Effects Of Grazing,Wind Erosion,and Dust Deposition On Plant Productivity In A Typical Steppe Of Northern China

As the largest terrestrial ecosystem,grasslands play vital roles in providing livestock production,preserving biodiversity,influencing regional carbon cycle,and regulating climate change.As one of the widely used grassland management practices,grazing can change plant community composition and structure by selective forging and trampling,with subsequent effects on net primary productivity.The semi-arid temperate steppe in Northern China is extremely vulnerable to wind erosion with insufficient precipitation and frequent wind.Wind erosion can suppress plant growth by destroying surface soil and shallow roots.In addition,given the reductions in plant cover under grazing,grassland is exposed to more risk of wind erosion,which will intensify the degradation of grassland.The dust deposition caused by wind erosion will also have unpredictable effects on ecosystem structure and functioning.Soil nutrient enrichment can facilitate plant growth,subsequently intensifying interspecific competition.Net primary productivity is one of the key indicators assessing the degree of the degradation and supporting ecosystem functioning.Previous studies mainly focus on the responses of plant community composition and structure to grazing and soil erosion(wind erosion or dust deposition),but the understanding of how grazing,soil erosion,and their interaction regulate the net primary productivity of grasslands remains unclear.This thesis analyzed five-year(2016-2020)data of a manipulative experiment which simulated grazing(control,light-intensity grazing)and soil erosion(control,wind erosion,and dust deposition)by removing and adding 1-cm topsoil in a temperate steppe in Inner Mongolia.One sheep was used to grazing once per month for 4 h from June to September in each plot of grazing.Topsoil of 1 cm was transported from plots of wind erosion to each plot of dust deposition,respectively.Results showed that:(1)Over the five years,grazing did not affect soil moisture,whereas wind erosion and dust deposition increased soil moisture by 2.61% and 0.85%(absolute change),respectively.In addition,grazing and wind erosion damaged soil total nitrogen content(TN)by 2.98% and 15.2%,respectively,but dust deposition elevated it by 9.28%.Though grazing and wind erosion increased species richness by 10.6% and14.3%,respectively,dust deposition decreased it by 10.3%.(2)Grazing decreased aboveground net primary production(ANPP)by 34.7% through directly inhibiting plant growth under livestock foraging and indirectly affecting plant growth resulted from reduced TN and increased species richness.Wind erosion suppressed ANPP by 33.2%,largely due to reduction in TN and increase in species richness.Given the stimulated TN and reduced species richness,dust deposition elevated ANPP by 17.9%.(3)When analyzed separately by plant functional groups,grazing stimulated aboveground biomass of shrubs and semi-shrubs by 73.4%,but decreased aboveground biomass of annuals and biennials(AB,58.0%),perennial forbs(PF,59.2%),and perennial grasses(PG,75.4%).Wind erosion did not affect aboveground biomass of AB or PF,but suppressed aboveground biomass of PR and SS by 47.3% and 51.6%,respectively.Aboveground biomass of SS was increased by 19.4% under dust deposition,but aboveground biomass of the others did not respond to dust deposition.(4)Grazing had no effect on belowground net primary production(BNPP).Wind erosion reduced BNPP by 35.9%,which resulted from decreased TN.However,the elevated BNPP(34.5%)under dust deposition could be driven by enhanced TN.Plants allocated more energy and resources to the roots,which maintained BNPP stability to adapt to the pressure of grazing,and led to an increase in the root-to-shoot ratio(35.8%)under grazing.The consistent response of ANPP and BNPP result in neutral responses of root-toshoot ratio to wind erosion and dust deposition,respectively.Given the whole plant community,soil moisture,TN,and species richness together leading the changes of ANPP.The findings quantified the effects of grazing,wind erosion,and dust deposition on grassland productivity,which would improve understanding of vegetation dynamics and carbon sinks under future anthropogenic and environment disturbance in temperate grassland,and further optimize grassland management practices.