Objective This study aims to investigate the effects of nitrogen fertilizer synergists on soil nitrogen transformation and rice photosynthetic physiological characteristics in a rice-loach co-culture system, and provide an efficient nitrogen management strategy for rice-fishery integrated systems.

Methods A pot experiment was conducted under the rice-loach co-culture model with four treatments: A control without synergists (CK), application of a urease inhibitor (N), application of a nitrification inhibitor (M), and the combined application of both inhibitors (N+M). Soil inorganic nitrogen content, activities of key nitrogen transformation enzymes, and rice leaf photosynthetic characteristics were measured at different growth stages.

Results Compared with CK, the nitrogen fertilizer synergist treatments improved rice photosynthesis, with the combined (N+M) treatment showing the best performance. The N+M treatment increased the net photosynthetic rate by 14.51%, 14.69%, and 7.71% at the booting, full heading, and grain filling stages, respectively. The synergist treatments significantly increased soil ammonium nitrogen content while decreasing nitrate nitrogen content, and they also inhibited soil urease and ammonia monooxygenase activities. Correlation analysis revealed that the net photosynthetic rate was significantly and positively correlated with soil ammonium nitrogen content but significantly and negatively correlated with nitrate nitrogen content, urease activity, and ammonia monooxygenase activity.

Conclusion In the rice-loach co-culture system, nitrogen fertilizer synergists can effectively regulate soil nitrogen transformation towards a high-ammonium and low-nitrate state by inhibiting nitrogen-transforming enzyme activities, thereby significantly promoting rice photosynthesis. The combined application of urease and nitrification inhibitors demonstrates the optimal effect.