Effects And Mechanism Of Robinia Pseudoacacia Forest Recovery In The Loess Plateau On Dissimilatory Nitrate Reduction Processes Of Root Zone Soil

The availability of soil mineral nitrogen is a key factor affecting the functional stability of aboveground vegetation communities and underground roots in fragile ecosystems,and the dissimilatory nitrate reduction process directly affects the storage and loss of soil mineral nitrogen.Therefore,from the perspective of ecological function stability and sustainable development of Robinia pseudoacacia forest vegetation widely distributed in loess hilly region,this paper takes the root zone soil of Robinia pseudoacacia forest with different restoration years as the research object,field soil samples were collected,and acetylene inhibition method,¹⁵N isotope tracing method,Illumina platform high-throughput sequencing technology and real-time fluorescence quantitative PCR technology were used.The paper quantified the dissimilatory nitrate reduction process rate in abandoned farmland,and explored the evolution characteristics of dissimilatory nitrate reduction microbial community,key environmental driving factors,expression levels and active expression levels of functional genes,and analyzed the quantitative coupling relationship between the rate of dissimilatory nitrate reduction process and the active functional gene group.The main conclusions are as follows:1.With the increase of the restoration years of Robinia pseudoacacia forest,the rate of dissimilatory nitrate reduction to ammonium(DNRA)gradually decreased;Compared with the farmland soil,the denitrification rate increased significantly after 35 years of restoration,indicating that the restoration of Robinia pseudoacacia forest inhibited the DNRA process and promoted the denitrification process.2.With the increase of the restoration years of Robinia pseudoacacia forest,the expression of functional gene nrf A in DNRA process decreased gradually,and the richness and diversity of microbial community decreased significantly,resulting in the decrease of DNRA rate.During the restoration of Robinia pseudoacacia forest,the main species in microbial community are Proteobacteria,Sorangium.The structure of microbial community has changed significantly.Organic carbon(OC)and microbial biomass carbon(MBC)are the key environmental factors affecting DNRA microbial community.The co-occurrence network mode is more complex after 10 years of restoration;With the increase of restoration years,the active expression of functional genes nap A?nar G?nir K?nir S and nos Z in denitrification process increased significantly in the late stage of restoration,and the richness of microbial community increased significantly,which led to the significant increase of denitrification rate.Proteobacteria,Azospirillum and Pseudogulbenkiania are the main species in the microbial community.There was no significant change in the microbial community structure.Organic carbon(OC),total phosphorus(TP),available phosphorus(AP)and ammonium nitrogen(NH₄⁺)were the key environmental factors affecting the microbial community,and the co-occurrence network mode is more complex after 35 years of restoration.3.The comprehensive analysis found that:1)DNRA rate was significantly positive correlation with TP,and negatively correlated with OC and MBC;Denitrification rate was positively correlated with OC and MBC.2)There are many coupling mechanisms between DNRA rate and expression and activity expression of nrf A gene,microbial community richness and diversity index,so DNRA rate is subject to the combined effect of microbial community diversity and functional genes.3)The ratio of gene activity expression nos Z/(nir K+nir S)is the key variable to predict denitrification rate,and the denitrification rate was limited by the combined effect of the abundance of bacteria and the functional genes of nir K,nir S and nos Z.In conclusion,the rate of dissimilatory nitrate reduction,microbial diversity,network stability and evolution characteristics in the root zone soil of Robinia pseudoacacia in loess hilly region were analyzed,the ecological association and quantitative response of dissimilatory nitrate reduction rate to environmental factors,microbial communities and functional genes were clarified.It has a certain scientific value for understanding the balance and fate of soil mineral nitrogen nutrient in Robinia pseudoacacia forest in theory,and can provide a basis for the formulation of soil nitrogen nutrient balance management strategy in practice.