Nutrient resorption responses of plant life forms to nitrogen addition in temperate shrublands

Atmospheric nitrogen (N) deposition has significantly altered nutrient availability in most terrestrial ecosystems, which may affect plant nutrient resorption and hence change nutrient cycling and community composition. Although studies on the effects of changed soil nutrients on leaf nutrient resorption have been extensively reported in forests and grasslands, much less known about how plants in temperate shrublands respond to nitrogen deposition across the world. A 4-year N addition experiment in Vitex negundo and Spiraea trilobata shrublands was conducted to investigate the potential impacts of N deposition on nutrient resorption in two life forms. The green and senesced leaf N ([N](g) and [N](s)) and phosphorus ([P](g) and [P](s)) concentrations were measured in four shrubs and two graminoids to calculate N (NRE) and P (PRE) resorption efficiency. We found markedly lower NRE and PRE in shrubs than in graminoids, implying that shrubs probably have a higher capability of acquiring nutrients from soil, whereas graminoids rely on acquiring more nutrients via resorption. N addition increased the [N](g), [N](s), and N:P ratios ([N:P]) and decreased the [P](g) of shrubs and all species, whereas it only increased [N](s) but had negligible effects on the [N](g), [P](g), and [N:P] of graminoids. The [P](s), NRE, and PRE did not change in either life form or all species combined after N fertilization. Our results suggest that shrubs increase N uptake but do not alter internal N cycling (i.e., nutrient resorption), whereas graminoids do not change N acquisition strategies, in response to increased N supply from the soil. Continuous N addition exacerbated the P limitation of the shrub plants. Both shrubs and graminoids were more likely to adopt nearly complete utilization of P from the soil and senesced leaves under P deficiency, leading to a noneffect of N addition on leaf PRE. Our findings also suggest that the [N](g) and [P](g) responses to N addition are life-form specific, implying that N addition may affect ecosystem carbon (C) and nutrient cycles by altering the dominance values of the two life forms and community composition.