Disentangling the effects of animal defoliation, trampling, and excretion deposition on plant nutrient resorption in a semi-arid steppe: The predominant role of defoliation

Abstract

Nutrient resorption is a key strategy of perennial plants for conservation and efficient use of nutrients. Previous studies show that semi-arid steppe may maintain its nutrient resorption flux under moderate grazing that substantially removes plant biomass and nutrients, while the mechanisms underlying the fact are unclear. We performed a three-year simulation experiment in the Inner Mongolian grassland to quantify the respective and combining effects of three grazing mechanisms, i.e., animal defoliation, trampling and excretion return, on plant nitrogen (N) and phosphorus (P) resorption traits (efficiency, proficiency and flux) at species and community levels. We found that defoliation reduced N resorption efficiency (NRE) of Leymus chinensis; excretion return reduced N resorption proficiency (NRP) of L. chinensis and Stipa krylovii; and trampling reduced P resorption proficiency (PRP) of Cleistogenes squarrosa in non-defoliated grassland. Defoliation reduced plant community nutrient (N and P) pools for resorption, but enhanced the pool-based plant community nutrient resorption efficiency, leading to a neutral effect on plant community nutrient resorption fluxes. Animal excretion return significantly enhanced plant nutrient resorption fluxes by enlarging the nutrient pools for resorption, not increasing resorption efficiency or proficiency. Simulated animal trampling at moderate intensity over three seasons does not alter plant community nutrient resorption, though it reduced nutrient resorption of a fragile species C. squarrosa. The negative effects of defoliation on plant nutrient resorption by removing plant aboveground biomass and nutrient pool, could be offset by the positive effects of animal excretion return that improved plant available nutrient in the soil, enhanced plant nutrient uptake and pool for resorption and actual nutrient resorption fluxes; however, the effects of animal excretion return and that of trampling were significant only in non-defoliated grassland, as these effects were masked in defoliated grassland. Our results suggest that animal defoliation and excretion return jointly explain the majority of overall grazing effects on plant nutrient resorption fluxes, and the defoliation plays a predominant role in mediating the interactive effects of three grazing mechanisms. Our results also provide insights into the divergent effects of animal grazing versus mowing for hay (defoliation only) practices on grassland ecosystems and have management implications.