[Responses of Soil Organic Carbon Fractions to Land Use Types in Hilly Red Soil Regions, China]

Land use type exerts important influences on soil organic carbon (SOC) and its fractions, and determines the stability of the carbon pool. Taking woodland as a reference, the content of SOC and its labile fractions[dissolved organic carbon (DOC), microbial biomass carbon (MBC), and particulate organic carbon (POC)] and non-labile fractions[mineral-associated organic carbon (MAOC)] in upland and paddy surface soils in hilly red soil regions were determined to explore the responses of SOC fractions to land use type. The results showed that the contents of SOC, MBC, POC, and MAOC ranked highest in paddy compared with upland and woodland. DOC content in woodland was significantly higher than in upland and paddy (P<0.001). The proportion of each SOC fraction, i.e. DOC/SOC, MBC/SOC, POC/SOC, and MAOC/SOC, was in the range of 0.22%-0.93%, 1.62%-2.70%, 31.08%-40.00%, and 43.22%-56.82%, respectively. The contents of labile fractions (MBC and POC) and their proportions (MBC/SOC and POC/SOC) were in the order of paddy > woodland > upland. MAOC content ranked the highest in paddy but the lowest in upland, while MAOC/SOC exhibited the opposite trend. The correlation suggested that the labile fractions (MBC and POC) and inert fraction (MAOC) were significantly positively correlated with SOC (P<0.001) in the three land use types, while no significant correlations were found between DOC and SOC and its fractions (P>0.05). There was a significant positive correlation between POC and MBC in upland and woodland (P<0.001). POC was significantly positively correlated with MAOC in the three land use types (P<0.001). MAOC and MBC in paddy and upland were significantly positively correlated (P<0.001). Therefore, compared with upland and woodland, SOC in paddy had a higher proportion of labile SOC fraction, but a lower proportion of inert fraction. Moreover, MBC content in paddy was not related to the accumulation of the labile fraction of POC, but positively related to the accumulation of the inert fraction of MAOC. In summary, agricultural land uses have great influence on SOC and its fractions in hilly red soil regions. Though paddy is beneficial for SOC sequestration, the proportions of labile fractions in its SOC are relatively higher, and thus it is vulnerable to loss due to improper agricultural management.