Soil organic carbon and nitrogen fractions as affected by straw and nitrogen management on the North China Plain

Straw incorporation and nitrogen (N) application play crucial roles in regulating soil carbon (C) and N storage and availability. However, their effects on soil C and N fractions and microbial community composition, as well as the relationships between microorganisms and soil organic C (SOC) and soil organic N (SON) fractions remain largely unexplored. Here, arable soils following 5-year straw (NS, nonamended controls; AS, wheat and maize straw incorporation) and N (0, 150 and 250 kg N ha −1 applied for each crop) management were collected to determine the organic C and N fractions and microbial community composition. Compared to NS, AS significantly increased the contents of SOC and total nitrogen (TN) by 36.26 % and 24.12 %, respectively, mainly by increasing the contents of dissolved organic carbon (DOC), particulate organic carbon (POC), hydrolysable unknown-N (HUN) and acid insoluble-N (AIN). Under straw incorporation, N addition significantly improved total phospholipid fatty acids (PLFAs), the contents of TN, DOC, amino acid-N (AAN), active SON and stable SON, and the ratios of POC/SOC and DOC/SOC. However, there was no significant difference in SOC pool, organic C and N fractions (except for AAN and active SON) and microbial community composition between the N150 and N250 levels under AS. Compared to straw removal, straw incorporation significantly changed the microbial community composition by increasing the accumulations of SOC, DOC, POC, and mineral-associated organic carbon (MAOC). Regression analysis indicated that high relative abundances of fungi and gram-positive bacteria (GP) were more conducive to stable SON accumulation. Collectively, straw incorporation coupled with 150 kg N ha −1 per crop growing season was a sustainable field management practice for effectively improving soil fertility on the North China Plain. • Straw incorporation improves the soil C pool by increasing organic C availability. • The fungal community composition is driven by the soil active organic C. • The gram-positive bacteria community is driven by the mineral-associated organic C. • Stable organic N accumulation is closely related to fungi and gram-positive bacteria. • Straw with N addition increases the soil N pool by enhancing organic N stability.