Forbs dominate plant nutrient resorption of plant community along a 34-year grazing exclusion gradient in a semiarid grassland

Abstract

Grazing exclusion has been commonly used to restore degraded grasslands and has shown positive effects on plant community structure, productivity, and soil properties. Nutrient resorption affects key ecosystem processes such as nutrient uptake and carbon cycling, and plays a key role in plants' nutrient conservation in nutrient-poor environments. However, how plant nutrient resorption varying with grazing exclusion duration remains unclear. In this study, leaf nitrogen (N) and phosphorus (P) resorption efficiencies were examined at plant functional group and community levels in grasslands along a 34-year grazing exclusion gradient. Along the grazing exclusion gradient, N resorption efficiency (NRE) of plant community remained unchanged, while P resorption efficiency (PRE) increased, accompanied with increased community biomass, the proportion of grasses and soil fertility. Nutrient concentrations and resorption efficiencies, as well as their responses to grazing exclusion, varied among plant functional groups. Both NRE and PRE were higher in grasses than in sedges or forbs, while forbs had higher N and P concentrations in both green and senesced leaves. NRE and PRE of grasses and sedges changed little along the grazing exclusion gradient except the decreased NRE of grasses after 34 years of grazing exclusion, while both NRE and PRE of forbs increased with the duration of grazing exclusion. Variation in community nutrient resorption efficiencies was largely driven by forbs, and nutrient resorption efficiencies were negatively correlated with senesced leaf nutrient concentrations. Nutrient concentrations and resorption efficiencies of different plant functional groups and plant communities varied in their relationships with soil properties. Overall, our study highlights the importance of forbs in plant nutrient use pattern of grassland communities along the grazing exclusion gradient, and offer insights for grassland management to boost ecosystem nutrient cycling.