Abstract:Phosphorus (P) loss from farmland is one of the important sources of agricultural non-point source pollution, and colloidal P (Pcoll) is an important form due to its large specific surface area, strong adsorption and migration capacity. In order to evaluate the activity and loss potential of Pcoll in paddy soil under different organic substitution methods, this study relied on a long-term experiment of rice-wheat rotation under two partial substitution methods of organic fertilizers (N and P fertilizer application control and N fertilizer application control only) to explore the changes in soil Pcoll content and its loss risk, as well as its relationship with different P components and soil chemical properties. The treatments of N and P fertilizer application control (4 a) included no P fertilizer (CK), chemical fertilizer (CF), and pig manure instead of 30% chemical P fertilizer (OF); the treatments of control N fertilizer application only (24 a) included no fertilizer (CK), chemical fertilizer (CF), and pig manure instead of 40% chemical N fertilizer (OF). The results showed that, for the fertilization of N and P control, there were no significant differences in soil organic carbon (SOC), Pcoll content and colloidal phosphorus loss potential (LPP) between the organic substitution treatment and the chemical fertilizer treatment. The proportion of soil MRPcoll (RMRP, MRPcoll/TPcoll) was significantly (P<0.05) reduced by 1.76% than that of the chemical fertilizer treatment. For the fertilization of N control only, soil Pcoll and SOC content of organic substitution treatment were 13.08 mg/kg and 20.19 g/kg, respectively, which were significantly (P<0.05) increased by 78% and 212.6% compared with the application of chemical fertilizer alone; soil LPP and RMRP were significantly (P<0.05) increased by 1.89% and 16.05%, respectively. Soil Pcoll was positively (P<0.01) correlated with soil TP, TSP, Olsen-P and CaCl2-P under the two organic substitution methods. Soil Pcoll had no significant correlation with SOC under the fertilization of N and P control, however showed a significant positive correlation (P<0.01) under the fertilization of N control only. Compared with the treatments of N and P fertilizer application control, the long-term application of organic fertilizer with only N control significantly increased the content of SOC, Pcoll and different active P components in the soil, thus may induce the risk of phosphorus loss. Therefore, equaling nutrient application is a feasible organic substitution fertilization to reduce the P loss in farmland.