Selenium Combined With Rhizobia Enhanced Nitrogen Accumulation In Leguminous Plants

Soil inorganic nitrogen loss（leaching,runoff,volatilization,etc.）is one of the main causes of agricultural non-point source pollution,such as soil acidification,water eutrophication and greenhouse gas emission.To improve plants to absorb nitrogen in improving soil nitrogen use efficiency,maintain agricultural ecological security and agricultural sustainable development is of great significance.Se is an essential trace element for human body and a beneficial element for plants.Adding proper concentration of Se can improve the physiological characteristics of crops and promote plant growth.This study through two pot experiment validation of leguminous plant biomass and n accumulation amount of Se,and the soil biochemical process of Se regulation,microbial proteomics and soil nitrogen cycle related process in comprehensive analysis of Se to delve into leguminous plant nitrogen efficient absorption and the effect of selenium on soil nitrogen cycle.The purpose of this study was to provide reference for efficient use of nitrogen fertilizer in legume crops and promote the application of Se in biofortification of legume crops.The main results obtained are as follows:（1）Se increased biomass and nitrogen accumulation of leguminous and non-leguminous plants.Under different Se concentrations（0.3 and 1 mg/kg Na₂Se O₃）,fresh weight of shoot and root parts increased by 11.35%～22.92%,7.12%～17.08%,and plant nitrogen accumulation increased by 17.63%～45.37%.The fresh weight of shoot of non-leguminous ryegrass increased by 7.12%～12.60%,while the fresh weight of root did not change significantly,and the nitrogen accumulation of plant increased by3.32%～14.24%.Se promotes legumes better than gramineae.（2）Se combined with Rhizobia promoted the biomass and nitrogen accumulation of leguminous plants in soil with different nitrogen levels.Under the condition of non-inoculated with Rhizobia,0.3 mg/kg Se had the best effect on promoting plant biomass and nitrogen accumulation,while after inoculated with Rhizobia,1 mg/kg Se had the best effect.The biomass of leguminous plants could be significantly increased by applying a small amount of nitrogen in the soil,while the application of Se had a more significant growth promoting effect on leguminous plants,and the combined application of 1 mg/kg Se and Rhizobia had the best effect.Se and Rhizobia in the regulation of the soil urease,the activity of nitrate reductase,improved the soil ammonium nitrate.Under different Se concentrations（0.3 and 1 mg/kg Na₂Se O₃）,the content of ammonium nitrogen in low nitrogen soil decreased by 18.66%to 31.07%,and the content of nitrate nitrogen increased by 52.26%to 68.04%in nitrogen application and Rhizobia inoculation.In low nitrogen soil,the nitrate reductase activity was decreased by 13.86%by inoculated Rhizobia,while Se had no significant effect on soil urease and nitrate reductase activity.In nitrogen application soil,the nitrate reductase activity was decreased by 31.35%-35.24%by inoculated Rhizobia.（3）Se combined with Rhizobia regulated the community structure of rhizosphere soil and root endophytic bacteria,and improved the nitrogen cycling process of soil.Se combined with Rhizobia enriched microorganisms related to nitrogen cycling in the rhizosphere,such as Rhizobiaceae,Frankia,etc.Ammonia oxidation process:Proteobacteria,etc.Nitrate assimilation process:Firmicutes et al.Nitrification process:Chloroflexi,etc.Some beneficial microorganisms such as Paraburkholderia,Lactobacillus,Bacillus and Thiobacillus were also enriched in the rhizosphere,and the relative abundance of some pathogenic bacteria such as Ralstonia was decreased.Se and Rhizobia treatment improved the modularization level of rhizosphere soil microbial network.In the symbiotic network diagram of endophytic bacteria in plant roots,Se also significantly increased the diversity of bacterial community and made the plant roots have stronger resistance to stress.In soil with different nitrogen levels,Se and Rhizobia regulated the main functional genes in soil nitrogen cycle.In soil with different nitrogen levels,Se and Rhizobia could improve the relative abundance of nif H gene in soil,and promote the soil nitrogen fixation process.In the nitrification process,the combination of Se and Rhizobia increased the relative abundance of Arch-amo A and nxr A genes in the nitrogen application soil,which was beneficial to the transformation of NH₄⁺to NO₃^-in the soil.In the soil without nitrogen application,Se and Rhizobia increased the relative abundance of Bact-amo A and nxr A genes,which also promoted the nitrification process in the soil.At the same time,Se and Rhizobia combined significantly reduced the relative abundance of nar G gene in soil,reduced the soil denitrification process,which can effectively prevent the loss of N₂O and NO.In conclusion,Se and Rhizobia regulate soil nitrogen cycling through rhizosphere microorganisms and thus affect the growth and nitrogen accumulation of leguminous plants.The results of this study can be used as a relatively green biofortification method for leguminous plants,and the addition of a small amount of Se and Rhizobia can promote the growth of leguminous plants and reduce non-point source pollution of agricultural nitrogen fertilizer.