Long-term nitrogen and phosphorus fertilization improved crop yield by influencing rhizosphere nitrogen transformation processes

Soil nitrogen (N) transformation plays a crucial role in enhancing farmland productivity. However, the impacts of long-term N and phosphorus (P) fertilization on soil N transformation and crop yield in farmland remain unclear. This study investigated the mechanisms by which crop root exudates, microbial N function genes, and soil N transformation characteristics influenced crop yield under different N and P fertilization regimes over 26 years. The results revealed that long-term N and P fertilization significantly increased millet root exudates and soil nutrient contents. Specifically, dicarboxylic acid exudates, total N, and ammonium N prominently affected the composition of microbial N function genes. Moreover, N and P fertilization markedly increased the abundance of genes responsible for soil N fixation and nitrification. The abundance of soil nitrification (amoA1, amoA2, and nxrA) and ammonification (ureC) functional genes substantially influenced soil nitrification and N mineralization rates. Enhanced soil N transformation rates facilitated N uptake of millet, and crop yield increased with the increasing of soil N transformation rates and nitrification genes abundance. Essentially, long-term N and P fertilization increased crop yield mainly by enhancing the root organic acid exudates, increasing the abundance of functional genes such as amoA2, nxrA, and ureC, and elevating soil available N content. This study emphasizes the importance of the rhizosphere N transformation process for the sustainable agricultural development of the Loess Plateau region.