The mechanism by which nitrate inhibits nitrogen fixation in soybean (Glycine max L.) is not fully understood. Accumulation of ureide in soybean plant tissues may regulate the nitrogen fixation capacity through a feedback pathway. In this study, unilaterally nodulated dual-root soybeans prepared by grafting were grown in sand culture. They were subjected to the removal of the nodulated side roots, and were given either nitrate supply or no supply to the non-nodulated side roots for 3 days (experiment I). Additionally, they received nitrate supply to the non-nodulated side roots for 1–14 days (experiment II). The results showed that nitrate supply increased the levels of asparagine and ureide in soybean shoots (Experiment I). In Experiment II, nodule dry weight, nodule number, nodule nitrogenase activity, and nodule urate oxidase activity decreased significantly after 3, 7, and 14 days of nitrate supply. Ureide content in the shoots and nodules increased after 1, 3, and 7 days of nitrate supply, but decreased after 14 days of nitrate supply. There was a significant positive correlation between urate oxidase activity and nitrogenase activity. Hence, we deduced that nitrate supply increased the asparagine content in soybean shoots, likely inhibiting ureide degradation, which induced the accumulation of ureide in soybean shoots and nodules, and, in turn, feedback inhibited the nodule nitrogen fixation. In addition, urate oxidase activity can be used to assess the nitrogen fixation capacity of nodules.

Soybean (Glycine max) nitrogen fixation is inhibited by nitrate, which has been linked to a reduction in carbon allocation and metabolism within nodules. However, the underlying mechanisms remain unclear. In this study, we tested the hypothesis that the nitrate-induced suppression of nitrogen fixation is mediated through altered sucrose allocation and catabolism in nodules. Using unilaterally nodulated dual-root soybean plants in sand-based systems, we applied 200 mg·L⁻¹ nitrate exclusively to the non-nodulated roots for 14 days. Nitrate supply enhanced the proportion of dry weight in leaves but reduced it in nodules at 3, 7, and 14 days. Similarly, nodule dry weight, nodule number, acetylene reduction activity (ARA), and specific nodule activity (SNA) all declined significantly during the same intervals. Notably, sucrose content in the nodules decreased significantly by 20.4% after 3 days but recovered at 7 and 14 days. In contrast, sucrose synthase (SuSy) cleavage activity and malate content in nodules decreased significantly following nitrate treatment, with reductions of 27.8% and 30.7% observed at 7 days, and further decreased to 38.5% and 39.2% at 14 days, respectively. These results suggest that transient sucrose scarcity may drive the initial decline in nitrogen fixation capacity, while restricted sucrose catabolism and decreased malate levels may be a consequence rather than a cause.

To elucidate the regulatory mechanisms underlying the interaction between straw return and nitrogen (N) fertilization on yield formation, nutrient uptake, and soil N cycling in a continuous maize cropping system, a two-year positioning experiment was conducted. The study established two straw treatments (S0: 0 g/box; S1: 84 g/box) combined with three N levels (N0: 0 g/box; N1: 1.24 g/box; N2: 2.47 g/box). (The box refers to the cylinder used for planting maize.) The responses of maize yield, plant nutrient accumulation and partitioning, fertilizer-derived N ratio, nitrogen fertilizer use efficiency (NUE), and soil microenvironment were analyzed. Results indicated that under N1 conditions, straw return had a negligible effect on crop growth and yield formation. Conversely, under N2 conditions, straw return significantly enhanced maize yield and promoted the accumulation of N, phosphorus (P), and potassium (K) in plant tissues. ¹⁵N isotope tracing revealed a novel mechanism: rather than significantly altering direct fertilizer nitrogen use efficiency, straw return improved crop yield primarily by elevating indigenous soil N content and boosting the activities of N-transforming enzymes, thereby beneficially altering the ultimate environmental fate of the fertilizer N. Furthermore, straw return significantly boosted the activities of enzymes involved in N transformation and optimized the soil microenvironment. Collectively, straw return coupled with increased N application (specifically the S1N2 treatment) significantly maximizes maize yield, providing a theoretical basis for rational straw utilization and N management.