Effects Of Precipitation Changes In Different Periods Of Growing Seasons On Ecosystems Carbon Fluxes Of Typical Grassland In China

Climate warming strengthens atmospheric circulation and hydrological cycle,leading to the changes in precipitation patterns.Increasing extreme precipitation events during the growing season in the temperate grasslands of northern China,where water is one of the main limiting factors,cause dramatical changes of the intra-and inter-annual precipitation.Net ecosystem gas exchange(NEE),as the difference between ecosystem respiration(ER)and gross ecosystem productivity(GEP),is an important indicator of carbon balance in terrestrial ecosystems.Most studies in recent years have focused on the responses of carbon cycle to changing precipitation magnitude,but few studies have explored the effects of variations in the timing of extreme precipitation during the growing season on the carbon cycle.Simulating changes in precipitation amounts in different periods of the growing seasons to study the effects of precipitation changes on ecosystems carbon cycle of typical grassland in northern China can enrich and predict the future net carbon balance of temperate grasslands in northern China.In 2015,a manipulation precipitation timing experiments were set up in the semi-arid grassland ecosystems in northern China to investigate the effects the timing of extreme precipitation occurrence on carbon flux.There were seven treatments,including control(C),60% decrease in precipitation in the early growing season(April-June,DEP),60% decrease in precipitation in the late growing season(JulySeptember,DLP),60% decrease in precipitation during the entire growing season(April-September,DP),60% increase in precipitation in the early growing season(April-June,IEP),60% increase in precipitation in the late growing season(July-September,ILP),and 60% increase in precipitation during the entire growing season(April-September,IP).Each treatment had five replicates resulting in 35 plots in total.The main results were as follow:(1)The results from 2015 to 2019 show that reduced precipitation have a significant negative effect on ecosystem carbon flux.Specifically,DEP significantly reduced GEP and ER by 12.52 and 12.72%,and DLP significantly reduced GEP and ER by 27.47 and 20.26%.Compared with DEP,DLP significantly reduced NEE by 34.72%.The decrease in plant community cover caused by the reduced precipitation explained the decline of GEP.Reduced plant community cover and the ratio of plant community carbon to nitrogen after precipitation reduction led to the decline of ER.Results indicate that drought in late growth season weakens the carbon sink function by reducing plant community cover in the semi-arid grassland ecosystems in northern China.In contrast,GEP,ER and NEE were not changed under increased precipitation during the early growing season.The effect of ILP on GEP and ER is not statistically significant.ILP significantly increased NEE by 10.14%,probably due to the increase of aboveground carbon absorption induced by increased plant community cover(total leaf area).(2)The result NEE shows a strong correlation with GEP irrespective of precipitation increases(R~2 = 0.72)or decreases(R~2 = 0.96),indicating that NEE is mainly regulated by GEP.Compared with the increased precipitation treatment,NEE responds more strongly to the precipitation reduction.Compared with DEP,NEE is more sensitive to DLP.There are no differences of NEE sensitivity between IEP and ILP.The reduced net carbon uptake under decreased precipitation in the late growing season indicates that drought in the late growing season can cause a reduction of ecosystem carbon sequestration.If such a situation persists for a long time,decreased precipitation in the late growing season may weaken the ecosystem carbon sink and even shift the semiarid grassland from a net carbon sink into a net carbon source,which will have a profound impact on terrestrial carbon balance at the regional and global scales.Therefore,the effects of changes in precipitation in the late growing season on carbon cycle indicators has an important role in predicting carbon balance in semi-arid grassland ecosystems under climate change scenario.