The Study On Biological Nitrogen Fixation In Typical Paddy Soils In South China

Rice is the predominant food crop in the world.Nitrogen is one of the limiting factor for rice growth.The nitrogen input into paddy soil includes fertilizer and biological nitrogen fixation.To fully exploit the biological nitrogen fixation is the key measure to reduce much fertilizer application in paddy soil.The study was conducted with three typical paddy soils in south China in a pot experiment.Soil nitrogenase activity,the quantity of nitrogen fixation and 16S r RNA gene,nif H genes in the paddy soils were examined using the methods of acetylene reduction,¹⁵N-labled gas and Illumina Miseq sequencing analysis.The results provide the theory evidence for fertilization application and field management for paddy soils.The main conclusions are as follows:（1）Field moisture capacity and soil bulk density were evidently influenced by the paddy soil types.The gleyed paddy soil had the higher field moisture capacity and soil porosity,while with the lower soil bulk.Clay content,total K,Fe,Cu,Zn contents were relatively even-distributed along the paddy soil profile for most samples.p H increased along the paddy soil profile,but SOM,total nitrogen content,cation exchange capacity and available Fe,Mn,Cu,Zn and B contents decreased along the paddy soil profile.The soil parent material,pedogenesis and field management had much impact on the physicochemical properties.（2）The growth stage had significant influence on nitrogenase activity in paddy soil,with the lower nitrogen fixing rate at early stage and increasing with the growth period extending.The nitrogenase activity was higher in the treatments without nitrogen fertilizer addition than that in nitrogen fertilizer addition.According to the RDA results,total Zn content and integrated fertility index were significantly correlated with nitrogenase activity,and available P and B contents had impact on nitrogenase activity.Temperature also effected nitrogenase activity,especially everage daily minimum temperature.（3）The types of paddy soil had much impact on quantities of nitrogen fixation,with gleyed paddy soil showing the maximum quantity of nitrogen fixation,while hydromorphic alluvial paddy soil showing the minimum quantity.The nitrogen fixation quantity in 0-1 cm soil layer was significantly higher than that in 1-15 cm soil layer,which suggested that 0-1 cm soil layer contributed mostly to nitrogen fixation quantity.According to the RDA results,total Cu content was significantly correlated with the quantity of nitrogen fixation,The total P and total K content had impact on the quantity of nitrogen fixation.（4）The Proteobacteria was the most dominant phylum in soil bacteria and nitrogen-fixing bacteria.The 16S r RNA gene information suggested the amino acid metabolism,carbohydrate metabolism and energy metabolism were the predominant metabolism in the paddy soil.The diversity index and hierarchical clustering trees demonstrated that nif H gene was less conserved than 16S r RNA gene and there was a interrelationship between soil bacteria and nitrogen-fixation bacteria.Total N and Mn content had significant effects on the diversity of soil bacteria and nitrogen-fixing bacteria.Exchangable Ca and Mg contents,clay mineral,CEC,available Fe,Zn and B contents had important impact on the population of soil bacteria and nitrogen-fixing bacteria.