Does foliar nutrient resorption regulate the coupled relationship between nitrogen and phosphorus in plant leaves in response to nitrogen deposition?

Nutrient resorption from senescing leaves is an important process of internal nutrient cycling in plants, but the patterns of nutrient resorption and the coupled relationship between nitrogen (N) and phosphorus (P) in plant leaves as affected by N deposition remain unclear. We analysed the effects of N addition on the nutrient resorption and coupled relationship between N and P in plant leaves under different nutrient-limited conditions based on a global meta-analysis. Globally, the mean N resorption efficiency (NRE) and P resorption efficiency (PRE) under natural conditions were 47.4% and 53.6%, respectively, which were significantly regulated by geographical and climatic factors as well as plant characteristics. Furthermore, N addition significantly decreased the NRE by 13.3% but slightly affected the PRE on a global scale, and N addition rates and latitude directly and negatively affected the effects of N addition on NRE. Specifically, N addition significantly decreased the NRE under all nutrient-limited conditions, while it had negative, positive, and neutral effects on the PRE under N-limited, P-limited, and N and P-co-limited conditions, respectively. Moreover, the relationships between N and P in green and senesced leaves were tightly coupled under different nutrient-limited conditions in natural ecosystems. However, N addition significantly weakened the relationships between N and P concentrations in green leaves but slightly affected the relationship in senesced leaves, which were mainly modulated by the effects of N addition on nutrient resorption efficiency, especially NRE. These results highlight that nutrient-limited conditions determine the response of nutrient resorption to N deposition and emphasize the effect of nutrient resorption regulation on the coupling of N and P responses to N enrichment. The findings are important for understanding plant nutrient use strategies and the mechanisms underlying the stoichiometric coupling of N and P in response to climate change, and can be used in global biogeochemical models.