Effects of 22-year fertilisation on the soil organic C, N, and theirs fractions under a rice-wheat cropping system

The quantification of soil organic carbon (SOC) and N fractions under the influence of long-term fertilisation is useful to understand soil organic matter (SOM) stabilization mechanism and to develop optimal management practices. This study was to investigate the responses of different SOC and N fractions to long-term fertilisation and their contributions to the total change in the bulk soil under a rice-wheat cropping system. Compared with soil with no fertilisation, the 22-year fertilisation improved the C and N content of the bulk soil and different fractions with the greatest increase in NPK fertilisers and rice straw treatment. The SOC and N changes of heavy fraction accounted for 92% and 88% of the total SOC increases, while the Fe/Al-bound organic carbon and humic organic carbon in heavy fraction accounted for 43% and 48%, respectively. The SOC and N contents of each fraction were positively correlated with the bulk soil and the others. It is concluded that the NPK fertilisers plus straw returning was efficient to enhance SOC and N accumulation; the chemical binding of Fe and Al in organic-mineral complexes and the further formation of recalcitrant humin might be the potential mechanism for SOM accumulation in the rice-wheat cropping system.Abbreviations: SOM: soil organic matter; SOC: soil organic carbon; STN: soil total nitrogen; POM: particulate organic matter; MOM: mineral associated organic matter; LF: light fraction; HF: heavy fraction; Ca-OC: Ca-bound organic carbon; Fe/Al-OC: Fe- or Al-bound organic carbon; Humin-OC: humic organic carbon