Phosphorus is one of the limiting nutrients for plant growth in terrestrial ecosystems.Nitrogen deposition can change the activity and bioavailability of phosphorus by regulating soil phosphorus cycle.However,the under-standing of changes in soil phosphorus fractions and their influencing factors under nitrogen deposition is still limit-ed.To reveal the effects of nitrogen addition on soil phosphorus fractions and its regulation pathways,we measured soil phosphorus fractions,soil physicochemical properties,soil microbial biomass,and phosphorus invertase activity in a three-year field experiment with different nitrogen addition levels in a loess hilly region.The results showed that the contents of moderately and highly labile phosphorus increased significantly with increasing nitrogen addition lev-els,while the contents of non-labile phosphorus decreased significantly.Organic phosphorus content significantly in-creased by 1.71％-5.14％,but inorganic phosphorus content significantly decreased.Nitrogen addition reduced soil pH.High nitrogen addition level promoted the accumulation of soil C,N,and P,and increased soil microbial bio-mass C,N,and P by 18.11％,38.27％,and 41.73％,respectively.With increasing nitrogen addition levels,the activities of soil alkaline phosphatase,phosphodiesterase and phytase decreased first and then increased.Soil nutri-ents,pH,microbial biomass,and enzyme activities explained 68.64％of the variation of soil phosphorus fractions.Structural equation model analysis showed that nitrogen addition induced changes in soil microbial metabolizing sub-strate nutrients,which had significant effects on microbial biomass and the activities of phosphorus conversion extra-cellular enzymes,and co-regulated soil phosphorus fractions through these two pathways.
nitrogen depositionphosphorus fractionmicroorganismphosphatase activityloess hilly region
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