Cultivated soybean is a vital source of protein and oil. Soybean obtains nitrogen (N) primarily from two sources: symbiotic nitrogen fixation (SNF) facilitated by microbes like Bradyrhizobium japonicum, and mineral nitrogen from soil or nitrate fertilizers. While nitrate fertilizers enhance plant growth, high nitrate levels inhibit nodule formation and SNF activity, reducing the proportion of nitrogen derived from SNF.
This study investigated the potential of nitrate-reducing bacteria (Bacillus subtilis and Priestia megaterium) to mitigate the nitrate-induced inhibition of SNF when co-inoculated with B. japonicum USDA 110 or USDA 6T strains. Nitrogenase activity, nodulation, root elongation, and plant biomass were evaluated under hydroponic conditions at 1 mM and 10 mM nitrate levels, with nitrate assimilation analyzed using the 15N stable isotope. The growth responses of B. japonicum to varying nitrate concentrations were also examined. The growth of B. japonicum peaked at 7 mM nitrate but declined at 10 mM, suggesting inhibitory effects at higher concentrations.
Nitrogenase activity significantly decreased under high-nitrate conditions. However, co-inoculation with B. subtilis partially alleviated this inhibition, enhancing nitrogenase activity by 15.23% (USDA 110) and 72.62% (USDA 6T) at 10 mM nitrate. Conversely, P. megaterium co-inoculation further reduced nitrogenase activity in some treatments. Nodulation was similarly inhibited by high nitrate levels but improved with the co-inoculation of B. subtilis under both nitrate conditions, while the effects of P. megaterium were strain-dependent. Root length and plant biomass responded positively to co-inoculation with both bacteria, although nitrate-induced reductions were observed in nodule and root N concentrations.
This study highlights the potential of B. subtilis to counteract the nitrate-induced inhibition of SNF and improve soybean growth under nitrogen-rich conditions. The findings provide insights into microbial interactions that could inform sustainable agricultural practices aimed at enhancing legume productivity in nitrogen-rich soils such as those found in intensively managed agricultural systems.
