Microbial biomass associated with aggregates; aggregate associated carbon and soil aggregate dynamics in rice-wheat cropping system: A review

Aggregate-size classes may have different microbial accessibility and therefore different decomposability of aggregate-associated soil organic matter (SOM). However, processes and mechanisms of soil organic carbon (SOC) mineralisation in different aggregate-size classes, and particularly, the interaction of aggregates with tillage intensity are poorly understood. The OC and N stocks in total soils were significantly correlated with the changes in the >0.053 mm aggregates. Adoption of CA increased OC stocks in the >0.053 mm size class of aggregates and N stocks in the >0.25 mm size class but decreased OC stocks in the P2 > P3 and T4 > T3 > T1 > T2 in wheat strip regarding tillage practices. However, the MWD decreased drastically in lower soil depth. The increases in OC and N stocks in these aggregates accounted for 99.5 and 98.7% of the total increases, respectively, in the continuous residue retention in rice –wheat cropping system. Organic amendments increased the proportion of macro-aggregate and mean weight diameter (MWD), especially in the plow layer. The macro-aggregates accounted for 43.87% and 49.96% of the total soil weight in the straw retention (SR) and manure application (MA). The distribution patterns of soil organic C (SOC) and microbial biomass C (MBC) within aggregate sizes was affected by organic treatments. The cropping increased the stocks of OC and N in total soils at mean rates of 13.2 g OCm-2 yr-1 and 0.8 g N m-2 yr-1 at the 0–20 cm depth and of 2.4 g OCm-2 yr-1 and 0.4 g N m-2 yr-1 at the 20–40 cm depth. Microbial biomass carbon showed a seasonal pattern. It was low initially, reached its peak during the flowering stages in both rice and wheat and declined thereafter. Microbial biomass carbon was linearly related to SOC in both rice and wheat indicating that SOC could be used as a proxy for MBC. The fine (0.053–0.25 mm) intra-aggregate particulate organic C (iPOM-C), in 0.25- to 2-mm aggregates, was also higher in ZT than conventional tillage. A higher amount of macro-aggregates along with greater accumulation of particulate organic C indicates the potential of ZT for improving soil carbon over the long-term in rice-wheat rotation.