Effects Of Yak Grazing On CO₂ Flux In Alpine Meadow During The Growing Season

As one of dominant vegetation types on the Qinghai-Tibetan Plateau?QTP?,alpine meadow is widely used for grazing.Over the past few decades,QTP has experienced obviously environmental changes,including increasing temperature and enhanced human activity,which have greatly altered carbon balance of alpine meadow ecosystem.Although numbers of warming experiment have been conducted on the QTP,there is still some controversies regards to how grazing influences carbon absorption and emission in alpine meadow ecosystem.Thus,in this study,the dynamics of carbon uptake and release in an alpine meadow ecosystem under yak grazing were detected during the growing season in 2015 and 2016 through long-term continuous observations.Second,the impacts of radiation change on the potential net ecosystem productivity?NEPpot?were evaluated by simulating the potential photosynthetic active radiation?PARpot?.Finally,the impacts of yak dung patches on the dynamics of alpine meadow CO₂ flux were explored.The results are as follows:1)Both net ecosystem production?NEP?and ecosystem respiration?Re?showed a similar changing trend in non-grazing and grazing plots during the whole growing season.NEP showed an obvious diurnal variation trend,i.e.carbon uptake occurred at 7:30 am to 19:00 pm and carbon emission was found at 19:00 pm to 7:30 am.The maximum appeared in between 12:00 pm and 14:00 pm,while the minimum Re was detected in between 6:00 am and 8:00am in non-grazing and grazing plots.Except the early stage of experiment,the daily carbon uptake rate of NEP,the carbon emission rate of NEP and the changing rate of Re in no-grazing plots were all significantly higher than that in grazing plots?P<0.05?.2)Grazing significantly decreased gross primary production?GPP?and Re by reducing aboveground biomass?P<0.05?,and thus resulted in a significant decrease of NEP?P<0.05?.The total GPP,Re,NEP of grazing plots was 319.08 g C·m^-2,230.28 g C·m^-2,88.65 g C·m^-2,which was 40.94%,35.01%,52.28% lower than that in no-grazing plots?P<0.05?.The effect of grazing on NEP is more than Re.3)The long-term continuous observations showed NEP was mainly affected by?photosynthetically active radiation?PAR.The relationship of NEP and PAR was logarithmic function.Re was mainly controlled by soil temperature at 5 cm depth and an exponentially functional relationship was detected between them.The correlation coefficients between Re and soil temperature at 5 cm depth in no-grazing plots were higher than that in grazing plots.Q10 value was higher in no-grazing plots than that in grazing plots,i.e.grazing reduced the sensitivity of Re to 5 cm soil temperature.4)Through simulating PARpot,we found the annual mean potential gross primary production?GPPpot?and GPP were 11.35 and 8.99 ?mol·m^-2·s^-1 in grazing plots,which were significantly lower than that in no-grazing plots by 51.91% and 48.69%?P<0.05?;the annual mean NEPpot and NEP were 6.79 and 3.20 ?mol·m^-2·s^-1 in grazing plots,which were significantly lower than that in no-grazing plots by 53.40% and 55.86%?P<0.05?;the annual mean Re was 5.79 ?mol·m^-2·s^-1 in grazing plots,which was significantly lower than that in no-grazing plots by 36.30%?P<0.05?.By simulating the maximum PAR,GPPpot significantly increased by 35.98% and 26.25%?P<0.05?than GPP in grazing and no-grazing plots;NEPpot significantly increased by 100.97% and 112.19%?P<0.05?than NEP in grazing and no-grazing plots.In both grazing and no-grazing plots,simulating PARpot reduced the correlation of NEPpot and PARpot.5)In grazing plots,NEPpot gradually increase with the increase of PARpot and 5 cm soil temperature.In no-grazing plots,an upper limit was found with the increase of PARpot and 5 cm soil temperature,i.e.NEPpot was observed when daily mean soil temperature at 5 cm depth reached 19.75? and daily mean PARpot reached 1875 ?mol·m^-2·s^-1.6)After covering by dung patches,the mean CO₂ release rate was found significantly increased by 0.61,2.96,3.16 times than normal in June,July,and August,respectively?P<0.05?.CO₂ fluxes in each month was significantly correlated with soil temperature at 5 cm depth.But the coverage of dung patches could reduce the sensitivity of CO₂ fluxes to soil temperature.This results may support to correctly evaluate the influences of yak grazing on QTP carbon uptake and release balance and greenhouse gases emission.