Study On Ecological And Hydrological Regulation Mechanism Of Grazing And Fertilization On Semi-arid Grassland

Global changes caused by human activities have caused profound responses in the terrestrial biosphere and affected the ecological products and services provided by the terrestrial biosphere to humans,which in turn threaten ecological security and sustainable socio-economic development.The temperate grasslands in the northern part of China are an important part of the Eurasian steppe,one of the largest biota in the world,and the most important supply base and ecological barrier for livestock products in China,but the ecosystem in this region is relatively fragile.Since 1998,China has implemented a large number of ecological restoration projects,and the main use of grasslands has changed from free grazing to rotational grazing or rest grazing.Since the industrial revolution,the rate of atmospheric nitrogen deposition has increased year by year due to the burning of fossil fuels and the extensive use of chemical fertilizers,which has had a profound impact on grassland soil and plant characteristics.Phosphorus is another major nutrient element necessary for plant growth,and the imbalance of long-term nitrogen and phosphorus deposition may lead to a shift from nitrogen limitation to phosphorus limitation of plant growth in China’s temperate grassland ecosystems.Coupled with the fact that plant growth in semi-arid grasslands is mainly constrained by scarce precipitation,high evaporation capacity,and low soil nutrient content,systematic evaluation of the response of grassland ecosystem community structure and ecohydrology to land use practices and climate change can help to develop reasonable grassland management measures.In this study,we analyzed the effects of land use practices and nitrogen and phosphorus nutrient additions on soil physicochemical characteristics and plant community structure in semi-arid grasslands in northern China,and obtained the following results.(1)The effects of Fertilization on soil infiltration characteristics and vegetation community of semi-arid grassland were expounded.The experimental system established in 2014 was used to study the response of soil infiltration characteristics and community structure to nitrogen and phosphorus additions.It was found that high N addition(10 g N/m2.a)significantly reduced soil bulk density(6.6%)and increased soil porosity(3.0%)and initial soil infiltration rate(12.1 mm/h)compared to no N addition treatment.Compared with the no-phosphorus treatment,the stable infiltration rate of soil was increased by 1.9 mm/h.The total community cover was significantly increased by 16.4%and 21.8%for the low and high N additions,respectively,and the grass cover was significantly increased by 11.9%and 22.2%,respectively,compared with the no-N additions.With the increase of N addition,the more severe soil acidification,the faster the decrease of species richness and diversity index.The effect of N and P additions on total community cover and functional group cover of miscellaneous grasses had an interactive effect,under the high N treatment,low P additions significantly increased total community cover and functional group cover of miscellaneous grasses.5 years of N and P additions did not change the soil total carbon content,indicating that the response of soil carbon sequestration to N and P additions in the study area takes a longer time.(2)The effects of different grazing practices on the ecological functions of semi-arid grasslands were elucidated.Fifteen sample plots with different grazing management measures,including free grazing,warm season grazing and fallow grazing,were selected to explore the response of plant and soil properties to grazing management.The results showed that the species richness,Shannon-Wiener index and the proportion of weed functional groups were higher in the warm season grazing sites than in the free grazing sites.Soil bulk density,pH,Pielou evenness index,and proportion of legume functional groups did not differ among the three different grazing practices samples,but aboveground biomass,root biomass,canopy cover,and height of vegetation communities were higher in the fallow area than in the free-grazed area.Soil organic carbon and total nitrogen content decreased with increasing grazing intensity,with the lowest and highest values occurring in the free-grazing and resting areas,respectively.In addition,soil organic carbon and total nitrogen content were positively correlated with root biomass.The study showed that the resting grazing method is a suitable management strategy for carbon and nitrogen fixation,and moderate grazing is a suitable management strategy for the conservation of grassland diversity in the temperate zone of northern China.(3)The effects of grazing and fertilization on biomass and species communities in semi-arid grasslands were elucidated.The responses of grassland productivity,plant diversity,community level and individual level plant stoichiometry to land use practices and nutrient addition were systematically investigated.Grazing significantly reduced above-ground biomass and apoplankton biomass of grassland,and nitrogen application significantly increased above-ground biomass.The proportion of functional groups of grasses significantly decreased by 36.55%and the proportion of functional groups of miscellaneous grasses significantly increased by 62.27%under the grazing treatment compared to the control.Nitrogen application under the no-grazing treatment resulted in 32.15%of the functional group percentage of grasses,while N application under the grazing treatment reduced the percentage of grasses in the grass by 47.98%.Grazing significantly increased the Shannon-wiener diversity index and Pielou evenness index.Nitrogen application significantly reduced the species richness of grassland,with about 4 species lost per square meters.Grazing had no effect on aboveground biomass carbon,apoplastic carbon,and root biomass carbon content of the grassland.Nitrogen application reduced the carbon to nitrogen ratio of each component at the community level by increasing the aboveground biomass,apoplankton,and root nitrogen contents at the community level.The effect of grazing on carbon content of different species differed significantly among species:grazing increased the carbon content of grasses such as needlegrass and cryptomeria,and decreased the carbon content of chrysanthemum and lentil plants,and the carbon content of plants in different functional groups responded differently to grazing disturbance.The effect of nitrogen application on the carbon content of different species differed significantly among species:nitrogen application increased the carbon content of Chrysanthemum and decreased the carbon content of Cryptomeria,but had no effect on the other three species.Moderate warm season grazing in this study could promote Shannon-wiener diversity index and Pielou evenness index,indicating that reasonable and moderate grazing in this area is a reasonable management strategy to maintain species diversity.Nitrogen application had a stimulating effect on above-ground biomass of grassland,indicating that N addition is one of the effective management measures to improve grassland productivity.(4)The regulatory mechanisms of grazing and fertilization on the ecohydrological characteristics of semi-arid grasslands were revealed.To address the current uncertainties about the effects of grazing and N application on soil structure and soil infiltration characteristics,the response of soil infiltration,soil aggregate composition,and soil chemical characteristics to grassland utilization and N addition was systematically investigated using a two-factor experimental platform of grazing and N application.The results showed that N application significantly increased the initial soil infiltration rate by reducing soil bulk density(P=0.02).The initial infiltration rate and mean infiltration rate of the soil were the highest under N application alone,at 306.46 ± 29.93 mm/h and 65.14 ± 3.94 mm/h,respectively.Nitrogen application increased the proportion of microagglomerates in the range of 0.053-0.25 mm and decreased the content of<0.053 mm particle size.Grazing significantly reduced the stable infiltration rate(P=0.004)and the mean infiltration rate(P=0.02)of the soil,and the mean infiltration rate of the grazing treatment decreased by 10.79 mm/h.Grazing reduced the content of 0.25-2.0 mm soil macroagglomerates,the proportion of 0.053-0.25 mm microagglomerates,and increased the content of<0.053 mm particle size.Moderate grazing utilization in this area did not reduce the soil carbon pool in this area and did not decrease the soil total nitrogen content,indicating that grazing is a reasonable utilization method.