Microbial Mechanisms Of Phosphorus Fraction Response To Nitrogen Addition In Grassland Soil Aggregates From Loess Hilly Region

The low phosphorus（P）content and low effectiveness of soils in semi-arid zones make P a key limiting factor for grassland growth,while N input due to atmospheric N deposition in the context of global climate change exacerbates P limitation and impacts soil P conversion processes.Microorganisms are the main drivers of soil phosphorus cycling,but the processes of phosphorus fraction transformation in response to N addition at the soil aggregate level in semi-arid grasslands and their microbial mechanisms are still unclear,limiting the in-depth understanding of nutrient cycling in grasslands.In this study,the grassland in the Loess Hilly Region was selected as the research object,a three years of field nitrogen addition control experiments at different levels(0,10,20,40,80 kg N ha^-1yr^-1)was conducted to determine the characteristics of phosphorus components in soil aggregates and pho D gene phosphorus transformation functional microorganisms,and to measure the basic physical and chemical properties of soil as well as the activities of key enzymes for phosphorus transformation.We analyzed the response patterns of phosphorus fraction in different soil aggregates to N addition,explored the main ways of functional microbial communities in different soil aggregates affecting phosphorus fraction after N addition,and finally revealed the microbial mechanisms of N addition affecting phosphorus fraction in grassland soil aggregates.The main results of the study are as follows:（1）Nitrogen addition changed the phosphorus content of soil aggregates and their fractions,promoting the activity of phosphorus in macro and medium aggregates,while increasing the stability of phosphorus in micro aggregates.Grassland soil phosphorus was dominated by low active phosphorus（Water-soluble inorganic phosphorus,sodium bicarbonate extracted state inorganic phosphorus and organic phosphorus）fractions,accounting for 86.47%-89.14%of the total phosphorus content;while moderately active（Sodium hydroxide extraction state inorganic and organic phosphorus）and highly active（Hydrochloric acid extraction state inorganic phosphorus and organic phosphorus,residual state phosphorus）phosphorus fractions accounted for only 5.55%-7.01%and 4.81%-6.94%,respectively.In addition,N addition improved soil aggregate stability,reduced soil pH,and significantly altered soil carbon（C）and nitrogen（N）contents and nutrient stoichiometry characteristics.（2）Nitrogen addition significantly changed the diversity,community composition and co-occurrence network characteristics of phosphorus transforming functional microorganisms in soil aggregates,and the differences were observed in soil aggregates of different grain sizes.In general,the Chao1 index and ACE index of functional microorganisms showed a trend of decreasing and then increasing with the increase of nitrogen addition concentration.Moreover,there were significant inter group differences in functional microbial communities under nitrogen addition treatment.The soil phosphorus transforming microbial communities were dominated by the phylum Actinobacteria,Proteobacteria and Planctomycetes under different treatments,and their abundance accounted for more than 70%;nitrogen addition caused a significant difference between groups in the microbial communities of Actinobacteria,Firmicutes,Gemmatimonadetes,and N addition caused significant changes in the abundance of microbial phylum such as Actinobacteria,Firmicutes,Gemmatimonadetes,Deinococcus-Thermus and Cyanobacteria,while the difference in agglomerate particle size significantly changed the abundance of microbial phylum such as Actinobacteria and Planctomycetes.Cooccurrence network analysis of phosphorus transforming functional microorganisms showed that the number of microbial network modules for macro,medium,and micro aggregates was 9,4,and 2,respectively,and the number of network connections,mean degree,clustering coefficient,and degree centrality increased with decreasing particle size.（5）The pathways through which N addition drives changes in different active phosphorus fractions of the soil and their microbial mechanisms differ significantly among particle-size aggregates.In micro aggregates,N addition mainly caused changes in nutrients and pH to affect the soil pho D microbial community,which in turn regulated the highly and low active phosphorus fractions;in medium aggregates,it mainly affected the moderately active phosphorus fractions by changing microbial biomass and pho D microbial community;while in macro aggregates,N addition mainly changed the pho D microbial community through pH and nutrients,which in turn regulated the medium and low active phosphorus fractions.The relationships between soil phosphorus fraction,phosphorus conversion functional microbial community,microbial load,key enzyme activities and nutrients after N addition were significantly different in different grain size soil aggregates.In summary,this study revealed the changes in phosphorus fractions of grassland soil aggregates in loess hills after N addition,and revealed the microbial mechanisms by which N addition affects phosphorus fractions of grassland soil aggregates,highlighting the large differences in phosphorus fractions changes and microbial processes in different grain size aggregates.The results of the study can help to further understand the laws of nitrogen deposition on nutrient cycling processes in semi-arid grasslands,and also contribute to sustainable management of grassland ecosystems.