Characteristics of soil organic carbon and nitrogen distributed in different density fractions of mollisols under long-term continuous cropping and natural restoration

Land use change has a long-term impact on terrestrial ecosystems, resulting in variations in soil carbon pool and carbon cycle. This has triggered a contentions research on the state of soil organic carbon. A long-term experiment was conducted at the National Observation Station of Hailun Agroecosystem, Chinese Academy of Sciences. The experiment included three land use types in five treatments ― i.e., farmland (continuous cropped maize, continuous cropped soybean and continuous cropped wheat), grassland and bare-land, with mollisol as the soil type. The objective of the study was to determine how land use influences the characteristics of soil organic carbon (SOC) and nitrogen distribution among different density fractions (light and heavy). The results showed no significant differences in total SOC among the three farmland treatments. The difference in FLF (free light fraction) SOC content between 0~10 cm and 10~20 cm farmland soil layers was significant (P < 0.05). Whereas FLF SOC mainly accumulated in the 0~10 cm soil layer under continuous cropped soybean treatment, it mainly accumulated in the 10~20 cm soil layer under continuous cropped wheat treatment. There were also no significant differences in OLF (occluded light fraction) SOC content among the farmland treatments. Accumulated FLF and HF (heavy fraction) were critical for net enhancement of TOC (total organic carbon) and TN (total nitrogen). OLF lessly influenced TOC and TN accumulation. The distribution characteristics of soil nitrogen in different density fractions were similar to those of SOC. In terms of C/N ratio, there was significant negative correlation between FLF and OLF. TOC and TN were significantly higher in grassland than in farmland and bare-land treatments. FLF SOC, OLF and HF contents in grassland were much higher than in farmland and bare-land treatments. Vegetation conversion caused not only soil organic matter redistribution, but also carbon and nitrogen sequestration variation in mollisols.