Effect of Long-term Fertilization on Active Organic Carbon and Nitrogen in Paddy Soils

Soil active organic carbon and nitrogen play important roles in soil carbon and nitrogen cycle as the most active parts of soil organic carbon and nitrogen.Soil microbial biomass carbon(MBC)and microbial biomass nitrogen(MBN),dissolved organic carbon(DOC)and dissolved organic nitrogen(DON)were measured to investigate the effects of long-term fertilization on active soil organic carbon(MBC+DOC) and nitrogen(MBN+DON)from three long-term field experiments(Taoyuan,Ningxiang and Taojiang)in Hunan Province,China.Results showed that chemical fertilizer or combined with organic manure applications increased the contents of soil active organic carbon and nitrogen,as compared to non-fertilizer treatments.The applications of chemical fertilizer increased active soil organic carbon and nitrogen contents by 3.3%～21.0% and 3.3%～27.1%,respectively.Similarly,the applications of organic manure increased active soil organic carbon and nitrogen contents by 48.7%～84.8% and 17.9%～105.8%,respectively.Furthermore positive correlation between the active soil organic carbon and nitrogen contents and organic fertilizer application amounts were found in all the three sites.The contents of active soil organic carbon between three sites followed the track of TaoyuanNingxiangTaojiang.The correlations relationship between active soil organic carbon contents and soil organic carbon accumulation rates was significantly positive(P0.05),while correlation relationship between active soil organic carbon contents and soil organic carbon contents was insignificant.High soil clay contents at Taoyuan site could be the reason why SOC accumulation rates at Taoyuan site were higher than other sites.Although the clay contents and amounts of organic carbon input at Ningxiang and Taojiang sites were similar,the difference of SOC accumulation rates at the two sites was significant.Since high original SOC content at Tao-jiang site was a regulated factor of SOC accumulation rates.Comparable active soil organic nitrogen contents were gained at Taoyuan and Ningxiang sites which were about 1.8 times that in Taoijang site.The significant correlation between active soil organic nitrogen contents and total nitrogen accumulated rates(P0.01)was found in the study.