Effects Of Grazing On Plant Community And Soil Characteristics Of Robinia Pseudoacacia Forest In The Loess Hilly-gully Region

After the project of converting farmland to forest（grassland）,a large area of forest and grassland has been formed in the Loess Plateau,and its rational utilization and sustainable management are the practical problems concerned by many scientists and government departments.As a traditional mode of production,grazing occurs not only in grassland ecosystems,but also in forest ecosystems.Aiming at the feasibility of grazing under robinia pseudoacacia forest in the hilly region of the Loess Plateau,this paper takes the effect of grazing on the plant community and soil of the artificial forest as the entry point,takes the artificial robinia pseudoacacia forest over 20 years old as the research object,divides four grazing intensities according to the density of sheep dung,and selects 40grazing observation plots of the artificial forest.The total community coverage,plant density,aboveground biomass and litter amount of understory vegetation in autumn and spring were obtained through field investigation.Soil samples were collected to determine soil physical and chemical properties.The effects of grazing on community and soil properties of robinia pseudoacacia forest were studied,and the feasibility and suitability of grazing of artificial robinia pseudoacacia forest were analyzed.In order to provide the basis for the appropriate utilization and sustainable management of artificial robinia pseudoacacia forest in the Loess hilly-gully region.The main conclusions are as follows:（1）The effects of grazing on Robinia pseudoacacia plant communities in the Loess hilly-gully region were related to grazing intensity and grazing season.The grazing had no significant effect on tree height,DBH and H_cbof Robinia pseudoacacia trees above 20years of age.The grazing in summer and autumn had no effect on the total coverage and aboveground biomass of the understory plant community,and did not affect the diversity of the plant community.Grazing in winter and spring can reduce understory vegetation biomass,and the total coverage of understory plant community in light and moderate grazing is 47.30%and 47.35%lower than that without grazing.Therefore,grazing in winter and spring is not recommended.Grazing in summer and autumn increased the plant density,and the plant density of lightly grazed plots,moderately grazed plots and heavily grazed plots was 1.95 times,2.05 times and 1.88 times that of ungrazed plots,respectively.Grazing in winter and spring could reduce plant density,which was 16.06%and 15.87%for light and moderate grazing.（2）Grazing had a significant effect on soil physical properties under Robinia pseudoacacia forest in the Loess hilly-gully region.The soil moisture content of 0～200cm was increased by grazing,and the influence degree was related to grazing intensity.When the soil depth was 0～100 cm,the soil water content of each grazing intensity was heavy>light>moderate>no grazing.When the soil depth was 100～200 cm,it was heavy>moderate>light>no grazing.Except for ungrazed plots,soil moisture content increased with the increase of soil depth.The bulk density of 0～10 cm soil under moderate grazing was 1.26 times of that without grazing.The soil bulk density of 10～20 cm in lightly grazed,moderately grazed and heavily grazed plots was 1.08,1.14 and 1.09 times that of ungrazed plots,respectively.The soil bulk density of 10～20 cm was higher than that of0～10 cm under no grazing and light grazing.The reverse was true for moderate and heavy grazing.With the increase of grazing intensity,the soil water storage of 0～10 cm increased first and then decreased.The soil water storage of lightly,moderately and heavily grazed plots was 1.65 times,1.48 times and 1.32 times of that of ungrazed plots,respectively.In light and moderate grazing plots,soil water storage of 0～10 cm was higher than that of10～20 cm.The reverse was true in the ungrazed and heavily grazed plots.（3）With the increase of grazing intensity,the contents of soil organic matter and total nitrogen decreased,while the contents of total phosphorus increased first and then decreased,and all three decreased with the increase of soil depth.In the 0～10cm soil layer,soil organic matter in light,medium and heavy grazing was 84.12%,93.20%and65.77%of that in ungrazed plots,total nitrogen was 83.04%,92.88%and 67.22%of that in ungrazed plots,and total phosphorus was 3.40%,9.03%and 0.13%higher than that in ungrazed plots,respectively.In the 10～20cm soil layer,soil organic matter in light,moderate and heavy grazing was 66.57%,72.92%and 54.65%of that in ungrazed plots,total nitrogen was 67.11%,74.55%and 56.36%of that in ungrazed plots,and total phosphorus was 5.48%,10.83%and 2.79%higher than that in ungrazed plots,respectively.（4）Grazing intensity changed the interaction between plant community and soil physicochemical indexes in Robinia pseudoacacia forest.The correlation between plant community indexes was more obvious than soil indexes under different grazing intensities.Grazing affected the aboveground biomass of the community by affecting the total community coverage,litter amount,soil water content of 100～200 cm and soil bulk density.In addition to directly affecting the aboveground biomass of the community,soil moisture content of 100～200 cm affected the aboveground biomass of the community indirectly by affecting soil moisture content and soil organic matter of 0～100 cm,and soil moisture content of 0～100 cm affected the aboveground biomass of the community indirectly by affecting soil bulk density.The amount of soil organic matter and litter indirectly affected the aboveground biomass by affecting the total community coverage.Considering the effects of different grazing intensity on plant community and soil physical and chemical properties of Robinia pseudoacacia forest and understory,the understory grazing of mature Robinia pseudoacacia forest can maintain the plant community and water nutrient status of robinia pseudoacacia forest when the grazing is controlled below 3 sheep·hm^-2(the density of sheep dung is 10.0 ind·m^-2).