Effects Of Experimental Warming On The Key Processes Of Carbon Cycling In The Yellow River Delta

Climate warming threatens the structure and function of wetland ecosystems.Due to the high primary productivity and low organic matter decomposition rate,coastal wetlands are the densest carbon sinks on earth.The carbon burial rate per unit area of coastal wetland is about 4.5,3 and 4.8 times that of temperate forest,tropical forest and boreal forest.The sustainability of ecosystem carbon sink mainly depends on the rate and stability of carbon in multiple cycles,including carbon input,storage,and output.Therefore,the evolution of carbon sink in coastal wetlands under climate warming mainly depends on the response of a series of key carbon processes,including ecosystem productivity and its stability,soil respiration and its temperature sensitivity,ecosystem carbon exchange and its seasonality,as well as soil methane emissions,to climate warming.In addition,the extreme hydrological events can affect the plant growth of wetlands,and may change the response of ecosystem carbon cycle processes to climate warming.Therefore,how the carbon processes of coastal wetlands respond to climate warming is an important scientific issue for understanding and predicting the dynamics of coastal wetland carbon sink under climate change.Focusing on this scientific issue,based on a field warming test platform in the Yellow River Delta,this paper deeply analyzed the response of key processes of carbon cycling in coastal wetland ecosystems to climate warming.The main results are as follows:(1)Since November 2014,experimental warming has significantly increased soil temperature by 2.4 ^oC.Extreme rainfall caused a 100-year extreme flooding event.in August 2016.Experimental warming significantly decreased aboveground net primary productivity(ANPP)by 31.0%and the temporal stability of ANPP by 74.8%due to the effects of community composition and its response to the extreme flooding event.The increase in soil salinity under warming shifted the community from P.australis to S.glauca.The altered community was less resistant to the extreme flooding event because the height of S.glauca was less than the 72-cm flooding.As a result,experimental warming decreased the resistance and resilience by 44.7%and 40.9%,respectivily.Therefore,this study shows that changes in plant community structure are key factors regulating wetland ecosystem productivity in response to climate warming.Predicting the wetland stability in future should consider ecosystem resistance and resilience to extreme flooding.(2)After the extreme flooding year(2017-2020),warming significantly reduced total soil respiration(R_s,-19.9%)and heterotrophic respiration(R_h,-25.8%),but had no significant difference in autotrophic respiration.The negative effects of warming on R_s and R_h were limited by the increase in soil salinity and the decrease in ANPP caused by warming.The effect of warming on R_a was mainly affected by the belowground net primary productivity.In addition,this study found that warming had no significant effect on the temperature sensitivity(Q₁₀)of R_s and R_h in the growing season,but decreased the Q₁₀ in the non-growing season,which indicated that R_s and R_h were adaptive to warming in the non-growing season.(3)After the extreme flooding year(2017-2020),experimental warming significantly reduced ecosystem net CO₂ exchange(NEE,-25.0%),gross primary productivity(GPP,-25.4%),ecosystem respiration(ER,-29.6%)and soil methane emissions(-84.9%).These response to warming were mainly due to the limitation of warming on vegetation productivity.In addition,this study found that warming changed the seasonality of NEE.The negative effect of warming on NEE mainly occurred in summer,and the positive effect occurred in autumn.This was because warming increased soil salinity,benefited the growth of plant species with high salt tolerance and late peak growth.This study provides experimental evidence on the reduced magnitude of net CO₂ exchange under climate warming in coastal wetlands.These findings underscore the high uncertainty of wetland CO₂ sink in coastal regions under future climate change.To sum up,this study showed that warming rapidly altered plant community structure by increasing the dominance of low-canopy species.Then,it showed that warming reduced the resistance and resilience of vegetation productivity to an extreme flooding event.This study monitored that the experimental warming after extreme flooding significantly reduced the rates of carbon cycling,such as gross ecosystem productivity,ecosystem respiration,soil respiration,and soil methane emission.These results suggest that climate warming and extreme flooding events can affect the stability of vegetation productivity by altering vegetation community structure,and have a profound impact on ecosystem carbon cycling in the Yellow River Delta.