Soil tillage can affect the stability and formation of soil aggregates by disrupting soil structure. Frequent tillage deteriorates soil structure and weakens soil aggregates, causing them to be susceptible to decay. Different types of tillage systems affect soil physical properties and organic matter content, in turn influencing the formation of aggregates. The objective of this review study was to evaluate the effect of tillage and fertilization on soil aggregates and associated organic carbon fractions to a subtropical soil and to identify the optimal conservation tillage in this system. The average concentration of particulate organic carbon (POC), dissolved organic carbon (DOC) and microbial biomass carbon (MBC) in organic manure plus inorganic fertilizer treatments (NP+S and NP+FYM) in 0–60 cm depth were increased by 64.9–91.9%, 42.5–56.9%, and 74.7–99.4%, respectively, over the CK treatment. Average SOC concentration of the control treatment was 0.54%, which increased to 0.65% in the RDF treatment and 0.82% in the RDF+FYM treatment. Compared to F1 control treatment the RDF+FYM treatment sequestered 0.28 Mg C ha-1 yr-1 whereas the NPK treatment sequestered 0.13 Mg C ha-1 yr-1. As tillage intensity increased there was a redistribution of SOC in the profile, but it occurred only between ZT and PRB since under CT, SOC stock decreased even below the plow layer. Increased SOC stock in the surface 50 kg m-2 under ZT and PRB was compensated by greater SOC stocks in the 50-200 and 200-400 kg m-2 interval under residue retained, but SOC stocks under CT were consistently lower in the surface 400 kg m-2. Over the last 16 years, CT lost 0.83 ±0.2 kg of C m-2 while ZT gain 1.98 ±0.3 and PRB gain 0.97±0.2 kg of C m-2 in the 1200 kg of soil m-2 profile. However increasing the quantity of C input could enhance soil C sequestration or reduce the rate of soil C loss, depending largely on the local soil and climate conditions. SOC can be best preserved by crop rotations with conservation tillage practices such as no or reduced tillage, and with additions of residues, chemical fertilizers and manure SOC change was significantly influenced by the crop residue retention rate and the edaphic variable of initial SOC content. Soil disturbance by tillage leads to destruction of the protective soil aggregate. This in turn exposes the labile C occluded in these aggregates to microbial breakdown. A higher amount of macro-aggregates along with greater accumulation of particulate organic C indicates the potential of conservation tillage for improving soil carbon over the long-term in rice-wheat rotation in North India.

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