Purpose The escalation of nitrogen (N) deposition has resulted in phosphorus (P) limitation in alpine grasslands on the Qinghai–Tibetan Plateau (QTP). However, the impact of N deposition affects soil P transformations in alpine grasslands, and whether there is a universal pattern of N-induced soil P fraction change in terrestrial ecosystems is still not well understood.

Methods We performed field experiments in two alpine grasslands on the QTP and a meta-analysis including 1033 records worldwide to analyze the responses of soil P fractions to N addition.

Results We found that N addition significantly altered soil P fractions in alpine meadow, whereas it induced a minor response in alpine steppe. The N addition induced a decrease in soil inorganic phosphorus (Pi) in the alpine meadow, resulting from occluded P (i.e., C.HCl-Pt and residual-Pt). Though N addition did not change total organic P (Po) concentration, there were remarkable changes among soil organic P fractions (C.HCl-Po, NaOH-Po, and NaHCO3-Po) in the alpine meadow, with an increase in NaOH-Po but a decrease in C.HCl-Po. Soil inorganic P in the alpine meadow was associated with Ca2+ and soil pH that was also reduced by N addition. By contrast, meta-analysis results showed that N addition significantly increased the lnRR of NaOH-Pi, but decreased lnRR of C.HCl-Pt and marginally reduced lnRR of NaHCO3-Po across all terrestrial ecosystems. Among multiple environmental and experimental variables, soil pH, mean annual temperature (MAT), mean annual precipitation (MAP), N forms, and soil phosphatase activity mainly drove the response of NaHCO3-Po to N addition at the large scale. Structural equation model (SEM) further showed that soil phosphatase activity was the main direct factor controlling NaHCO3-Po response.

Conclusions Our results suggest that soil P fractions are more sensitive to N addition in alpine meadow than in alpine steppe. The reduction of inorganic P fractions and uneven changes of organic P fractions in alpine meadow suggested that N addition may accelerate inorganic P dissolution but depress organic P mineralization. Environmental factor (e.g., MAP) and experimental variables (N rate) affected soil P fractions in response to N addition mediated by soil pH and enzymatic activities. Collectively, these findings improved our understanding of the consequences of N addition on soil organic and inorganic P transformations and predicted the trajectory of soil phosphorus fraction change under increasing N deposition.