The Impacts And Mechanisms Of Grazing On Plant Nutrient Resorption In A Typical Steppe Of Inner Mongolia

The steppes on the Inner Mongolian Plateau are of crucial importance for the ecological security in Northern China,and for the pastoral production in the region.However,over the past several decades,overgrazing has markedly altered nutrient cycling in grassland ecosystems and induced their degradation.The nutrient resorption of perennial plants,that is,nutrient transfer from senescing tissues to living tissues,is a crucial component of plant internal nutrient cycling,and plant strategy for improving nutrient use efficiency,especially in nutrient-poor habitats.Plant nutrient resorption is affected by various environmental factors,while the effects of land use on the plant nutrient processes are under explored.The existing studies on the nutrient cycling of grassland ecosystems have a focus on the cycling of nutrients among plants,animals and soils,but not the internal cycling of nutrients in plants.In addition,most of process-based grassland ecosystem models are lack of the precise simulation of the plant internal nutrient cycling processes.Therefore,it is imperative to estimate the effects of animal grazing on plant nutrient resorption in grazed grassland ecosystems and understanding the underling mechanisms,for more accurately modelling the nutrient cycling within plants and among in ecosystem components,and supporting the development of grassland management regimes.In the present study,taking the typical steppe of Inner Mongolia as a model,we studied the effects and mechanisms of grazing on plant nutrient resorption at plant species and community levels;we also examined the the effects of defoliation timing on plant nutrient resorption in a cutting simulation experiment,and in an experiment of seasonal grazing.The major results are as follows:(1)Plant nitrogen(N)and phosphorus(P)resorption responses to summer grazing varied among species.Summer grazing promoted plant community N and P resorption efficiency(NRE and PRE),but did not alter N and P resorption fluxes,thus did not affect plant nutrient conservation.Summer grazing promoted plant aboveground net primary productivity(ANPP)and nutrient acquisition,and increased nutrient input into soil by animal excretion return,which would accelerate nutrient cycling in grassland ecosystem.These findings provided the key evidence to support the summer grazing at moderate intensity as a sustainable grassland management regime for plant production while maintaining plant nutrient conservation and soil properties.(2)Defoliation was the major grazing mechanism for plant nutrient resorption.Defoliation elevated plant community N and P resorption efficiency,and consequently maintained plant community N and P resorption fluxes to meet the needs of plant nutrient conservation.Animal excretion return improved plant nutrient pool for resorption by increasing nutrient availability in soil,leading to an increase in N and P resorption fluxes.Trampling had little effect on plant nutrient resorption,except that it inhibited N and P resorption of Cleistogenes squarrosa.The interactions of three grazing mechanisms were observed,with defoliation playing a dominant role,that is,the effects of animal excretion return and trampling were mediated by defoliation.(3)Defoliation after the start of nutrient resorption reduced plant community NRE,and defoliation at plant biomass peak period or after nutrient resorption starting reduced plant community N and P resorption proficiency and fluxes,as well as litter nutrient return fluxes;while defoliation before plant biomass peak period had no such effects.The 1-year defoliation treatment had no effect on plant root nutrient conservation.In mowing grasslands,defoliation at plant biomass peak period benefited to plant production,but defoliation before plant biomass benifited to the nutrient conservation of plant and soil,suggesting that early mowing is preferred for a sustainable grassland management regime.(4)Seasonal grazing affected the nutrient resorption flux of Stipa grandis by altering plant nutrient pool for resorption,and the strength of the flux was the biggest under spring grazing,medium under summer grazing,and the smallest under autumn grazing.N and P resorption proficiency were highest under spring grazing and lowest under summer grazing.Spring and summer grazing reduced root nutrient conservation,while,autumn grazing had no effect as root nutrient uptake from soil compensated for the reduction in nutrient resorption.Seasonal grazing had different effects on S.grandis growth and nutrient accumulation in the coming spring,and the effects was weaker by spring grazing than summer grazing,and than autumn grazing.The mediation of seasonal grazing on S.grandis growth was achieved mainly by its alteration of soil water supply,but not plant nutrient resorption and root nutrient conservation.These results provide new insights into the effects and mechanisms of grazing on plant internal nutrient cycling,and evidences for the development of grassland management regimes to meet the balance of plant production and conservation.