Effects Of Long-term Application Of New Nitrogen Fertilizers On Nitrogen Utilization And NifH Gene Community Of Nitrogen Fixing Bacteria In Maize

The application of new fertilizers can achieve the reduction of nitrogen fertilizer application while meeting the nutrient demand of crop growth,improving the utilization rate of nitrogen fertilizer,and controlling agricultural surface pollution,becoming an important way to maintain the goal of sustainable agricultural development.In this study,three new nitrogen fertilizers,controlled-release fertilizer（SU）,stable nitrogen fertilizer（SF）and sulfur-coated fertilizer（SCU）,were selected for a 8-year long-term locational trial in a northeastern black soil area starting in2014,and no nitrogen fertilizer（N0）was used as a control,compared with conventional urea（CU）,to investigate the effects of long-term application of new nitrogen fertilizers on soil properties,maize growth,nitrogen utilization and nitrogen-fixing bacteria nifH gene community by high-throughput sequencing technology,and to provide a theoretical basis for the development of reasonable new nitrogen fertilizer application measures in a northeastern black soil area.The results are as follows:1.Compared with CU,the new nitrogen fertilizer treatment（SCU,SU,SF）significantly increased the yield of maize by 13.69%～23.41%,and the yield of SF treatment was the highest（12040 kg/hm²）.The new nitrogen fertilizer significantly increased the nitrogen uptake of grain（17.49～28.30 kg/hm²）,and the partial productivity,agronomic efficiency and nitrogen fertilizer utilization index were higher than those of conventional urea.Compared with N0,nitrogen application reduced soil p H（0.12～0.31）,but increased soil AN,NH₄⁺-N,NO₃^--N content and soil urease activity.The nutrient content and urease activity of maize rhizosphere soil were higher than those of non-rhizosphere soil.2.In rhizosphere and non-rhizosphere soils,nitrogen application increased the nifH gene abundance of nitrogen-fixing bacteria in rhizosphere and non-rhizosphere soils.Compared with CU,the copy number of nifH gene of nitrogen-fixing bacteria of the new nitrogen fertilizers showed an increasing trend.NO₃^--N and AN contents in rhizosphere soil were the main influencing factors of nifH gene copy number of nitrogen-fixing bacteria.p H and NO₃^--N content in non-rhizosphere soils were the main factors affecting the copy number of nifH gene in nitrogen-fixing bacteria.In rhizosphere and non-rhizosphere soils,the new nitrogen fertilizer increased the richness index of nifH gene of nitrogen-fixing bacteria,but decreased the diversity index.3.There were differences in the community composition of nifH genes in rhizosphere soil and non-rhizosphere soil nitrogen fixing bacteria,showing a trend of increasing richness from rhizosphere to non-rhizosphere species,which demonstrated the selection strategy of maize roots on soil nifH genes.The main dominant bacteria in rhizosphere soil were Stenitrophomonas（17.69～34.80%）,and the abundance of SU treatment was significantly increased by 16.15%compared with CU treatment.Compared with CU treatment,the abundance of Stenitrophomonas was increased by 18.89%～23.35%,and the SU and SF treatments reached a significant level（P<0.05）.Different fertilization treatments had a certain impact on the enrichment degree of the dominant genus,and the enrichment of the dominant genus was the most significant in the SF treatment.4.According to NMDS and ATOMIS analysis,the community structure of nifH gene in non-rhizosphere soil of maize was significantly affected by nitrogen application rate,but the community structure of nifH gene in rhizosphere soil was not significantly different compared with CU.Redundancy analysis（RDA）showed that soil AN and NO₃^--N content were the key factors affecting the rhizosphere soil community structure,and soil p H and NO₃^--N content were the key factors affecting the non-rhizosphere soil community structure.The results of network co-occurrence showed that the new nitrogen fertilizer had a more stable community structure than CU,and the information transmission between species was more sensitive.The structural equation results showed that the nifH gene diversity（pathway coefficient=1.277）,soil p H（diameter coefficient=0.857）and soil nitrate nitrogen（diameter coefficient=0.483）of soil nitrogen-fixing bacteria had a positive and direct impact on nitrogen fertilizer utilization.The community structure of nifH gene of soil nitrogen-fixing bacteria（pathway coefficient=-0.387）and soil alkaline hydrolyzable nitrogen（pathway coefficient=-0.345）had direct and negative effects on nitrogen fertilizer utilization.Soil p H（pathway coefficient=-0.439）and nitrate nitrogen（pathway coefficient=-0.577）have a direct negative impact on the nifH gene diversity of soil nitrogen-fixing bacteria.Soil alkaline nitrogen hydrolysis（diameter coefficient=-0.575）had direct and negative effects on the community structure of nifH gene application of soil nitrogen-fixing bacteria.Individually and synergistically,they affect N use efficiency.In summary,we found that the long term application of new nitrogen fertilizers improved the nitrogen fertilizer utilization rate and maize yield,changed the community structure and richness of soil nitrogen-fixing bacteria nifH gene,and soil nutrient and plant nitrogen uptake.Under the choice of one-time application which generally accepted by farmers,controlled release fertilizer（SU）,stable fertilizer（SF）and sulfur coating（SCU）can can be used to ensure good results of stable yield increase and relatively less soil residues and environmental pollution.Among the three new N fertilizers,SF treatment had the most significant yield increase,the highest reduction in the proportion of inorganic N residues,and significantly improved nutrient utilization efficiency such as apparent N fertilizer utilization,with high N supply level for the whole fertility period of maize.