Effects Of Foraging And Feces Of Large Herbivores On Vegetation And Nitrogen Mineralization In A Grassland Dominated By Stipa Grandis

Grasslands,one of the most extensive vegetation types in the world,are often grazed by larger herbivores.These animals can impact the plant community structure,soil characteristics and nutrient cycling in grasslands by grazing,trampling and feces.Many previous studies focused on the effects of herbivore grazing on grassland vegetation,soil source availability and nutrient cycling,but more studies are needed to distinguish the differences in the effects on the vegetation and soil characteristics between herbivores grazing and feces.Moreover,most terrestrial studies focus on litter decomposition and often ignore feces decomposition.However,and especially in grasslands,where 30~50% of aboveground plant biomass is consumed annually by herbivores,feces decomposition is a significant,co-dominant pathway for nutrient cycling within those intensive gazing systems.Here,we measured how large herbivores might respectively influence plant community structure and soil nutrients in grassland by grazing and feces.Our study was carried out at the field observation station of National Climate Observatory in Xilinhot,Inner Mongolia Autonomous Region,China.In this study,we first evaluated whether different herbivore grazing(no grazing,cattle grazing,sheep grazing and mixed grazing)could influence vegetation characteristics and soil N mineralization.We then determined whether feces addition had effects on plant community structure,aboveground biomass and soil nutrient availability.We finally investigated the processes of feces decomposition and nutrient releases from cattle and sheep,and assessed how herbivores affect C and N flux via specific plant selection and their respective digestive abilities.We found that:(1)Different herbivore grazing could alter plant species diversity in grasslands.The mixed grazing might promote plant species diversity in the long term by decreasing light limitation and weakening nutrient competition from grasses.Sheep favor the plants with high content of nutrients,while cattle demand a greater quantity of food to maximize performance.Accompanying by other negative factors,sheep grazing resulted in the decreases of plant species diversity.(2)Both of the herbivore grazing and precipitation had significant effects on the aboveground biomass.Though compensatory growth after defoliation can overrode the negative effect of biomass loss,but time available for recovery had a major impact on the outcome of defoliation.With a short time for recovery,herbivores grazing resulted in the significant decrease of the aboveground biomass in the plots.Additionally,foraging behavior of herbivores is density-dependent,and thus there was no significant difference in the aboveground biomass between cattle and sheep for high diet overlap.(3)Herbivores had different effects on soil N mineralization.The results indicated that soil net N mineralization rates were related to herbivores species.Cattle could promote soil N mineralization,which was suppressed by sheep grazing.However,there was no difference in the soil mineral N contents between cattle and sheep grazing.The contradiction between soil mineral N contents and net N mineralization rates suggested that feces from sheep offset the negative effects of sheep grazing.(4)Our results show that fecal decomposition process is closely related to the chemical composition of feces,which was potentially regulated by the diet selection of large herbivores.Generally,there is a negative correlation between body size of herbivores and nutrient contents in their feces,which reflects a tendency for declining diet quality with increasing body size.This diverse diet quality resulted in a C/N ratio of cattle feces that was higher than that of sheep feces.Relatively higher labile C availability in the cattle feces accelerated cattle feces decomposition,and the feces from cattle mineralized about twice as much N as feces from sheep,despite the latter having slightly higher N content.The study results indicated that herbivore grazing and feces could impact vegetation and soil characteristics in grassland ecosystem via different pathway,and there was a closely relationship between foraging behavior and feces.Our study increased understanding of the interaction among herbivores,plants and soil,and helped people maintaining plant species diversity and ecosystem functioning in grasslands.It is significant for optimal foraging theory and grazing management.