Effects Of Straw Returning Methods On Ammonia-oxidizing Microbial Communities And Their Functions In Red Soil

Red soil is widely distributed in China and is the main soil type in Southeast China.Due to its sufficient water and heat,long illumination time,good climate conditions and uniform distribution,it is suitable for the growth of subtropical economic crops.However,due to the acidity and heavy clay of red soil,the phenomenon of desilication and iron-rich aluminization is serious,which leads to the lack of soil nutrients.Excessive application of nitrogen fertilizer in pursuit of high yield in farmland aggravates soil acidification,reduces the number of microorganisms,has poor activity of nitrogen transformation function,and has low plant nutrient utilization rate,which limits the potential productivity of red soil and brings burden to the environment.Straw,as the source of soil nutrients and organic matter,decomposes and releases nutrients to maintain soil and improve crop yield.Therefore,straw returning is an effective measure to improve soil quality,crop quality and nutrient utilization.Nitrogen(N)is the key limiting factor of plant growth.The mechanism of microbial transformation of nitrogen is the key goal of studying nitrogen loss.Nitrogen loss is the rate limiting step in the process of nitrification,which is carried out by bacteria ammonia oxidizing archaea(AOA)and ammonia oxidizing bacteria(AOB).At the same time,the community structure of ammonia oxidizing microorganisms also has a certain impact on crop growth and fertilizer utilization.Therefore,it is of great significance to explore the action mechanism of ammonia oxidizing microorganisms under different returning conditions and analyze the potential ways to promote crop growth and nitrogen utilization,so as to guide the rational fertilization in red soil area and improve crop yield and nitrogen utilization efficiency.Taking the typical red soil in South China as the research object,five fertilization systems were determined through long-term equal carbon straw returning experiment:CK(no fertilization),N(conventional NPK),NS(conventional NPK+straw returning),NSM(conventional NPK+straw returning),NSM(conventional NPK+straw/pig manure)and NB(conventional NPK+biochar),combined with High-throughput sequencing and Quantitative PCR,the effects of different straw returning treatments on the physical and chemical properties of red soil were studied,and the community structure characteristics such as the richness and diversity of ammonia oxidizing microbial community under different straw returning treatments were analyzed.The main results are as follows:1.Through the analysis of soil physical and chemical properties,it was found that long-term application of different forms of straw significantly increased the fertility of red soil.The contents of total nitrogen(TN),total phosphorus(TP)and available phosphorus(AP)in NSM treatment were the highest,which were 2.14,1.44 and 3.77 times of those in N treatment,respectively.The contents of soil organic matter(SOM)and ammonium nitrogen(NH₄⁺-N)were the highest under NB treatment,which were 1.74and 1.35 times of those under N treatment,respectively.The content of nitrate nitrogen(NO₃^--N)was significantly increased,which was 1.53 times that of N treatment.2.Quantitative PCR analysis showed that the abundance of AOA was higher than that of AOB under acidic conditions.Compared with N treatment,the three straw returning methods significantly increased the abundance of AOA gene and AOB gene.The abundance of AOA in NSM treatment had the highest abundance(9.08*10⁶ copies/g dry soil),the abundance of AOB in NB treatment was the highest(3.26*10⁴ copies/g dry soil),which was 12.83 and 3.48 times of that in N treatment,respectively.Different straw returning treatments had no significant effect on AOA diversity,but significantly increased AOB diversity.Compared with N treatment,NSM treatment increased Shannon index by 1.61 times and Chao1 index by 3.39 times.The principal co-ordinates analysis(PCo A)showed that the community structure of AOA and AOB under different straw returning treatments also changed significantly,and the community structure of AOA and AOB was obviously separated along the PC1 axis.Nitrossotalea Cluster 1.1 was the dominant group in AOA community,and the relative abundance of Nitrososphaera Cluster 9 increased significantly under NB treatment,which was 3.31 times that of N treatment.Nitrosospira Cluster 3a was the dominant group of AOB community,and the relative abundance of Nitrosospira Cluster 3a was the highest under NSM treatment,which was 1.69 times of that under N treatment.Random forest analysis showed that p H,SOM and TN content were the main predictors of ammonia oxidizing microbial diversity and community structure changes.3.The results showed that straw tillage reduced the positive and negative connection ratio of AOA-AOB network,and the interaction pattern among microorganisms changed to a competitive one.Compared with N treatment,straw returning changed the network structure,phylogenetic distribution and topological structure of the module members.The main factors affecting the network module are p H,AK and NO₃^--N.The?NTI(?-nearest taxa index)values of ammonia-oxidizing microorganisms were calculated,it was found that the?NTI values of AOA and AOB communities decreased gradually in different straw returning treatments,and the proportion of random process in the construction process increased gradually.The community assembly process was affected by a variety of abiotic factors,such as p H,SOM and NH₄⁺-N content.The absolute value of?NTI was negatively correlated with the Bray-Curtis distance,suggesting that the community assembly process might be more dominated by random processes with the increase of the diversity of the ammonia oxidizing microbial community.4.The ammonia oxidation potential(PAO)of AOA and AOB were measured by soil culture method.It was found that the PAO of AOA and AOB increased significantly under the three straw returning methods.Correlation analysis and random forest model showed that the abundance of AOA and AOB functional genes,AP,TN,SOM and p H were the main controlling factors of soil organic matter content PAO.SEM model showed that soil physical and chemical properties(SOM,AP and NO₃^--N)could not only directly affect N use efficiency,but also indirectly affect maize yield and N use efficiency by affecting the abundance of AOA and AOB,network structure and PAO.