Greenhouse Gas Emissions Of Alpine Meadow Grazing Systems On The Qinghai-Tibet Plateau

The Qinghai-Tibet Plateau（QTP）is an important livestock production base in China,and yak and Tibetan sheep are the main grazing animals.GHG emissions from livestock grazing systems cannot be ignored for regional or global greenhouse gas emission inventories.This study was conducted in alpine meadow systems on the QTP.The GHG emissions from yak grazing systems were measured during 2012-2013.Meanwhile,the GHG emissions from Tibetan sheep grazing systems were observed by buliding grazing gradient and different doses of nitrogen addition during2012-2014.Moreover,we also measured the GHG emissions of non-growing season and the contribution to the annual total GHG emissions in alpine meadow during 2016-2017.The main conclusions are as follows:（1）The GHG fluxes of N₂O and CH₄ from night pens and composting manure heaps per unit area were expectedly high,with yak night pens being an important N₂O emission source,in the yak grazing alpine meadow systems on the QTP.Total annual GWP（CO₂-eq.）from the typical 83 ha family farm with an average stocking rate of 1.05 yaks ha^-1 is 334.2 tonnes.The yak grazing family farm system has a GWP mitigation status during the early and peaking growing seasons,but is a large carbon source during the non-growing season.The contributions of CH₄ and N₂O（21.1%and44.8%,respectively）to the annual total GHG budget strongly suggest these GHG should not be ignored when estimating GWP（CO₂-eq.）in the grazing alpine meadows on the QTP for the purposes of determining national and regional land use policies or compiling global GHG inventories.（2）The GHG emissions from Tibetan sheep systems were investigated during 2012-2014.The objectives of these experiments were to 1)examine the effect of grazing intensity and nitrogen addition on the GHG fluxes in sheep grazed alpine meadow ecosystems;2)examine the dynamics of vegetation characteristics and soil physical and chemical properties on GHG emissions;3)identify the key factors affect the GHG fluxes.The CO₂ fluxes between alpine meadow soil and ecosystem respiration follow a similar pattern,the CO₂ fluxes in peak growing season>early growing seasom>non-growing season.The CO₂ fluxes from ecosystem respiration were larger than soil respiration.Grazing and nitrogen addition both promoted soil and ecosystem respiration of alpine meadow,however,the interaction of grazing and nitrogen addition inhibited the soil and ecosystem respiration,the larger the grazing intensity,the stronger the inhibition.The alpine meadow soil and ecosystem absorbed CH₄,which were the CH₄ sink,in the early growing season,peak growing season and non-growing season.Regardless of growing or non-growing season,grazing reduced the CH₄ absorption,nitrogen addition promoted CH₄ absorption,but both did not change the CH₄ sink funtion of alpine meadow.The alpine meadow soil and ecosystem were N₂O emissions sources in the whole year;grazing,nitrogen addition and the interaction of both can significantly promote N₂O emissions.Nitrogen addition increased the aboveground biomass（AGB）of alpine meadow,the higher the level of nitrogen addition,the stronger the promotion.Light stocking rate（LSR）increased the AGB,but the high stocking rate（HSR）decreased.The interaction between nitrogen addition and grazing weakened the effect of HSR on the decreasing of AGB.The influence of grazing and nitrogen addition on the belowground biomass（BGB）of alpine meadow（0-30 cm）varied greatly.The ratios of root to shoot ratios of alpine meadow were increased by several year continuous nitrogen addition.The effects of grazing,nitrogen addition and their interaction on the alpine meadow soil microbial biomass carbon and nitrogen（SMBC&N）in 0-5 cm and 5-10 cm depth were not significant.Soil SMBC&N content in 0-5 cm depth was higher than 5-10 cm.The content of soil organic carbon（SOC）and total nitrogen（TN）in alpine meadow decreased with the increase of soil depth.The influences of grazing and nitrogen addition on SOC and TN content in 0-5 cm and 5-10 cm depth were larger than 10-20 cm and 20-30 cm depth,that is the effect of grazing and nitrogen addition on SOC and TN content gradually decreased with increasing soil depth.The aboveground biomass is the key factor of soil and ecosystem respiration and the ecosystem CH₄ absorption.Soil microbial biomass nitrogen（SMBN10）in 5-10 cm depth is the key factor for N₂O emission fluxes of the ecosystem,and the total nitrogen（TN10）in 5-10cm depth is the key factor for the soil and ecosystem N₂O emission fluxes.The soil microbial biomass carbon（SMNC5）in 0-5 cm depth,total nitrogen（TN10）in 5-10 cm depth and aboveground biomass（AGB）were the key factors for soil CH₄absorption.The analysis of structural equation model shows that ST is the most important predictor of CO₂ flux,and plant factor and soil factor are important predictors of CH₄ flux and N₂O flux,respectively.（3）The contribution of cumulative CO₂ fluxes of non-growing season to the annual total fluxes is small in alpine meadow systems on the QTP.Alpine meadow in non-growing season can absorb CH₄,which is the CH₄ sink.The contribution of cumulative CH₄ absorption fluxes and cumulative N₂O emission fluxes in non-growing season to the annual total emissions were 45.5%and 44.4%,respectively,which were higher than cumulative CO₂ emission fluxes and were significant to the total annual emissions.The cumulative fluxes of CO₂,CH₄ and N₂O during the freeze-thaw period accounted for larger proportions of the cumulative fluxes of the non-growing season,with 56.6%,60.4%and 43.6%,respectively.The effect of grazing on CO₂,CH₄ and N₂O fluxes were not significant during the whole year study,however,nitrogen addition can significantly promote the alpine meadow ecosystem N₂O emission,the higher the nitrogen addition level,the stronger the promotion.