Nitrogen enrichment regulates straw decomposition and its associated microbial community in a double-rice cropping system.

Litter bag method was conducted to investigate the decomposition characteristics of rice straw (6000 kg ha-1) and its associated microbial community under different nitrogen (N) addition rates (0, 90, 180 and 270 kg N ha-1) under double-rice rotation. Generally, straw mass reduction and nutrient release of rice straw were faster in early stage of decomposition (0-14 days after decomposition), when easily-utilized carbohydrates and amines were the preferential substrates for involved decomposers. Straw-associated N-acetyl-glucosamidase and L-leucine aminopeptidase activities, which were higher under 180 and 270 kg N ha-1 addition, showed more activities in the early stage of decomposition. Gram-positive bacteria were the quantitatively predominant microorganisms, while fungi and actinomycetes played a key role in decomposing recalcitrant compounds in late decomposition stage. Straw residue at middle decomposition stage was associated with greater cbhI and GH48 abundance and was followed by stronger β-glucosidase, β-cellobiohydrolase and β-xylosidase activities. Although enzyme activities and cellulolytic gene abundances were enhanced by 180 and 270 kg N ha-1 application, microbial communities and metabolic capability associated with rice straw were grouped by sampling time rather than specific fertilizer treatments. Thus, we recommended 180 kg N ha-1 application should be the economical rate for the current 6000 kg ha-1 rice straw returning.