Effects Of Grazing Exclusion On Carbon Exchange In Alpine Shrubland-Grassland Ecotone In The Eastern Qilian Mountains

Qilian Mountains,as an important source of water to the Yellow River,is an important component of the ecological barrier of west China.Qilian Mountains play irreplaceable role in maintaining sustainable development in the Hexi Corridor and preventing desertification.Al-pine shrubland is the main vegetation type in the Qilian Mountains,and play important ecolog-ical roles in maintaining the soil and water conservation,carbon storage,animal husbandry production and the stability and balance of terrestrial ecosystems.Transitions between alpine shrubland to alpine meadow are shrubland-grassland ecotone,which is not only an occurrence area of high plant biodiversity but also is a special response area for amplifying external dis-turbance signals.Furthermore,this area is most susceptible to climate change,especially cli-mate warming,which is regarded as potential indicator of environmental change.Thus,the ecotones are ideal research area for studying anthropogenic disturbance and forecasting climate change.However,the alpine shrubland is a very fragile ecosystem,which is not able to with-stand trampling and is also vulnerable to climate change and grazing disturbance.Therefore,the adverse natural conditions and human activities result in alpine shrubland degradation.Grass-land degradation not only affects economic sustainability and development but also threatens social stability and ecological security.Grazing exclusion using mesh fencing is considered the most effective management strategy applied to restore degraded grasslands worldwide.Howev-er,the effects of grazing exclusion on carbon source-sink relationship remain unclear of a de-graded alpine shrubland.In order to understand the impacts of grazing exclusion on the carbon fluxes and ecosystem carbon storage of degraded alpine shrubland this study was carried out.In this study,we selected an alpine shrubland with different degradation levels（lightly degraded and heavily degraded）to determine the effects of grazing exclusion on vegetation composition and structure,soil characteristics,gross ecosystem primary productivity（GEP）,ecosystem res-piration（Re）and net ecosystem exchange for CO₂（NEE）at different ecocline（shrubland area,ecotone zone,grassland area）of the degraded shrubland.The main findings are as follows:（1）Grazing exclusion significantly improved the herbaceous and shrub height,coverage,above-and below-ground biomass,litter biomass,soil moisture content,soil organic carbon,soil total nitrogen,soil total phosphorus,soil available nitrogen,soil available phosphorus,soil available potassium,soil ammonia nitrogen and soil ammonia nitrogen contents in different ecocline of lightly and heavily degraded shrubland.Although grazing exclusion could be effec-tive in the restoration of lightly and heavily degraded shrublands,the recovery effect is better for heavily degraded alpine shrub than lightly degraded shrubland.Grazing exclusion signifi-cantly reduced the shannon-wiener diversity index,evenness index,richness index,simpson index of the herbaceous and the whole community,soil bulk density and soil compaction in each ecocline of lightly and heavily degraded shrubland.However,grazing exclusion had no significant effect on diversity of shrub and bacterial community.Structural equation modeling showed that soil physical properties were the most important determining factors leading to an increase in above-ground biomass in each ecocline of the lightly and heavily degraded alpine shrubland after grazing exclusion,whereas the soil chemical properties were the most important determining factors leading to a decrease in biodiversity.（2）During the whole growing season（May-October）,the GEE exhibited a hump-shaped pattern with a July threshold in each ecocline of the lightly and heavily degraded shrubland.Meanwhile,grazing exclusion significantly increased the GEE in each ecocline of lightly and heavily degraded alpine shrubland,but the recovery effect is better for heavily degraded alpine shrubland than lightly degraded shrubland.The GEP inside the fence was significantly in-creased by 9.2%-9.4%,10.0%-17.1%and 8.0%-12.1%,respectively,in the grassland area,eco-tone zone and shrubland area of the lightly degraded shrubland,compared to the outside the fence.The GEP inside the fence was significantly increased by 921.4%-30.3%,23.3%-28.7%and 25.3%-27.8%,respectively,in the grassland area,ecotone zone and shrubland area of the heavily degraded shrubland,compared to the outside the fence.Structural equation modeling showed that soil chemical properties were the most important determining factors leading to an increase in GEE in each ecocline of the lightly and heavily degraded alpine shrublands after grazing exclusion（3）During the whole growing season,the carbon output exhibited a hump-shaped pattern with a July threshold in each ecocline of the lightly and heavily degraded shrubland.Meanwhile,grazing exclusion significantly increased the carbon output in each ecocline of lightly and heav-ily degraded alpine shrubland.Structural equation modeling showed that soil chemical proper-ties were the most important determining factors leading to an increase in carbon output in each ecocline of the lightly and heavily degraded alpine shrubland after grazing exclusion.（4）During the whole growing season,both the lightly and heavily degraded alpine shrub-land were carbon sink,and the NEE exhibited a hump-shaped pattern with a July threshold in each ecocline of the lightly and heavily degraded shrubland.Meanwhile,grazing exclusion sig-nificantly increased the NEE in each ecocline of lightly and heavily degraded alpine shrubland,but the recovery effect is better for heavily degraded alpine shrub than lightly degraded shrub-land.Meanwhile,the carbon sink capacity of shrubland area was higher than that of ecotone zone and grassland area.The NEE in the grassland area,ecotone zone and shrubland area was4.7-4.9,4.9-5.0 and 5.5-5.7μmol·m^-2·s^-1,respectively,in the lightly degraded shrubland.The NEE in the grassland area,ecotone zone and shrubland area was 4.0-4.1,4.3-4.4 and 4.7-5.0μmol·m^-2·s^-1,respectively,in the heavily degraded shrubland.Structural equation modeling showed that soil chemical properties were the most important determining factors leading to an increase in carbon sink capacity in each ecocline of the lightly and heavily degraded alpine shrubland after grazing exclusion.（5）Grazing exclusion significantly increased the vegetation and 0-30 cm layer soil organic carbon storage in each ecocline of the lightly and heavily degraded shrubland.Grazing exclu-sion significantly increased the ecosystem organic carbon storage in each ecocline of the heavi-ly degraded shrubland,and only significantly increased the ecosystem organic carbon storage in grassland area of the lightly degraded shrubland.The ecosystem carbon storage inside the fence was significantly increased by 16.9%-18.5%,10.3%-13.0%and 16.0%-18.1%,respectively,in the grassland area,ecotone zone and shrubland area of the heavily degraded shrubland,com-pared to the outside the fence.The ecosystem carbon storage inside the fence was significantly increased by 12.2%-16%in the grassland area of the lightly degraded shrubland,compared to the outside the fence.Although grazing exclusion could effectively restore the vegetation and soil characters,and improve the NEE and ecosystem organic carbon storage of the lightly and heavily degraded shrubland,the recovery effects were better for heavily degraded alpine shrub than lightly de-graded shrubland.However,considering the effective utilization of grassland resources,update,and the economic income of herdsmen,when we manage alpine shrub grassland and when we conducted functional zoning of Qilian Mountain national park,the enclosure measures of heav-ily degraded alpine shrubland can be selected as the priority.But for the lightly degraded alpine shrubland,measures such as rested grazing in the growing season can be considered to gradu-ally improve its ecological and production functions.