Nutrient resorption exacerbates nitrogen-phosphorus imbalances in plants under increasing nitrogen addition in a saline-alkaline grassland

Reabsorbing nutrients from senescent tissues before leaf falling has been recognized as a strategy to adapt to nutrient deficiency. However, how nutrient resorption modulates the nitrogen (N)-phosphorus (P) balance inside plants remains unclear, especially under increased soil N availability. We examined the impacts of N addition at varying rates (0-32 g N m(-2) yr(-1)) on nutrient resorption and the performance of nutrient resorption on controlling the internal N-P balance in the leaf and stem of a dominant grass species, Leymus secalinus, in a saline-alkaline grassland in northern China. After 6 years of N addition, N concentration and N:P ratio in green and senesced tissues (leaf and stem) rose with increasing N addition. The P concentration in green tissues decreased, but did not significantly change in senesced tissues with increasing N addition. The N resorption efficiency (NRE), P resorption efficiency (PRE), and NRE:PRE ratio significantly decreased along the N addition gradient. Moreover, we found more sensitive responses of N:P ratio in senesced tissues than in green tissues; such exacerbation of plant internal N-P imbalances mainly resulted from a disproportionate reduction in nutrient resorption, especially NRE. Overall, our study suggested that differences in NRE and PRE further exacerbated the internal N-P imbalances in plant litters.