Convergent responses of nitrogen and phosphorus resorption to nitrogen inputs in a semi-arid grassland

Background/Question/Methods

Human activities have significantly altered the nitrogen (N) availability in most terrestrial ecosystems, with consequences for community composition and ecosystem functioning. While studies of how changes in N availability affect biodiversity and community composition are relatively common, less is known about the effects of N inputs on the coupled biogeochemical cycling of N and phosphorus (P), and still fewer data regarding how increased N inputs affect the internal cycling of these two elements in plants. Nutrient resorption is an important driver for plant nutrient economy and substrate quality of plant litter, with ecological implications for plant population and community fitness and for ecosystem nutrient cycling. We studied the effects of a wide-range of N inputs on foliar nutrient resorption of two dominant grasses, Leymus chinensis and Stipa grandis, in a semi-arid grassland of northern China.

Results/Conclusions

After four years of treatments, N and P availability in soil, and N and P concentrations in green and senesced leaves of grasses increased with the rates of N addition. Foliar N and P resorption significantly decreased along the N addition gradient, implying a resorption-mediated positive plant-soil feedback induced by N inputs. Furthermore, N:P resorption ratios were negatively correlated with the rates of N addition, indicating the sensitivity of stoichiometric coupling of N and P in plants to N inputs. The results demonstrate that N addition accelerates ecosystem uptake and turnover rates of both N and P in the temperate steppe and that N and P cycles are coupled in dynamic ways. The convergence of N and P resorption in response to N inputs emphasize the importance of nutrient resorption as a pathway by which plant and ecosystem can adjust in face of the increasing N availability.