Impact of conservation tillage and intensifying crop rotations in enhancing soil carbon, microbial cycling and aggregation in semiarid agro-eco systems: A review

Tillage influenced SOC and microbial cycling but most effects were observed at 0–5 cm rather than 5–15 cm. Reduced tillage (RT) in a continuous monoculture increased SOC by 24% compared to conventional tillage (CT) at 0–5 cm, but tillage had no effect at 5–15 cm. Crop rotation increased soil C contents compared to continuous monocropping rotation under CT increased SOC by 28% at 0–5 cm compared to CT continuous monocropping. Reduced tillage increased soil microbial biomass C (MBC) by an average of 11 and 18% compared to CT continuous monocropping and the adoption of crop rotation, respectively, while microbial biomass N (MBN) for RT was 62% greater than for CT. NT (no-till with residue) and RTW (rotary tillage with residue) treatments significantly increased the proportion of macroaggregate fractions (>2000 and 250-2000 im) compared with the MP-R (moldboard plow without residue) and MP+R (moldboard plow with residue) treatments. Averaged across all depths, mean weight diameters of aggregates (MWD) in NT and RT were 47 and 20% higher than that in MP+R. Aggregate stability in continuous rotations was about twice that in wheat-fallow rotations. Fungal biomass was three times greater in continuous rotations than wheat fallow, but was not significantly different from mid-intensity rotations.

Tillage and cropping systems can change 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. However, higher SOC content of 8.14 gkg^-1 of 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^-1 of soil were found in puddled transplanted rice followed by wheat planted under conventionally tilled plots.