Effects Of Grazing And Mowing On Grassland Soils, Plants And Insects: Community Properties And Ecological Stoichiometric Linkages

Understanding the organization and assembly of ecological communities is an important goal of ecology.Less known is how human disturbances can alter the community properties and ecological stoichiometric linkages among multiple trophic levels.In grassland ecosystems,grazing and mowing are the two primary management strategies across the globe,which can exert profound effects on properties of soil,plants,insects,as well as their carbon(C),nitrogen(N),and phosphorus(P)contents and ratios.Grazing and mowing can affect the structure and composition of plant and insect communities,including their species diversity,abundance,and biomass.Grazing and mowing continue to remove C and nutrients from plants,which can in turn affect the sizes of C and nutrient pools and their cycling processes in the soils,with potential consequences for ecosystem energy and nutrient dynamics in the long term.In history,numerous studies have been conducted to investigate the independent effects of grazing and mowing,yet very few studies have compared how they can differently affect the properties of multiple biotic and abiotic components,including soils,plants,and insects,inhibiting both our understanding of the organization and assembly of grassland ecosystems,as well as the formation of successful management and conservation strategies for these biome.For this study,by using a 4-year field manipulative experiment,we investigated how moderate intensity of domestic livestock(cattle,Bos tarurs)grazing and mowing can affect the physicochemical characteristics of soils,and the community properties of plants,and insects;Moreover,we further explored how these two management methods can differently affect the C:N:P stoichiometric patterns and linkages of soils,the dominant plants(Leymus chinesis grasses),and the dominant insect herbivores(Euchorthippus grasshoppers)of Songnen grassland,a semi-arid meadow steppe in Northeastern China.We found that grazing and mowing can differently affect the physicochemical characteristics of soils,and the community properties of plants and insects.(1)For soils,grazing significantly increased soil bulk density and moisture by 7% and 12%,respectively,whereas mowing had no impacts on these two variables.Grazing also enhanced soil N concentration by 14% at the depth of 0-10 cm,however,grazing failed to affect the C and P concentration as well as their C:N,C:P,and N:P ratio of soils at the depth of neither 0-10 cm nor10-20 cm.Compared to grazing,mowing have much weaker influences on the soil C,N,P concentrations and their stoichiometric ratio at both depths,as it failed to significantly affect any of these variables.(2)For plant community properties,grazing significantly increased plant species richness by 9%,whereas mowing have no impacts on plant diversity.However,mowing significantlyreduced total plant biomass by 18%,while grazing failed to affect this variable.Shoot biomass were not affect by both grazing and mowing,yet root biomass is reduced by mowing by 12%,whereas they were not affected by grazing.Both grazing and mowing failed to affect grass biomass,yet mowing significantly altered plant community composition by reducing forb biomass,and increasing grass:forb biomass ratio by 11% and 32%,respectively.(3)Grazing and mowing exerted different impacts on community properties of insects.Grazing and mowing did not affect total abundance of insects.Grazing and mowing also failed to affect insect species evenness.However,grazing significantly increased insect species richness by 17%,whereas mowing failed to affect this variable.Grazing and mowing differently affect the functional composition of insect community: mowing reduced predator insect abundance by27%,whereas grazing increased detritivores insect abundance by 18%.Grazing and mowing have relatively weak effects on the abundance of herbivore and parasitoid insects.(4)More importantly,we found that grazing and mowing can differently alter the C,N,P concentrations and their C:N:P stoichiometric patterns across soils,the dominant L.chinesis grass,and the dominant insect herbivores,Euchorthippus grasshoppers in this ecosystem.Grazing significantly enhanced the total N concentrations of soils and L.chinensis grass,and reduced C:N of L.chinensis grass.However,grazing did not affect total C,N,or P concentrations nor their ratios in either female or male Euchorthippus grasshoppers.Compared to gazing,mowing have a much weaker influences on the C:N:P stoichiometric properties,as it failed to affect all these variables in soils,L.chinensis grass,and Euchorthippus grasshoppers.Our results reveal that the effects of grazing disturbances on elemental composition attenuated from lower to higher trophic levels in a grassland ecosystem.These findings support the theory that organisms from higher trophic levels have relatively stronger stoichiometric homeostasis compared to those from the lower trophic levels.In summary,this study documented that moderate livestock grazing tends to have different effects on the physicochemical characteristics of soils,and the community properties of plants and insects.Compared to mowing,cattle grazing generally tended to increase soil N concentrations,the most important limiting nutrient element in grassland ecosystems,which probably due to their foraging and trampling activities,as well as the urine and feces deposition that accelerate nutrient cycling.Moderate cattle grazing also have limited negative impacts on plant biomass and insect abundance,and tended to increase both the diversity of plants and insects,probably because they are generalist feeders and can balance the feeding pressure on different plant species.In contrast,mowing tended to have negative impacts on plants by decreasing their total biomass,but have limited impacts on soil properties and insect community structure.This study suggested that,at least short-term and moderate intensity of management,cattle grazing is a better management method.Compared to mowing,cattle grazing tend to improvesoil physicochemical characteristics,the community composition and structure of plants and insect communities,with potential benefiting consequences for the ecosystem functioning,processes,and services of grasslands.However,this study only looked at the short-term(4-year)effects of grazing and mowing,further longer term experiments with different intensity of livestock grazing and mowing experiments are needed to explore their influences on properties of multiple biotic and abiotic components,including soils,plants,and insects,which may enable us to better predict the consequences of environmental changes for community assembly,organizations,ecosystem functioning,and thus help us to choose the optimal management method for grasslands in northeastern China.