Soil labile organic carbon fractions; microbial community composition and soil organic carbon stocks as affected by conservation tillage and high fertilizer practices input in sub-tropical ecosystems: A review

To improve C sequestration in soils and mitigate climate change, it is essential to understand how nutrient management strategies impact on soil organic carbon (SOC) stocks and labile fractions. Compared to conventional tillage, conservation tillage (no-tillage coupled with straw return) increased water-stable large macro-aggregates (>2 mm) by 35.18%, small macro-aggregates (2–0.25 mm) by 33.52% and micro-aggregates by 25.10% in the topsoil (0–20 cm). The subsoil (20–40 cm) also showed the same trend. Compared to conventional tillage without straw return, large and, small macro-aggregates and micro-aggregates in conservation tillage were increased by 24.52%, 28.48% and 18.12%, respectively. Straw return also caused a significant increase in aggregate-associated carbon (aggregate-associated C). No-tillage coupled with straw return had more total aggregate-associated C within all the aggregate fractions in the topsoil. But the different is that conventional tillage with straw return resulted in more aggregate-associated C than conservation tillage in the subsoil. A considerable proportion of the SOC was found to be stocked in the small macro-aggregates under both topsoil (74.56%) and subsoil (67.09%). Tillage systems can changes in soil organic carbon dynamics and soil microbial biomass by changing aggregate formation and C distribution within the aggregate. Wheat straw ditch-buried returning (WD) had significantly higher total organic carbon than did wheat straw returning with ploughing (WP) and no straw returning (CK) in wheat season. Soil dissolved organic carbon and easily oxidizable carbon contents were significantly increased by 21.3%, 24.3%, 38.6%, and 43.5% under wheat straw returning with rotary tillage (WR) than that under CK in rice and wheat seasons, respectively. Soil microbial biomass carbon (MBC) content was highest under WP in rice season, but in wheat season, WR had significantly higher MBC than WP and WD. The content of SOC in WSAma increased on average in the following order: T 5 mm, 5‒3 mm, 2‒1 mm, 1‒0.5 mm and 0.5‒0.25 mm, respectively. The content of non-labile carbon reflected the contents of SOC in WSA. The highest labile carbon (CL) in WSAma, as compared to others, was found in T+FYM. Overall, application of higher NPK doses resulted in higher content of CL in WSAma compared with the lower applications of NPK. On the other hand, lower applications of NPK to soil increased the content of CL in WSAmi, as compared to G+NPK3.However, higher SOC content of 8.14 gkg-1of soil was found in reduced tilled residue retained plots followed by 10.34 g kg-1in furrow irrigated raised beds with residue retained plots. Whereas, the lowest level of SOC content of 5.49 gkg-1of soil were found in puddled transplanted rice followed by wheat planted under conventionally tilled plots. 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 F1control treatment the RDF+FYM treatment sequestered 0.33 Mg C ha-1yr-1 whereas the NPK treatment sequestered 0.16 Mg Cha-1yr-1. Long term conservation tillage significantly increased 66.1%, 57.9%, 50.9%,38.3%,37.3% and 32% LFOC, SOC, PON, TN, LFON, DOC and POC, over conventional tillage (T7) treatment and WSC 39.6% in surface soil and 37.4% in subsurface soil. Under RWCS, TOC contents were similar in 50% RDN as CF+50% RDN as GM/SPM (F6) and1/3rdN as CF+1/3rdN as FYM+1/3rdN as GM/SPM (F7) and 75% RDN as CF+25% RDN as FYM (F3) plots and significantly higher than those in control (no manure and fertilizer) (F1) plots (by 50.4% 48.3%, and 43.3% respectively).