Effects And Mechanisms Of Grassland Utilization On Ecosystem Carbon Sequestration Function In Global Change Scenarios

The global warming caused by greenhouse gas emissions has seriously threatened the living environment and health of human beings,so the global carbon cycle has become a hot issue that people are concerned about.Grassland ecosystem is an important part of terrestrial ecosystems and plays an important role in the global carbon cycle and mitigation of greenhouse gas emissions.Grazing and mowing are the main measures of grassland management,which play important roles in regulating the carbon cycle of grassland ecosystem.A large number of previous studies have shown that moderate utilization can improve the carbon sequestration of grassland ecosystem.At present,grasslands are facing multiple global changes,such as global nitrogen deposition and biodiversity loss.These global changes will have serious impacts on the carbon sequestration of grassland ecosystem.However,it is remained unclear about whether appropriate grassland utilization can still be an important grassland management method to maintain the grassland carbon sequestration in the global change scenarios.Therefore,this study examined the interaction effects of global change factors（simulating plant diversity losses and atmospheric nitrogen deposition）and moderate grassland grazing or mowing utilization on soil organic carbon stocks,ecosystem carbon fluxes（gross ecosystem productivity,ecosystem respiration,soil respiration,above-ground respiration and net ecosystem CO₂exchange）and plant carbon allocation of above-ground and below-ground.Based on the changes of plant and soil characteristics,we analyzed the regulation mechanisms of moderate grassland utilization on ecosystem carbon sequestration under global change scenarios（plant diversity losses and atmospheric nitrogen deposition）.Moreover,based on three-years（2016,2018,2020）at 8 sites,we further evaluated the potential impact and mechanism of over-utilization on soil organic carbon across three main grassland types（meadow steppe,typical steppe and desert steppe）in northern China.We obtained important results as follows.（1）Based on the study about the effects of moderate grassland utilization（sheep grazing in from 2015 to 2018）on soil organic carbon stocks under simulated nitrogen deposition and plant diversity losses scenarios,we found that the effect of nitrogen deposition on soil organic carbon stocks depends on grassland plant diversity.In low-diversity grasslands,nitrogen deposition had a negative effect on soil organic carbon stocks,while it had no significant effect on soil organic carbon stocks in high-diversity grasslands.Moderate grazing significantly improved soil organic carbon stocks,and its effect is independent of grassland plant diversity.Meanwhile,moderate grazing could alleviate the negative effect of nitrogen deposition on soil organic carbon in low-diversity grassland.Moderate grazing mainly altered the negative effects of nitrogen deposition on soil organic carbon stocks through direct pathways（such as changing root exudates）and indirect pathways mediated by below-ground biomass and soil p H.This study showed that the loss of plant diversity could aggravate the negative effect of nitrogen deposition on soil carbon storage.Moderate grazing could improve soil carbon sequestration,thus alleviating the negative effect of nitrogen deposition on soil carbon storage under plant diversity losses scenarios.（2）Based on the study about the effects of moderate grassland utilization（mowing in autumn from 2018 to 2020）on soil organic carbon stocks under simulated nitrogen deposition and plant diversity losses scenarios,we found that the variation trend of ecosystem carbon fluxes were consistent under each treatment with the peak in July and August.Plant diversity losses reduced gross ecosystem productivity（GEP）and net ecosystem CO₂exchange（NEE）,and moderate mowing can alleviate the negative effects of plant diversity losses on GEP and NEE under nitrogen deposition scenarios.In addition,nitrogen deposition and moderate mowing had interactive effects on ecosystem carbon fluxes.Nitrogen deposition had no significant effects on ecosystem carbon flux in high-diversity grasslands.Under plant diversity losses scenarios,nitrogen deposition had no significant effect on GEP,but it increased ecosystem respiration（ER）and thus decreased NEE.In the absence of nitrogen deposition,moderate mowing significantly increased NEE in high-diversity grasslands,but it had no effect on NEE in low-diversity grasslands.Moderate mowing significantly increased GEP,ER and NEE in both high and low-diversity grasslands in the presence of nitrogen deposition.Our study further indicated that above-ground biomass,below-ground biomass and leaf nitrogen content were important factors to regulate the responses of GEP,ER and NEE to nitrogen deposition and mowing.Therefore,this study suggests that moderate mowing can alleviate the negative effects of plant diversity losses and nitrogen deposition on grassland ecosystem carbon sequestration.（3）Based on the study about the effects of moderate grassland utilization（mowing in autumn）on plant carbon allocation of above-ground and below-ground under simulated nitrogen deposition and plant diversity losses scenarios,we found that the effect of nitrogen deposition on above-ground biomass depended on the inter-annual precipitation variation,which increased above-ground biomass in the wet years（2019 and 2021）,but had no effects in the dry years（2018 and 2020）.Thus nitrogen deposition decreased above-ground biomass stability of plant communities.Moderate mowing had no significant effect on above-ground biomass in the absence of nitrogen deposition,while in the presence of nitrogen deposition,moderate mowing significantly increased above-ground biomass in both dry and wet years,and thus significantly improved plant community stability.In addition,nitrogen deposition and moderate mowing altered allocation strategies of above-ground and below-ground biomass,and the effects depended on plant diversity.In high-diversity communities,nitrogen deposition,mowing and their interactions had no significant effects on allocation strategies of above-ground and below-ground biomass.This is consistent with"allometric partitioning theory".However,nitrogen deposition under plant diversity losses scenarios increased allocation to above-ground biomass in the wet years,but had no significant effects in the dry years,which is in line with"multiple constraint hypothesis".However,in the presence of N deposition,moderate mowing significantly increased above-ground biomass allocation,and it was independent of annual precipitation.The results were consistent with the"optimal allocation theory".Therefore,nitrogen deposition may change plant carbon allocation strategies of above-ground and below-ground and reduce the stability of above-ground biomass,while moderate mowing can alleviate the negative effects of nitrogen deposition and improve above-ground biomass and the stability of plant communities.（4）Based on the study about the effects of long-term overgrazing on soil organic carbon in northern China,we found that long-term overgrazing significantly reduced soil organic carbon concentration,and changed the relative contributions of plant biomass（above-ground and below-ground biomass）,biodiversity（plant diversity and microbial diversity）,and soil environment（soil moisture and bulk density）to soil organic carbon concentration.In the long-term enclosure grasslands,plant biomass,biodiversity and soil environment jointly regulated soil organic carbon concentration.However,overgrazing can greatly reduce the regulation effect of biodiversity on soil organic carbon concentration and the change of the regulation effect of biodiversity on soil organic carbon concentration was mainly attributed to the decrease in the regulation effect of microbial diversity on soil organic carbon concentration caused by overgrazing.Moreover,overgrazing increased the regulation effect of soil environment on soil organic carbon concentration.Therefore,we suggest that in the face of the current over-utilization of large-scale grasslands,improving the soil environment may be the key way to maintain the grassland carbon sequestration.In conclusion,moderate grazing and mowing can be an effective grassland management method under global changes scenarios to maintain grassland carbon sequestration and mitigate the negative effects of plant diversity losses and nitrogen deposition on grassland carbon sequestration.However,over-utilization will greatly reduce soil organic carbon,weaken the regulation effect of microbial diversity on soil carbon sequestration,and enhance the regulation effect of soil environmental factors on soil carbon.Our study explored the effects of grassland utilization on multiple components of ecosystem carbon sequestration（ecosystem carbon fluxes,plant carbon pools,soil carbon stocks）under the multiple global change scenarios（atmospheric nitrogen deposition and plant diversity losses）,revealed the regulation mechanism of ecosystem carbon sequestration.The results further provide important theoretical basis and guiding significance for the adaptive management of grassland ecosystems,the mitigation of global warming,and the realization of"emission peak"and"carbon neutrality"in China.