Effects Of Grazing And Nitrogen Addition On Structure And Function Of Plant Community In A Typical Steppe

Grazing governs more than half of the global terrestrial ecosystems.The increase in nitrogen(N)deposition caused by human activities is a global concern.Grazing and nitrogen deposition are key drivers of ecosystem structure and function of grasslands,and both of them occur simultaneously in grasslands.However,how the combined effects of grazing and N deposition on plant community structure and function are not well studied.To this end,based on the long-term zoned rotational grazing experiment platform(started in 2001)of Huanxian Grassland Agricultural Experiment Station of Lanzhou University to conduct a nitrogen addition experiment.The experimental platform consisted of warm-season rotational grazing and cool-season grazing grasslands,each of them with four grazing intensities of 0(SR0),2.7(SR2.7),5.3(SR5.3)and 8.7(SR8.7)sheep/ha.Four N additions levels of 0(N0),5(N5),10(N10)and 20(N20)g N/m~2/yr was applied on plots of each grazing intensity.Warm season grazing and N additions experiment began in 2012,and the cool season grazing and N additions experiment began in 2019.The effects of grazing and N addition on biodiversity,productivity,community stability and their interrelationships of typical steppe community were studied.The aim of the study is to provide theoretical and practical support for the sustainable management of grassland in the context of global change.The main results are listed below:Effects of grazing and N addition on communityα-diversity.Warm season grazing with SR8.7 increased species richness of plant community,and increased species richness of legumes or grazing tolerant plants.Cold season grazing reduced species richness of community,which was mainly caused by the reduction of species richness of non-grass and non-grazing tolerant plants.N addition and the interaction between grazing and N addition did not alter the species richness in this typical steppe regardless of warm-or cold-season grazing.Effects of grazing and N addition on communityβ-diversity.Warm season grazing with SR5.3 and SR 8.7 reduced the totalβ-diversity among communities;cold season grazing with SR2.7 reduced the totalβ-diversity in cold season grazing grassland.The reduction ofβ-diversity turnover components explained the decrease in totalβ-diversity.Nitrogen addition reduced theβ-diversity turnover components and increased nestednessed components ofβ-diversity in warm-season grazing sites,especially in wet years.N addition increased the totalβ-diversity mainly by increasing the nestednessed components ofβ-diversity in cold season grazing grassland.The interaction effects between grazing and N addition on communityβ-diversity and its components varied with grazing season.SR2.7N5 reduced nestednessed components of beta diversity in the warm season grazed grasslands;SR2.7N10 reduced theβ-diversity turnover fraction and increased theβ-diversity nestnessed components in the cold season grazed grasslands.The response of aboveground net primary productivity to grazing varied with stocking rate and grazing seasons.In warm-season grazing grasslands,aboveground net primary productivity increased with the increasing stocking rate;and aboveground standing biomass decreased with the increasing stocking rate.This was primarily caused by a decrease in the standing biomass of graminoids and moderately grazing-tolerant plants.In cold-season grazed grasslands,grazing reduced aboveground net primary productivity and aboveground standing biomass,primarily due to a reduction in standing biomass of graminoid plants.N addition had no significant effects on above-ground standing biomass and net primary productivity in warm-season grazed grassland.N addition promoted above-ground standing biomass,litter biomass and above-ground net primary productivity in cold-season grazing grassland,which were primarily caused by the increase in standing biomass of Graminoid plants.Grazing and N addition had no interactive effects on above-ground standing biomass and net primary productivity in both warm-and cold-season grazed grassland.Community structure was changed dramatically when stocking rate exceeded SR5.3 in the warm season grazed grassland.Community structure was shifted dramatically when N addition level exceeded 5 g N/m~2/yr in the cold season grassland.The combined effects of grazing and N addition enhanced the positive diversity-productivity relationship in the typical steppe.SR2.7N10 enhanced the positiveβdiversity-productivity relationship in warm-season grazed grassland;SR5.3N10strengthened the relationship betweenβ-diversity and productivity in cold-season grazing grassland.SR8.7 enhanced the temporal stability of aboveground net primary productivity and reduced the temporal stability of aboveground standing biomass in warm-season grazed grassland.Grazing reduced the temporal stability of community standing biomass mainly through negative effects onβ-diversity turnover components,temporal stability of life-type functional group standing biomass(Graminoid,Fabaceae and Others),temporal stability of grazing tolerant(grazing tolerant plants)functional group standing biomass and species asynchrony.Grazing did not significantly alter the temporal stability of standing biomass and net primary productivity in cold-season grazed grassland.Grazing maintained community stable production and ecosystem stability mainly by reducing the diversity of dominant species.N addition had no significant effects on the temporal stability of aboveground standing biomass and aboveground net primary productivity in warm-season grazed grassland.In cold-season grazed grassland,N addition had negative effects on community stability by decreasing the temporal stability of Artemisia capillaris biomass and thus the temporal stability of community standing biomass,while it had positive effects on the temporal stability of community standing biomass by reducing the community dominance.Grazing and N addition had significant interactive effects on the temporal stability of above-ground standing biomass and above-ground net primary productivity.In summary,stocking rate with 2.7 sheep/ha combined with N addition with 10 g N/m~2/yr is beneficial in maintaining plant diversity,productivity and community stability in warm-season grazed grasslands;stocking rate with 2.7 sheep/ha combined with N addition with 5 g N/m~2/yr is beneficial in maintaining productivity and community stability in cold-season grazed grasslands.Future research needs to focus on the response of above-and below-ground feedback in grassland ecosystems to grazing and nitrogen addition.This study has theoretical and practical implications for the sustainable management of grassland ecosystems in the context of global change.