Straw residue incorporation: Influence on soil microbial biomass and carbon–nitrogen dynamics in an oilseed rape–rice rotation

Abstract Soil C and N input drives microbial biomass in paddy–upland rotations. We explored how crop straw return impacts microbial C and N (MBC and MBN) dynamics and their role in rice–oilseed rape rotations at three sites in Hubei Province, China. These sites, under a subtropical monsoon climate, were categorized (using the modified Nemoro equation) as high, moderate and low soil fertility respectively. We found that with straw return, total nitrogen (TN) and soil organic carbon (SOC) increased to the greatest degree in the high fertility soils. Notably, soil MBC and MBN levels varied with soil fertility and were seasonably dependent, being lower in the rice season compared to the oilseed rape season. Structural equation modelling (SEM) revealed diminishing effects of MBC and MBN on soil N accumulation during the oilseed rape season compared to the rice season. In summary, our results highlight the role that microbial biomass can play in C and N accumulation as a soil fertility indicator in paddy–upland rotations.