The Mechanism Of Simulated Grazing And Excreta Return On Plant Diversity And Community Assembly In Grasslands

Biodiversity is the prerequisite for the functional stability of grassland ecosystem and the basis for human survival.However,human activities have caused serious loss of global biodiversity,and livestock grazing is one of the important interference factors.Therefore,scientific livestock grazing management is the key to maintain or restore grassland biodiversity.The maintenance of plant diversity is the core goal of global biodiversity protection,but the uncertainty and conditional dependence of livestock grazing on plant diversity make the current grazing management face great challenges.Therefore,it is very important to reveal the regulation mechanism of livestock grazing on grassland plant diversity.Under the condition of natural grazing,there are many ways of grazing(foraging,trampling and excreting)at the same time,and there may be complex interactions,which makes it very difficult to analyze the regulation mechanism of grazing.In the past,the research on the effect of grazing on plant diversity paid more attention to the overall effect of grazing.Some control experiments analyzed that grazing was mostly simulated by cutting,and the way of excreta return was still unclear.In addition,traditional biodiversity research mostly focuses on the changes of species diversity.In recent years,functional diversity,which focuses on species functional traits,has been proved to better reflect the response of ecosystems to environmental changes.Therefore,it is of great significance to in-depth analyze the regulation mechanism of multi-dimensional diversity of grassland plants by different ways of livestock grazing for grassland grazing management and biodiversity maintenance.In this study,the Songnen grassland in northern China was taken as the research object,and the field in-situ simulation of livestock grazing experiment was used to study the two main pathways of livestock grazing affecting grassland plant diversity-foraging pathway and excreta return pathway and their interaction on the multi-dimensional(species diversity and functional diversity),multi-scale(α diversity and β diversity)diversity of plant communities and community construction process.In addition,for the return pathway of livestock excrement,this study not only tested the changes of soil resource concentration caused by excrement,which was widely concerned in most studies,but also verified the effect of soil resource heterogeneity caused by excrement by setting ideal control.The main conclusions are as follows:(1)The effect of simulated livestock foraging and excreta return on plant species diversity is scale-dependent.Simulated livestock foraging significantly increased species α diversity,but did not affect species β diversity.Excretion addition had no effect on species α diversity,but significantly increased species β diversity,species turnover reached 79.71 %.At the same time,although both uniform addition and heterogeneous addition of excreta reduced species turnover and increased species nesting,heterogeneous addition of excreta significantly increased species βdiversity compared with uniform addition of excreta.Therefore,livestock foraging can alleviate the light competition between plants,reduce the competitive exclusion between species,and promote the coexistence of regional species.The addition of excreta may change the living environment of plants and improve the effect of environmental screening on plant communities by increasing soil heterogeneity,thus promoting species β diversity.(2)Livestock foraging significantly reduced the community-weighted mean and intraspecific trait variation of plant height and leaf dry matter content,while increasing the intraspecific trait variation of plant specific root length,indicating that livestock foraging makes plant communities lower above ground and allocate more biomass to underground to compete for soil nutrients and improve resource absorption efficiency.Excrement addition had no significant effect on plant functional traits,but its interaction with ingestion significantly reduced the community weighted average and intraspecific trait variation of plant height and leaf dry matter content,and increased the intraspecific trait variation of plant specific root length.In addition,the results of this study showed that there was a significant negative correlation between plant height and specific root length,and simulated foraging and simulated foraging and excreta interactions would remove this trade-off.There was a significant positive correlation between leaf dry matter content and root dry matter content,and excreta addition would weaken this positive correlation.The results of this study support that there is a trade-off between the ability of plants to obtain resources aboveground and underground,and the addition of restricted resources will relieve the trade-off between aboveground and underground parts of plants.(3)Simulated livestock foraging and excreta return had little effect on plant functional α diversity,but significantly changed plant functional β diversity,and they had opposite effects on plant above ground and underground functional diversity.Simulated foraging significantly increased the underground functional β diversity and total functional β diversity,and its function nesting reached 61.10% and 73.37%.The return of excreta significantly increased the above g round functional diversity,and its functional nesting reached 67.79%.Compared with the nutrient addition of excreta,the heterogeneity of excreta had a significant effect on the above ground functional β diversity of plants.Livestock foraging and excreta addition may change the environment of plant survival by alleviating the restrictive light resources and soil resources in the environment,and enhance the environmental screening effect of plant communities,thereby increasing the spatial heterogeneity of plant functions and increasing the β diversity of plant functions.(4)Simulated livestock foraging and excreta return significantly changed the process of plant community construction.Simulated foraging and excreta addition promoted the relative strength of the random process to the community assembly process.This study shows that diffusion plays a major role in the construction of plant communities.In addition,livestock foraging significantly increased the niche overlap between species,while excreta addition significantly reduced the niche breadth of plant communities.At the same time,the uniform addition of excreta had a strong effect on plant niche,which significantly increased the niche breadth and niche overlap of plants,while the heterogeneous addition of excreta only significantly reduced the niche breadth.This may be due to the fact that although excreta addition promotes species β diversity,it also increases plant functional diversity by promoting functional nesting,that is,plants gather functional characteristics into characteristics related to competition for light resources,thus promoting the homogenization of plant community functions.Livestock foraging may promote the survival and reproduction of bottom plants by increasing the available light resources,thus increasing the niche overlap of plant communities.In summary,this study deeply analyzed the mechanism of the two main pathways of livestock grazing-foraging pathway and excreta return on different dimensions(species diversity and functional diversity),different scale diversity(α diversity and β diversity)and community assembly process,and innovatively explored the effect of excreta on plant diversity by increasing the effectiveness and heterogeneity of soil limiting resources.It comprehensively revealed the process and regulation mechanism of livestock foraging and excreta return on grassland plant diversity,and improved our understanding of the biodiversity maintenance mechanism of grazing grassland ecosystem.It is of great significance to accurately guide grassland grazing management.The results of this study showed that livestock foraging and excretion had different effects on plant diversity at different scales and dimensions,and there was interaction between them.In the past,the simulation of grazing for mowing treatment was not comprehensive,and the excreta of livestock should be considered.In addition,in the past,grassland grazing management mostly determined the optimal grazing intensity through the intake of livestock.This study suggested that the determination of grassland grazing intensity should not only take into account the foraging effect of livestock,but also pay attention to the changes of soil resource concentration and heterogeneity caused by the return of livestock excreta to comprehensively determine the optimal grazing intensity.