Effect Of Straw Addition On Soil Biological N₂ Fixation Rate And Diazotroph Community Characteristics

Diazotrophs provide 24% of nitrogen input for crop production,which is one of the important sources of nitrogen input for agricultural ecosystems.However,the diazotroph community and its role in nitrogen fixation are affected by a variety of soil nutrients,environmental factors,and farmland management measures.Most previous studies used acetylene reduction methods to indirectly estimate the rate of biological N₂ fixation,and lack direct quantification of the rate of N₂ fixation.The response of N₂ fixation rate and diazotroph community to the straw addition amount and the mechanism is still unclear.At the same time,the effects of straw on soil nitrogen-fixing rate and characteristics of diazotroph community in nationwide soils and soils with different long-term fertilizations were scarily investigated.Therefore,we firstly used the ¹⁵N₂ incorporation method and high-throughput sequencing method to explore the effects of different straw additions on the rate of N₂ fixation and the characteristics of diazotroph communities to identify the most suitable amount of straw addition;then,we studied the influences of straw addition on the rate of N₂ fixation and the characteristics of the diazotroph community in soils with different fertilization measures and 40 farmland soils across China.The main research results obtained are as follows:(1)The effect of straw addition on N₂ fixation rate,diazotroph diversity,diazotroph community structure is mainly regulated by carbon availability.Under ideal conditions(25?,60%maximum field capacity),the N₂ fixation rate without straw(C0)is about 63 kg N·hm^-2·a^-1,and the addition of 4.0 mg·g^-1 straw can significantly increase the soil biological N₂ fixation rate by 26 kg N·hm^-2·a^-1.The diversity of diazotrophs reached the peak when the straw was added at 2.0 mg·g^-1and then they declined when the straw addition increased further.The relative abundance of some dominant functional diazotrophs that are significantly positively correlated with the rate of N₂ fixation decreased significantly when the amount of straw added reaches 4.0 mg·g^-1.The community structure of diazotrophs and the relative abundance of dominant functional diazotrophs are closely related to the rate of N₂ fixation and the content of NO₃^--N.(2)The long-term fertilization treatment changed the physical and chemical properties of the soil,the diversity of diazotrophs,and the community structure of diazotrophs.Compared with the treatment without fertilizer applied,the soil biological N₂ fixation rate was significantly increased by Straw+NPK(SNPK)by 79.31%,which was about 16.27 kg N·hm^-2·a^-1.After straw addition,NO₃^--N decreased significantly,and the N₂ fixation rate increased significantly in No Fertilization(NF)and Manure+NPK(MNPK)treatments,which was about 18.46—27.22 kg N·hm^-2·a^-1.The addition of straw significantly increased the diversity of diazotrophs in SNPK and MNPK treatments.Burkholderia,Desulfovibrionaceae,Azospirillum,Amorphomonas,Paenibacillus,etc.,which are positively correlated with biological N₂ fixation,all tended to increase their relative abundance after adding straw,thereby promoting biological N₂ fixation.(3)Generally,straw addition tended to increase the N₂ fixation rate of the 40 soils collected across China.The biological N₂ fixation rate of the treatment without straw addition is about 0.09—16.46 kg N·hm^-2·a^-1,and the rate of the biological N₂ fixation rate of the treatment with straw addition is about0.53—25.37 kg N·hm^-2·a^-1.After adding straw,the soil respiration rate was significantly increased,and NO₃^--N was significantly reduced.The abundance and community of diazotrophs and N₂ fixation rate in different areas without straw addition are mainly affected by climate,soil texture(silt,sand),p H,C/N,NH₄⁺-N,and available Fe;after straw addition,diazotrophs abundance and N₂ fixation rate are mainly affected by soil texture(silt,sand)and soil properties(p H,TC,TN,and C/N).The community structure of diazotrophs in different regions is divided into different groups due to latitude and whether straw is added or not,and is also affected by soil p H,TC,C/N,texture,and other factors.Bradyrhizobium was abundant in most areas.Paenibacillus,Desulfovibrionaceae,and Rhizobium were abundant in most of the soils.After adding straw in some areas,the relative abundance of Rhizobium,Rhodobacter,and Azospirillum was significantly increased.On a large scale,spatial distance,climate,and soil texture have a major impact on the rate of biological N₂ fixation by affecting soil properties,abundance,and structure of diazotrophs communities.After the straw was added,the abundant organic carbon sources stimulated diazotrophs to use NO₃^--N,changed the community structure of diazotrophs,and promoted the biological N₂ fixation of Rhizobium,Azospirillum,and Rhodobacter.In summary,this study shows that the addition of an appropriate amount of straw could stimulate diazotrophs to increase the use of NO₃^--N,and significantly increased the diversity and activity of some diazotrophs,thereby promoting biological N₂ fixation.However,the rate of N₂ fixation,the diversity and the structure of communities were affected by various environmental factors such as spatial distance,climate,soil texture,soil properties,and the quantity and quality of organic carbon sources.This result provides a scientific basis for the utilization of straw resources and the improvement of biological N₂fixation in China farmland.