Regulation Mechanisms Of Grazing On Grassland Soil Resources Heterogeneity,Plant Diversity And Productivity

The intensification of human activities and global climate changes have caused increasingly severe land degradation and the decline in grassland functions.How to maintain grassland productivity and the sustainable development of grassland are major problems faced by grassland science.Grazing is an important measure of grassland management,and it is also the main source of grassland degradation and the decline in productivity.A thorough understanding of the regulation mechanism of livestock grazing on productivity is an important theoretical basis for achieving sustainable grassland management and utilization.With the increasing loss of global biodiversity,the important role of plant diversity in maintaining grassland productivity and other ecosystem functions have attracted increasing attention.As the foundation of plant growth,soil resources play important role in regulating grassland productivity.The quantity and heterogeneity of soil limited resources(such as nitrogen)are important factors that affect and regulate plant diversity.Livestock grazing can affect competition between plants through selective foraging,thereby affecting plant diversity.On the other hand,grazing can also change soil resource availability and heterogeneity through excreta return and trampling,which can indirectly affect plant diversity.There has been a lack of systematic and in-depth understanding about the effects of grazing on grassland plant diversity,soil resource quantity,resource heterogeneity,and the regulation mechanisms of these processes on grassland productivity.These deficiencies have greatly restricted the scientific management about the maintenance and improvement of productivity in grazing ecosystems.This study was conducted on the long-term control grazing platform and the in-situ overgrazing platform,which spanned three main grassland types of the northern China(meadow steppe,typical steppe,desert steppe).We systematically studied the regulation mechanism of grazing on grassland soil heterogeneity,plant diversity and productivity,which based on the livestock-soil-plant multi-layer system level,considering the key characteristics that controlling grassland productivity,such as the plant diversity,soil heterogeneity and the selective foraging characteristics of herbivores,and combined with multiple grazing factors(moderate Grazing,overgrazing,herbivore species identity,as well as single grazing and mixed grazing methods).We obtained some important results as follows.(1)Based on the study about the impact of different grassland types and herbivore assemblages on soil resource heterogeneity,we found that moderate livestock grazing can increase the spatial heterogeneity of soil limited resources-nitrogen,and the effect size depended on herbivore assemblages and grassland types.Sheep grazing significantly increase the spatial heterogeneity of soil nitrogen in meadow steppe,typical steppe and desert steppe,while cattle grazing and mixed grazing with cattle and sheep can significantly increase the spatial heterogeneity of soil nitrogen only in typical grassland and desert grassland.Grazing can modulate soil nitrogen heterogeneity through changing plant community heterogeneity and excreta return,and grazing-induced changes in plant community heterogeneity can only affect soil nitrogen heterogeneity in typical steppe.The consistently increased heterogeneity of soil N availability was primarily due to the effect of animal excreta return in all the three grasslands.We found that the effects of animal excreta gradually weakened with the decrease of grassland precipitation,that is,the effect size of excreta return in the meadow steppe and typical steppe with relatively high precipitation was significantly higher than that of desert steppe with relatively low precipitation.Moreover,sheep excreta contributed more to soil N heterogeneity than that of other herbivore assemblages,and the effect was statistically significant in meadow steppe and typical steppe with relative high precipitation.We therefore suggest that managing above-ground herbivore species is important for conserving and promoting grassland soil heterogeneity,and that the management should also take into account local environmental conditions such as precipitation.Future climate change,particularly changes in precipitation amount,may affect soil heterogeneity in grazing grasslands.(2)Based on the study about the impact of different herbivore assemblages on grassland plant diversity at different scale,we found that herbivore species identity significantly affected both plant ?-diversity and ?-diversity.Grazing can affect plant diversity directly through selective foraging,and can also indirectly affect plant diversity through changing soil resource availability and heterogeneity,and plant ?-diversity was related to soil N quantity while plant ?-diversity was related to its heterogeneity.We found that grazing-induced changes in soil resource quantity and heterogeneity(i.e.combined effects of herbivore and soil resource)are important drivers of plant diversity change,especially for plant ?-diversity.Given the total effects of herbivore species identity from direct and indirect pathways,top-down effects of herbivore species identity on both plant ?-diversity and ?-diversity were stronger than bottom-up effects of soil resource.This research enriches the biodiversity maintenance mechanism in grazing grassland ecosystem,also provides a theoretical basis for formulating reasonable grazing management systems to improve grassland plant diversity.(3)Based on the study about the relationship between different dimensions of plant diversity and productivity in the free grazing scenario,we found that grazing-induced changes in plant diversity played an important role in controlling grassland productivity,and the effects depended on the dimensions of plant diversity considered.Plant species diversity has minor effect on productivity,while there is a positive relationship between plant functional diversity and productivity in the grazed scenario,and the results supported the mass-effect hypothesis.Further,the relationship between plant functional diversity and productivity was not affected by soil environmental factors(the quantity and heterogeneity of soil available nitrogen,the quantity and heterogeneity of soil available phosphorus,soil water content and its variation,soil p H and electrical conductivity).This study further confirmed the important role of grassland plant diversity in grazing ecosystem and emphasized the importance of protecting grassland plant diversity.(4)Based on the study about the main regulatory factors of grassland productivity under overgrazing scenario,we found that overgrazing can modulate the relative contribution of plant diversity and soil abiotic factors to productivity.In enclosed scenario,both plant diversity and soil abiotic factors can modify the ANPP.However,soil abiotic factors showed a greater and dominant contribution to the changes in ANPP in overgrazed scenario compared with that in enclosed scenario.Overgrazing had led to the decrease in plant functional diversity and the changes in plant community composition.Overgrazed grasslands differed from the enclosed grasslands by displaying greater importance of C3 and C4 species in terms of relative contributions to ANPP.Soil abiotic factors mainly control the productivity due to their large contributions to the biomass of C3 and C4 functional groups in overgrazed scenario.Considering the overgrazing situation faced by grassland,we must adopt the plant-soil coupled management to improve grassland productivity.In conclusion,grazing significantly improved the heterogeneity of grassland soil nitrogen through the excreta addition,and the effect size depended on the type of grassland and the herbivore assemblages.Grazing-induced changes in the soil resource quantity and heterogeneity can further regulate different scale plant diversity(?-diversity and ?-diversity),especially for plant ?-diversity.Grazing-induced changes in plant functional diversity had an important impact on grassland plant productivity.Overgrazing can restrain the capacity of plant functional diversity to sustain productivity,and strengthen the effect of soil environment on productivity.This study innovatively integrated the quantity and heterogeneity characteristics of soil limited resources and different scale plant diversity(?-diversity and ?-diversity).We accurately described the effects of grazing-induced changes in quantity and heterogeneity of soil resources on plant diversity,the top-down and bottom-up effects of grassland plant diversity,and fully revealed the process and regulation mechanism of grazing on grassland plant diversity.Further,we examnined the effects of grazing-induced changes in soil and plant diversity on productivity,and further determined the key factors that regulating grassland productivity in the situation of overgrazing.This study has improved our understanding about the mechanism of biodiversity and productivity maintenance in grazing grassland ecosystems,also enrichs the theory of grassland biodiversity(diversity-productivity theory),that is,grazing can regulate the diversity-productivity relationship.On the other hand,this study further provide important scientific basis for guiding the grassland management.