nitrogen addition rates, Qinghai-Tibetan Plateau, unimodal response, microbial biomass phosphorus, soil phosphorus fractions, Agriculture (General), S1-972

Nitrogen (N) enrichment is expected to induce a greater phosphorus (P) limitation, despite the acceleration of soil P cycling. However, the changing patterns in plant P and soil available P after N enrichment, and their regulatory mechanisms, remain poorly understood in alpine meadows. Here, we conducted a field experiment with four N addition rates (0, 5, 10, and 15 g N m–2 yr–1) in an alpine meadow, and investigated the P in plants, microorganisms, and soil to determine their patterns of change after short-term N addition. Our results showed that N addition significantly increased plant biomass, and the plant P pool showed a non-linear response to the N addition gradient. Soil available P initially increased and then declined with increasing N addition, whereas the occluded inorganic P decreased markedly. The critical factors for soil available P varied with different N addition rates. At lower N addition levels (0 and 5 g N m–2 yr–1), soil acidification facilitated the mobilization of occluded inorganic P to increase soil available P. Conversely, at higher N addition levels (10 and 15 g N m–2 yr–1), the elevated soil microbial biomass P intensified the competition with plants for soil P, leading to a decline in soil available P. This study highlights the non-linear responses of the plant P pool and soil available P concentration to N addition rates. These responses suggest the need for developing ecosystem models to assess different effects of increasing N rates, which would enable more accurate predictions of the plant P supply and soil P cycling under N enrichment.