Quantitative Determination of Nitrogen Fixed by Soybean and Its Uptake by Winter Wheat as Aftercrops Within Sustainable Agricultural Systems

The future of agricultural production involves sustainable production systems with a balance between nutrients in soil–plant systems. These production systems are based on limiting the use of mineral fertilizers while introducing natural sources that increase soil fertility. The best example of such a system is plant rotation, including legumes as a forecrop for cereal plants. For this reason, the goal of the present study was to determine the possibility of obtaining nitrogen from the air using 15N isotopes and to determine the quantity of nitrogen biologically fixed and taken up by winter wheat cultivated as a succeeding plant. In field experiments, we investigated the cycle of nitrogen fixed by legume plants in rotation under sustainable conditions, as follows: soybean–winter wheat–winter wheat. After soybean seedling emergence, a mineral fertilizer (15NH4)2SO4 containing 20.1 at% 15N (a dose of 30 kg∙ha−1) was applied, with summer wheat as a reference plant. The yield of soybean reached 2.48 t∙ha−1 for seeds and 8.73 t∙ha−1 for crop residue (CR), providing a total yield of 11.21 t∙ha−1. The total biomass of soybean contained 149.1 kg∙ha−1 of total nitrogen, with 108.1 kg∙ha−1 in the seeds and 41.0 kg∙ha−1 in the residue, of which 34.0 kg∙ha−1 in the seeds and 11.4 kg∙ha−1 in the residue was biologically fixed. CR was ploughed into the soil. Plots with winter wheat cultivated after soybean (2017) were divided into two sub-plots for the application of 0 and 100 kg∙ha−1 of mineral N. The scheme was repeated in 2018. Overall, winter wheat cultivated for two subsequent years took up 8.12 kg∙ha−1 of the total nitrogen from the CR from the control sub-plot and 15.51 kg∙ha−1 from the fertilized sub-plot, of which 2.61 and 2.98 kg∙ha−1 was biologically fixed by soybean plants, respectively. The dose of fertilizer contained 5.920 kg∙ha−1 of 15N, of which 3.024 kg∙ha−1 was accumulated in soybean. In wheat cultivated as the first subsequent crop, the accumulation of 15N was as follows: 0 kg N (control)—0.088 kg∙ha−1; 100 kg N—0.158 kg∙ha−1. Meanwhile, in winter wheat cultivated as the second aftercrop, 0.052 and 0.163 kg∙ha−1 of 15N was accumulated, respectively. This study demonstrates that biological nitrogen fixation in soybeans is an underappreciated solution for enhancing crop productivity within sustainable agricultural systems. It holds significant implications for planning rational fertilizer management, reducing the application of chemical fertilizers, and improving nitrogen use efficiency within crop rotation systems.