Changes in Yellow Paddy Soil Organic Carbon Fractions Under Long-Term Fertilization

【Objective】 Soil organic carbon represents an high heterogeneity. Because of different chemistry and existing forms, soil organic carbon fractions show different decomposibility and fertility functions. Therefore, investigating soil organic carbon fractions is important for better understanding the stabilization and fertility mechanims of soil organic carbon. This study was conducted in order to examine the effects of long-term fertilization on yellow paddy soil organic carbon fractions and to guide proper fertilization. 【Method】 An 18-year paddy field experiment was conducted to investigate the effects of long-term fertilization on soil organic carbon fractions and allocation, and also to discuss the soil organic carbon phenomenon by quantifying the relationship between soil carbon levels with annual carbon inputs under different fertilizer treatments. The designed treatments were: no fertilizer control(CK), chemical fertilizer(NPK), organic manure(M), low application rate of manure combined with chemical fertilizer(0.5MNPK) and high application rate of manure combined with chemical fertilizer(MNPK). The average annual carbon inputs ranged from 0.87(in CK treatment) to 6.02 t·hm-2·a-1(in MNPK treatment). 【Result】 The results showed that compared with CK, soil total organic carbon content was significantly increased by 10% under the NPK treatment, and by 24%-46% under the manuring treatments(0.5MNPK/M/MNPK). Compared with CK, soil free coarse particulate carbon, chemically-protected silt carbon and biochemically-protected silt carbon contents were significantly increased under the NPK treatment, and soil free coarse particulate carbon, physically-protected carbon, chemically-protected silt and clay carbon and biochemically-protected silt and clay carbon contents were significantly increased under the manuring treatments(0.5MNPK/M/MNPK). The proportion of physically-protected carbon to total carbon was higher under the treatments of 0.5MNPK, M and MNPK than under the CK and NPK trteatments. Soil free coarse particulate carbon concentration showed a significant linear increase with mean annual carbon input. Soil chemically-protected and biochemically-protected carbon concentrations and soil total carbon content showed a significant saturation curve increase with mean annual carbon input. 【Conclusion】 The data from the experiment indicate that the combined manure and chemical fertilizer is the best option for increasing the paddy soil organic carbon content, and that the relatively passive soil organic carbon fractions(chemically-protected, biochemically- protected) and total organic carbon existed phenomenon and limit under current conditions.