Abstract:An indoor soil column simulation experiment was conducted to examine the effect of adding straw and biochar on the nitrogen leaching loss of Hainan tropical paddy soil under different moisture conditions. Four treatments were set up: control (CK), addition of biochar (B), biochar + rice straw (BCS), and rice straw (CS) at two moisture condition: 75% field water holding capacity (WHC, simulated dry farming soil) and flooding conditions (simulated paddy field). The results showed that both biochar and straw addition could increase soil pH, soil organic matter, total nitrogen, available potassium and available phosphorus contents. Under 75% WHC conditions, compared with CK, BCS and CS treatments significantly increased soil NH4+—N leaching by 16.30% and 48.56% respectively, and no significant difference was found in B treatment. The treatment of CS increased soil NO3-—N leaching compared with CK, and BCS treatment reduced soil NO3-—N leaching, while there was no significant difference in B treatment. BCS and CS treatments significantly increased soil nitrate-ammonium nitrogen (S) leaching compared with CK, while there was on significant effect in B treatment. Under flooding conditions, compared with CK, the leaching loss of NH4+—N and S in the soil was reduced by 16.30% and 12.81%, respectively, but had no significant effect on the NO3-—N leaching loss in B treatment. CS treatment significantly decreased the leaching loss of soil NH4+—N, NO3-—N and S by 19.26%, 33.96% and 22.37%, respectively. BCS treatment reduced the leaching loss of soil NH4+—N and S by 14.52% and 14.19%, respectively, but had no influence of NO3-—N leaching. In summary, returning straw to the field under dry farming conditions would increase soil leaching loss of NH4+—N and NO3-—N, but biochar had no effect on nitrate ammonium leaching under the rice-vegetable rotation planting model in tropical areas of Hainan. In the paddy field, biochar could reduce the leaching loss of NH4+—N, but had no effect on NO3-—N, while the straw could decrease the leaching loss of NH4+—N and NO3-—N.