Objectives Long-term fertilization affects the soil carbon sequestration and fertility. The responses of soil organic carbon and nitrogen fractions long-term organic manure application under rice-wheat rotation were investigated. The objective is to provide theoretical support for carbon sequestration and emission reduction.

Methods The long-term experiment had lasted for 36-years with wheat and rice rotation in Wuhan, Hubei Province of China. The treatments included no fertilizer control (CK), inorganic fertilizer N (N), inorganic fertilizer N and P (NP), inorganic fertilizer N, P and K (NPK), manure (M), and manure plus inorganic fertilizer N, P and K (NPKM). After wheat harvested in 2017, the soil samples of 0−20 cm deep were collected. The contents of soil organic carbon (SOC), total nitrogen (TN), microbial biomass carbon and nitrogen (MBC, MBN), dissolved organic carbon (DOC), hot-water soluble carbon (HWSC), particulate organic carbon and nitrogen (POC, PON), light fraction of organic carbon and nitrogen (LFOC, LFON) and potassium chloride extracted nitrogen (KEN, as water-soluble inorganic nitrogen) were measured and analyzed.

Results 1) Except the KEN, the long-term organic fertilization significantly increased the contents of carbon and nitrogen fractions in surface soil, especially the NPKM treatment. 2) The percentages of each labile organic carbon fraction in SOC were in descendent ordered as POC > LFOC > HWSC > MBC > DOC, and the percentage of POC was as high as 24.04%−37.63%. The percentages of each nitrogen fraction in TN were in descendent ordered as POC > LFOC > MBN > KEN, with that of the PON of 12.09%−20.24%. The POC/SOC and PON/TN ratios in manure treatment were significantly higher than in the other treatments. 3) Through the analysis of sensitivity index of SOC and labile carbon fractions to fertilization, the results found that the sensitivity index of labile carbon fractions was significant higher than SOC, especially DOC. 4) Through the correlation analysis of each component, this study found that all fractions, except for the KEN, significantly and positively correlated whit each other, especially the correlation between DOC and SOC, PON and TN.

Conclusions In the tested region, long-term organic and inorganic fertilization significantly increase the contents of soil carbon and nitrogen pool and stimulate their accumulation, especially the particulate organic carbon and nitrogen (POC and PON). Moreover, the dissolved organic carbon can be regarded as a sensitive indicator of early change of organic matter because of its high sensitivity to fertilization.