Seed treatment, leaf spraying or soil fertilization? Integration of Ni application methods increases the efficiency of biological N2-fixation, N metabolism and soybean yield

Abstract Background and Aims In soybean production systems, increasing the efficiency of biological nitrogen fixation (BNF) is essential to ensure sustainability, which can be achieved through nutritional management. Fertilization with nickel (Ni) can positively affect BNF due to the role of this micronutrient in nitrogen (N) metabolism – urease and NiFe-hydrogenase. However, studies on Ni application methods are incipient. Our study aimed to investigate which Ni application method is most appropriate to promote soybean plant development, with an emphasis on BNF and N metabolism. Methods The isolated and combined effects of Ni application methods – seed treatment, leaf spraying and/or soil fertilization – were investigated in soybean plants grown under different edaphoclimatic conditions (field and greenhouse). Results Positive effects of the treatments Soil, Soil + Leaf and Seed + Leaf were observed, with gains of 7 to 20% in BNF, 1.5-fold in ureide, 14% in shoot dry weight and yield increases of up to 1,161 kg ha− 1. All Ni application methods increased the SPAD index, photosynthesis, nitrogenase and urease activity. The edaphoclimatic conditions, especially the soil classes, influenced the response of soybean to the Ni application methods. Conclusion The integrated Ni application methods, i.e., leaf spraying combined with soil or seed fertilization, as well as soil application alone, were more suitable to promote productivity in soybean cropping systems. The definition of the method is essential to ensure absorption and ideal use of this micronutrient, representing a viable and sustainable management technology. More studies are needed to elucidate an official recommendation for Ni in agriculture.