Responses and drivers of leaf nutrients and resorption to nitrogen enrichment across northern China’s grasslands: A meta-analysis

A meta-analysis was preformed to reveal how environmental and experimental factors alter the effects of N enrichment on the concentrations and stoichiometry of N and P in both mature and senesced foliage and nutrient resorption, with published studies from Chinese grasslands. N enrichment, when averaged across the studies, significantly increased the mature leaf N, P and N:P by 27.89%, 2.55% and 20.85%, respectively. The N, P and N:P ratio for senesced foliage under N enrichment increased by 41.35%, 3.28% and 20.11%, respectively, but N enrichment significantly decreased the leaf N resorption efficiency (NRE) and P resorption efficiency (PRE) by 6.85% and 3.41%, respectively. They were significantly controlled by the rates of addition of N, humidity index and the ratio of N addition rates to ambient N deposition, which suggested that ambient N deposition can indirectly influence plant internal nutrient cycling in this system. The effects of N enrichment on the N, P, stoichiometry and nutrient resorption in plants varied greatly among the types of grassland. In addition, the rates of N addition, forms of N application, experimental duration and application frequency of N had different effects on the leaf nutrient status. Specifically, the response of a critical threshold value of leaf P to N addition is 50–100 kg N ha−1 year−1, at which range with a shift from negative to positive impacts. Moreover, the addition of N significantly changed N and P coupling in mature leaves but not of senesced leaves, which were primarily regulated by N enrichment impacts on nutrient resorption. Our findings suggest that N enrichment significantly affected the status of leaf nutrients and then altered plant internal nutrient cycling across the grasslands of northern China.