Abstract: 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 understanding of changes in soil phosphorus fractions and their influencing factors under nitrogen deposition is still limited. 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 levels, while the contents of non-labile phosphorus decreased significantly. Organic phosphorus content significantly increased 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 biomass 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 nutrients, 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 substrate nutrients, which had significant effects on microbial biomass and the activities of phosphorus conversion extracellular enzymes, and co-regulated soil phosphorus fractions through these two pathways.

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