Tillage and straw-returning practices effect on soil dissolved organic matter, aggregate fraction and bacteria community under rice-rice-rapeseed rotation system

It is well recognized that soil tillage practice and straw returning treatment redistribute carbon (C) in soil and promote the bacterial biomass and diversity. Yet, to date the relative impact of soil tillage practice and straw returning on C in soil different fractions and bacteria community composition remains to be tested. The objective of this study was to investigate the influence of long-term cultivation practices (no-tillage (NT) and conventional tillage (CT)) and crop straw returning on dissolved organic matter (DOM), aggregate fraction and bacteria community composition in a 12-year-olds rice-rice-rapeseed (RRR) rotation system. The study included four different cultivation methods, i.e., NT with or without crop straw returning (NT+S and NT-S) and CT with or without crop straw returning (CT+S and CT-S). Fresh soil samples were collected after harvesting early-rice and rapeseed to analyze the carbon (C) and nitrogen (N) in DOM, aggregate fractions, and bacterial community composition. The results showed that the soil organic matter (SOM) and bacterial community structure diverged under different agriculture practices after12 years of RRR rotation. The combination of straw returning and NT had a more positive impact on C and N storage in the total SOM, DOM, and macro-aggregate fraction (> 2000 mu m) than the combination of straw returning and CT in both the early-rice and rapeseed seasons. Among the seasons in the RRR rotation system, the C and N contents in DOM were the highest in the early-rice season, while C and N contents in the macro-aggregate were the highest in the rapeseed season. The abundance and diversity of the soil bacterial community were influenced by the growing season (early-rice or rapeseed). No-tillage significantly increased the soil bacterial abundance and diversity in the rapeseed season, but had no considerable impacts in the early-rice season. Moreover, the data showed positive effects of straw returning on soil bacterial abundance and diversity in both growing season (early-rice or rapeseed), and the positive effect of straw returning on the soil bacterial community structure probably resulted from improved different soil organic matter fractions. A multivariate analysis (partial correlation analysis and redundancy analysis) indicated that total N (TN), C and N in the DOM (DOC, DON), and C in the macro-aggregate (Macro-C) were all significantly (P<0.05) correlated with soil bacterial communities, suggesting that TN, DOC, DON and Macro-C were sensitive to the changes in soil bacterial community composition under long-term straw returning and NT in our study.