Enhancement of nutrient resorption efficiency increases plant production and helps maintain soil nutrients under summer grazing in a semi-arid steppe

Abstract

Grazing regime is the most important factor that alters nutrient cycling in grassland ecosystems. Grazing exclusion has been used for restoration of degraded natural grasslands in Northern China during past two decades, while we propose that 'summer grazing' could be a better management than grazing exclusion for this purpose. Summer grazing allows plants to resorb more nutrients in autumn, thus preventing grazing-induced grassland degradation while keeping grassland in production. However, our understanding of grazing effects on nutrient resorption remains limited. In the present study, we examined plant nitrogen (N) and phosphorous (P) resorption at plant species and community levels in a natural steppe grassland that had experienced contrasting 'summer grazing' versus no-grazing treatments over a 6-year period in central Inner Mongolia. Plant community nutrient resorption efficiency (RE) and resorption fluxes were estimated, along with nutrient return fluxes via animal excreta and litter deposition. We found that (1) Summer grazing at moderate intensity increased plant production, but did not alter soil quality indicators such as soil organic carbon, total nitrogen, and total and plant available phosphorous. (2) Summer grazing increased plant community nutrient RE as the post-grazing plant re-growth with elevated nutrient concentration compensated for the nutrient removal by defoliation. Summer grazing increased nitrogen RE in Leymus chinensis and Cleistogenes squarrosa, but not in Stipa krylovii; and increased phosphorous RE in L. chinensis, but decreased that in S. krylovii and C. squarrosa. (3) Summer grazing accelerated nutrient cycling via animal excreta and litter deposition, but did not reduce plant nutrient resorption fluxes in autumn, which is a major mechanism for plant production enhancement and soil nutrient maintenance under the grazing regime. Our findings provide the key evidence to support moderate-intensity summer grazing as a sustainable grassland management regime to improve plant production while maintaining soil nutrients.