soil structural change for this subtropical soil. Soil organic C decreased with increasing soil depth but was greater under tree than others and was mainly concentrated in the topsoil layer (0–20 cm). In comparison to topsoil, deep soil aggregates generally exhibited a lower C , and higher min SOC . The highest SOC was in the 1.00–0.25 mm fraction, while the lowest SOC was in micro-aggregate (<0.025 mm) and min silt + clay (<0.053 mm) fractions and CT, respectively. Tillage did not influence the patterns in SOC across aggregates but did change the aggregate-size distribution, indicating that tillage affected soil fertility primarily by changing soil structure. The percentage of soil OCmineralized (SOC , % SOC) was in general higher in larger aggregates than in smaller min aggregates. Meanwhile, SOC was greater in coniferous forests (CF) than in broad-leaved forests (BF) at topsoil and deep min soil aggregates. In comparison to topsoil, deep soil aggregates generally exhibited a lower C , and higher SOC . The sum min min of macro-aggregate contributing rates for clay-humus stability of soil organic C (SOC) was significantly superior to that of the micro-aggregates. Water-stable aggregates increased by 34.5% in the CA with residue retention treatment, effectively improving the soil structure. Furthermore, 0.25–1.00 and 1–2mm aggregates had the highest SOC microbial biomass storage and responded rapidly to the various tillage treatments. Greater proportion of micro-aggregates within macro-aggregates in the plots under NT–NTcompared with CT–CTwas also observed in the surface layer only. Plots under NT–NThad about 10% higher coarse (250–2000 μm) intra-aggregate particulate organic matter-C (iPOM–C) within >2000 μm sand free aggregates in the 0-to 5-cm soil layer compared with CT–CTplots. The fine (53–250 μm) iPOM–C within the 250-to 2000-μm aggregates was also higher in the continuous NTplots compared with CTwithin both >2000 and 250 to 2000 μm sand free aggregate size classes in that soil layer.
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Introduction
Soil organic carbon (SOC) is the largest constituent of the Earth’sterrestrial carbon pool (Stockmann et al., 2013), and slight C losses from the soil may lead to considerable changes in atmospheric CO concentration (Wang et al., 2002), which would 2 affect the magnitude of future climate change (Davidson, and Janssens, 2006). Increasing anthropogenic disturbances especially, on land use/cover change is the major cause of soil quality deterioration in the world (Haynes, 2005). Soil organic carbon (SOC) has recently gained prominence in assessment of soil quality since it compound affects chemical, physical and biological aspects of the soil. Though described by some as the least most understood component of the soil because of its dynamism, (Lehmann and Kleber, 2015) SOC has been linked to its potential role in carbon sequestration through proper management of land use and cover types (Yang et al., 2012). Land use and cover types influence C fluxes in an ecosystem; through litter quality, deposition and turnover rate. Although SOC is an indicator of soil quality, conceptualization of soil fractions can be used to detect even slight changes in management and regulate degradation (Blair et al., 1995). As CO exchange between soil carbon and atmospheric CO varies strongly along climate gradients (Wang et al., 2010) focus 2 2 on whether there are enhanced response patterns in SOC stability along increasing latitudinal or altitudinal gradients. Numerous studies have implicated temperature as a primary controller of SOC stability by altering the quality and quantity of litter input into soil and soil physico-chemical characteristics (Bird et al., 2002; Garten et al., 2006). SOC stability was found to increase with increasing mean annual temperature (MAT) based on chemical sequential fractionation analysis Hilli et al., 2008). However, the components and stability of SOC were not always consistently related to variations on MAT (Djukic et al., 2210). Radiocarbon dating and 13C enrichment differentiation for soils indicated that SOC stability along latitudinal and altitudinal gradients was negatively related to MAT (Garten, 2011). Therefore, in addition to temperature affecting SOC stability, other factors must also contribute to SOC stability.
Conclusion
In topsoil, WSmacro-aggregate formation was highest (28.2 g of >250 mm aggregates per gram of C added) with the lowest residue input (2.5 gresidue-C kg-1 soil). In the subsoil, WSmacro-aggregate formation increased to 76.3 g of >250 mm aggregates per gram of C added with residue input of 5 gresidue-C kg-1 soil and decreased thereafter. The concentration of POC, MBC and HWC were higher under topsoil (0-10 cm) as compared to subsoil (10-20 cm) in CA practices. Organic carbon concentrations in the <0.053-, 0.053-to 0.25-, 0.25-to 2.0, and >2.0-mm fractions were 14.0, 12.0, 14.4, 24.1% greater, respectively, in CA than in CF. The contents of SOC,LOC, DOC, POC and EOC by 14.73%, 16.5%, 22.5%, 41.5% and 21% in the 0-40 cm soil layer, and by 17%, 14%, 19%, and 30% in the 0-100 cm soil layer. These results suggest that over time, the MBC and MBC-derived C under the fine-sized residue treatment may constitute a significant source of stable SOC through strong physical and chemical bonding to the mineral soil matrix. Conservation management in the NorthWest IGP is important in maintaining soil structure stability and conserving SOC from rapid decomposition with associated organic carbon fractions.
Soil microbial biomass, the active fraction of soil organic matter which plays a central role in the flow of C and N in ecosystems responds rapidly to management practices, and serves as an index of soil fertility. The practices of crop residue retention and tillage reduction provided an increased supply of C and N which was reflected in terms of increased levels of microbial biomass, N-mineralization rate in soil. Residue retention and tillage reduction both increased the proportion of organic C and total N present in soil organic matter as microbial biomass. The no-tillage system showed a trend to accumulate organic carbon near the soil surface layer. Conventional tillage reduced soil organic C stocks and that of its labile fractions both in top and subsoil (20-100 cm). POC reduction was mainly driven by a decrease in fine POC in topsoil, while DOC was mainly reduced in subsoil. Fine POC, LFOC and microbial biomass can be useful early indicators of changes in topsoil organic C. In contrast, LFOC and DOC are useful indicators for subsoil. Reduced proportions of fine POC, LFOC, DOC and microbial biomass to soil organic C reflected the decline in soil organic C quality caused by tillage. The LOC fractions to SOC ratios also decreased, indicating a reduction inC quality as a consequence of tillage and residue management. Reduced LOC fraction stocks in subsoil could partially be explained by the decrease in fine root biomass in subsoil, with consequences for SOC stock.