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Soil organic carbon (SOC) constitutes a significant proportion of the terrestrial carbon (C) store and has a pivotal role in several physical, chemical, and biological soil processes that contribute to soil productivity and sustainability. Applications of inorganic and organic materials are management options that have the potential to increase SOC in agricultural systems. A study was conducted in 3 long-term fertility experiments (Barrackpur, Mohanpur, and Cuttack) on agricultural soils in the eastern cereal belt of India, to examine the effect of cultivation and the application of inorganic and organic amendments on total soil organic carbon (TOC) and on the proportions of soil C fractions at these sites. A supplementary aim of this study was to determine the suitability of the loss-on-ignition (LOI) method to routinely estimate SOC (Walkley and Black, WB) in this region by determining relationships and conversion factors between the WB and LOI techniques. Soil was sampled at 3 depths (0-0.15, 0.15-0.30, and 0.30-0.45 m) from 4 treatments (conventional cultivation, NPK, NPK+FYM, and fallow) of the experimental sites and analysed for TOC and various soil C pools. There were differences in the magnitude of TOC values among the sites. Conventional cultivation had the lowest TOC contents (148 t/ha) and NPK+FYM amended soils the largest (207 t/ha), with intermediate values in the other treatments. The non-labile or residual SOC fraction (Cfrac4) constituted the largest percentage of SOC under all treatments and varied from 35-49%. A higher proportion of the labile Cfrac1 fraction was observed under the fallow, whereas the proportion of Cfrac4 was significantly larger under NPK+FYM. There was a significant decrease in SOC with increasing soil depth. SOC decreased up to 17% at 0.15-0.30 m and declined a further 21% at 0.30-0.45 m. The more labile C fractions (Cfrac1, Cfrac2, Cfrac3) dominated in the near surface soil layers, but decreased significantly in the deeper layers to be dominated by Cfrac4 at 0.30-0.45 m depth. We also observed a strong correlation between the WB and LOI methods (calibrated for each soil) irrespective of soil depths and conclude that this might be a suitable method to estimate SOC where other techniques are not available. We conclude that fertiliser application and especially manure application have the potential to significantly increase SOC in agricultural soils. |
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