Selenium enhanced nitrogen accumulation in legumes in soil with rhizobia bacteria

The loss of nitrogen in soil directly leads to ecological pollution such as water eutrophication. Urgent need to develop green and efficient approaches to increase nitrogen absorption in plants benefits to improve soil nitrogen use efficiency. Selenium is one of the beneficial elements of plants and has been proven to promote efficient nitrogen uptake by plants. However, previous studies focused on the effects of selenium on plant physiological functions while ignoring the regulation of selenium on the soil nitrogen cycle. This study aims to investigate the regulatory effect of selenium on legume rhizosphere microecology and soil nitrogen cycle process, and the key to legume biomass and nitrogen accumulation, soil physicochemical factors, microbiome characteristics and soil nitrogen cycle process. The results showed that selenium application increased the biomass (increased by 7.26%∼21.85% compared with CK) and nitrogen accumulation of legumes (increased by 18.29% ∼ 54.09% compared with CK). By 16S rRNA sequencing of rhizosphere soil and roots, we found selenium increased nitrogen-fixing bacteria such as Rhizobiaceae and Frankia and nitrifying bacteria such as Proteobacteria, Firmicutes and Chloroflex in rhizosphere. Moreover, selenium up-regulated soil nitrogen fixation genes (nifH) and nitrification genes (Arch-amoA, nxrA), and down-regulated denitrification genes (narG, nirS). These changes lead to a significant decrease in soil NO3−/NH4+, which was helpful for the uptake and utilization of legumes. When soil inoculated with rhizobia, the nitrogen accumulation in legumes promoted by Se was much higher than soil non-inoculated with rhizobia bacteria. This study explored the feasibility and great potential of selenium to promote nitrogen accumulation in legumes from the perspective of rhizosphere microecology, proposes and proves the approach to reduce the application of large amounts of elements by introducing selenium, which is conducive to reducing the use of nitrogen fertilizer from the source and alleviating nitrogen pollution.