The Impact of Bio-fertilizer Substitution on Nitrogen and Phosphorus Loss and Crop Yield in Farmland of the Danjiangkou Water Source Area

To safeguard the water quality of the Danjiangkou Reservoir—the water source for the Middle Route of the South-to-North Water Diversion Project—and explore source control technologies for agricultural non-point source pollution, this study selected typical crops (kumquat, corn, and pepper) in the surrounding Hujiashan small watershed. Through comparative field runoff plot experiments, the effects of partially replacing chemical fertilizers (20% reduction) with bio-fertilizers on nitrogen (N) and phosphorus (P) runoff losses and crop yields were evaluated. The results showed that: (1) Bio-fertilizer treatment effectively reduced the load of N and P runoff losses. The reduction rates for total nitrogen (TN), ammonia nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃⁻-N), and total phosphorus (TP) were 2.6%~16.4%, 4.5%~22.0%, 5.5%~46.7%, and 4.9%~10.6%, respectively. The reduction effect on NO₃⁻-N was the most significant, and NO₃⁻-N was the primary form of N loss (accounting for 33.1%~66.1% of total N loss). (2) The N and P reduction effects of bio-fertilizers varied among different crops. Corn demonstrated the best overall performance in reducing TN, NH₄⁺-N, and TP, while pepper showed the greatest potential for reducing NO₃⁻-N (with a reduction rate of 46.7%). (3) The impact of bio-fertilizers on yield was crop-specific. Pepper yield increased by 6.64%, whereas corn yield decreased by 8.42%. In conclusion, substituting with bio-fertilizers can effectively reduce the risk of N and P losses from farmland in the water source area, but the substitution ratio should be optimized according to the nutritional characteristics of the crops to achieve the synergistic goals of pollution reduction and yield increase.