The asynchronous changes of soil nitrogen and phosphorus along the altitude gradient in the southeastern Tibetan Plateau

The global nitrogen (N) and phosphorus (P) content of the soil exhibits distinct latitudinal variations, yet their variation along elevational gradients in alpine forest ecosystems remains poorly understood. Based on 28 sampling sites spanning an elevational range of 769-3,767 m in alpine forests on the southeastern Tibetan Plateau, we analyzed variations in soil N and P contents and their stoichiometric ratios, and explored the key driving factors. The results showed that soil N and P exhibited asynchronous nonlinear changes along the elevational gradient. Soil N increases with increasing altitude, reaching a peak at approximately 2,800 m above sea level, and then decreases as altitude continues to rise. In contrast, soil TP and AP exhibited an opposite nonlinear pattern. This N-P decoupling was reflected in the unimodal pattern of the soil N: P ratio, which reached a maximum value of 11.8 ± 1.86 at approximately 2,800 m. Elevated soil temperature resulted in a non-linear decrease in the N: P ratio. The collection of literature on a global scale also indicates that the N: P ratio of temperate evergreen coniferous forests is significantly higher than that of functional types of tropical and alpine plants. The asynchronous elevational patterns of soil N and P observed in this study provide empirical references for understanding nutrient cycling in montane ecosystems and for assessing potential soil nutrient dynamics under future climate change.