Returned straw reduces nitrogen runoff loss by influencing nitrification process through modulating soil C:N of different paddy systems

Nitrogen (N) runoff loss from paddy fields contributes significantly to non-point source pollution. Straw return to soil may affect N runoff loss by changing soil biochemistry. In situ N runoff monitoring combined with an analysis of soil biochemical indicators was implemented to explore the effect of straw return lasting for five or six years on N runoff loss and its potential mechanism in five rice rotation systems. The results showed that straw return reduced total N (TN) runoff losses by 2.29%− 26.10% through physical and biochemical pathways, and the reduction in inorganic N (IN) was the largest (53.56%–82.42%) through biochemistry. This was mainly achieved by the increase in soil C:N due to straw return, thereby increasing the immobilization of microbial N and reducing the soil IN. At the same time, the abundance of functional genes (AOA, AOB, nxrA) participating in the nitrification process was also conducive to the decrease in TN runoff concentration. More importantly, the reduction in IN and functional genes of the nitrification process did not affect N uptake by rice, and the increase in the atmospheric N fixation gene (nifH) was also beneficial to N supplementation in soil. However, the positive effects of straw return under the five rice rotation systems were different, and the TN loss reduction in double-cropping rice was the lowest. With the lower soil C:N in the single-cropping and paddy-upland systems, the increase in soil C:N after straw was more effective, and had greater potential to reduce TN runoff loss. This study provides a new perspective on N cycling in soil biochemistry by straw return, so as to select optimal measures to protect the water environment in different rice rotation systems.