Abstract:To explore the effects of long-term different fertilization treatments on organic carbon fractions and hydrolase activities related to carbon cycle of purple soil under rice wheat rotation in Southwest China, this work was based on the long-term fixed-point experiment of 29-years rice wheat rotation in purple soil of Beibei in Chongqing. Five treatments were applied to determine the contents of soil organic carbon fractions and the activities of hydrolases related to soil carbon cycle including no fertilization (CK), single application of NPK, combined application of organic and inorganic fertilizer (NPKM), straw returning with chlorinated fertilizer ((NPK) ClS), and straw returning with inorganic fertilizer (NPKS).Their correlations were further analyzed accordingly. Results showed that long-term fertilization (NPK, NPKM, and NPKS) significantly increased soil total organic carbon and different fractions of organic carbon content, the total organic carbon content of each fertilization treatment was enhanced by 11.6%~40.2% when compared with CK. Amongst them, NPKS treatment exhibited the most significant improvement effect. Coarse particle organic carbon (cPOC), particle organic carbon within microaggregates (i_POC), silt and clay particles organic carbon within macroaggregates(M_s+c) showed significant positive correlation with total organic carbon content. Notably, cPOC were the most sensitive to the change of soil carbon pool, 0.4 unit of soil organic carbon was stored in cPOC for each unit increase of soil organic carbon. In the three major organic carbon pools of chemical protected, unprotected, and physically protected, the chemical protected organic carbon pool was the main fractions of organic carbon in purple soil, accounting for 66.1% on average. The distribution of soil organic carbon in different carbon pools was affected by fertilization, as compared to CK, the distribution proportion of unprotected and physically protected organic carbon in soil treated with NPKM and NPKS was significantly improved, with an increased rate of 10.1%~24.1%. Moreover, ferlilization significantly increased hydrolase activities related to carbon cycle. The activities of α-glucosidase (AG), β-glucosidase (BG), and β-xylosidase (BDX) in soil treated by NPKM and NPKS were enhanced by 14.5%~89.1% in comparison with CK, and were the best enhanced enzymes. There was a significant positive correlation between hydrolase activities (AG, BG and BDX)and unprotected soil. In conclusion, the combined application of organic and inorganic fertilizer is the best fertilization measure to improve soil organic carbon content and hydrolase activities related to the soil carbon cycle.