Responses Of Ecosystem Carbon Exchange To Winterspring Warming And Summer-Autumn Rainfall Reduction In The Coastal Wetland Of The Yellow River Delta

Coastal wetland,the transitional zone between terrestrial and marine ecosystems,has valuable ecosystem services and functions.It provides habitats and food sources for many species,and possesses important ecosystem service functions.Meanwhile,coastal wetland becomes an important carbon sink system because of its high primary productivity and low soil organic carbon decomposition rate.However,climate change,which is dominated by global warming and regime shifts in precipitation patterns,has been impacting vegetation dynamics and community structure of coastal wetland ecosystems,thus affecting soil carbon storage and carbon sink of coastal wetland ecosystems.How the future climate warming and precipitation pattern change will jointly affect the ecosystem carbon exchange of coastal wetland ecosystem and its potential mechanism are still inconclusive.Since the 1960s,the coastal wetland in the Yellow River delta of China has been experiencing an increase in temperature during winter and spring associated with a reduction in precipitation during summer and autumn.Studying the impact and the underlying mechanisms of the seasonally asymmetric climate change on carbon processes of the coastal wetland ecosystem in the Yellow River Delta will not only provide data support for predicting the changing trend of carbon cycle of the coastal wetland ecosystem in the future,but also provide theoretical the basis for better management and protection of coastal wetland resources.Based on a field manipulative experiment of the Yellow River Delta to simulate winter and spring warming（+2°C）and summer and autumn precipitation（-50%rainfall）,this dissertation analyzes how key processes of the carbon cycle of coastal wetland ecosystem respond to climate change.The main results during the experimental period（June 2017 to May 2022）are as follows:（1）How winter-spring warming and summer-autumn rainfall reduction impact on the soil micro-environment.Soil sensors were used to record the temperature,salinity,and volumetric water content of the soil at a depth of 10cm underground.Under the treatment of winter-spring warming,the soil temperature increased by 1.39°C in the whole experiment period,2.98°C in winter,and 2.64°C in spring.The soil temperature in summer and autumn was not changed by winter-spring warming.Under the treatment of summer-autumn rainfall reduction,the soil salinity increased by 1.97 d S m^-1 in the whole experiment period,1.92 d S m^-1 in summer,1.88d S m^-1 in autumn,2.17 d S m^-1 in winter,and 1.92 d S m^-1 in spring.Consequently,the soil available water decreased by 11.23 V/V%m d S^-1 in the whole experiment period,17.18 V/V%m d S^-1 in winter,and 12.66 V/V%m d S^-1 in spring.These results suggest that winter-spring warming will increase the soil temperature in winter and spring,and summer-autumn rainfall reduction will change the water-salt conditions in the coastal wetland of the Yellow River Delta.（2）How winter-spring warming and summer-autumn rainfall reduction affect plant growth and community structure.Greenness of the canopy,biomass harvesting and investigation of species composition were measured regularly from2017 to 2021.Winter-spring warming advanced the start of the growing season（SOS）by 6.41 days,delayed the end of the growing season（EOS）by 7.03 days,and thus extended the length of the growing season（LOS）by 13.44 days.Winter-spring warming increased the relative greenness index（GCC,green chromatic coordinate）of the canopy by 1.99%in spring.Summer-autumn rainfall reduction increased the coarse root biomass by 37.51%,resulting in an increase of 29.97%in the belowground biomass and 22.91%in the total plant biomass.In addition,summer-autumn rainfall reduction increased the proportion of P.australia coverage in the middle growing season by 20.29%.The biodiversity indexes（Shannon Diversity Index and Simpson’s Diversity Index）were not changed significantly by winter-spring warming or summer-autumn rainfall reduction.These results suggest that winter-spring warming will change plant phenology,summer-autumn rainfall reduction will change the plant biomass and its allocation in the coastal wetlands,and there is no interaction between them.（3）How winter-spring warming and summer-autumn rainfall reduction affect soil respiration and its temperature sensitivity.The soil respiration and its components were measured regularly from June 2017 to May 2022.Summer-autumn rainfall reduction had no significant impact on soil respiration（R_s）,heterotrophic respiration（R_h）,autotrophic respiration（R_a）,or temperature sensitivity(Q₁₀).Winter-spring warming increased R_s（+8.88%）and R_a（+28.94%）,with no effect on R_h,leading to a reduced ratio of heterotrophic respiration to soil respiration（R_h:R_s）by 16.64%.The effect of winter-spring warming on soil respiration had interannual variation.The stimulation of warming on winter R_s showed an increasing trend with time,which was+23.34%,+16.66%,+27.92%,+37.90%,and+166.75%from 2017 to 2022,respectively.In addition,winter-spring warming reduced the Q₁₀ of R_s（+11.97%）and R_h（+14.74%）,with no effect on that of R_a.These results suggest that winter-spring warming have no significant effect on the heterotrophic respiration produced by microbial decomposition but promotes the autotrophic respiration of plants and then increases the total soil respiration of coastal wetlands,having no interaction with summer-autumn rainfall reduction.（4）How winter-spring warming and summer-autumn rainfall reduction affect ecosystem carbon fluxes.Net ecosystem CO₂ exchange（NEE）,gross primary productivity（GPP）,and ecosystem respiration（ER）were measured regularly by the static-chamber method from June 2017 to May 2022.Winter-spring warming increased NEE（+11.98%）,GPP（+10.77%）,and ER（+9.03%）.Moreover,summer-autumn rainfall reduction enhanced the positive effect of the warming on GPP and ER.For example,winter-spring warming in 2020 showed a greater stimulation on GPP under summer-autumn rainfall reduction（+55.81%）than the ambient rainfall condition（+8.28%）.Additionally,the effect of winter-spring warming and summer-autumn rainfall reduction on the ecosystem CO₂ fluxes had seasonal variations.Winter-spring warming always play significant positive effects on NEE and its components in spring,while summer-autumn rainfall reduction only had significant negative effects（under ambient temperature conditions）or positive effects（under winter-spring warming）in autumn.These results suggest that winter-spring warming will increase the NEE,GPP,and ER,but the magnitudes depend on plant growth stages and rainfall conditions.In sum,this study revealed that winter-spring warming would prolong the growing season,increase soil respiration,and promote the ecosystem carbon exchange in the coastal wetland.Moreover,summer-autumn rainfall reduction will change the community structure in the middle growing season and enhance the positive effect of winter-spring warming on GPP and ER.These results show how winter-spring warming and summer-autumn rainfall reduction affect the ecosystem carbon exchange and related carbon cycle processes of the coastal wetland ecosystem,emphasizing the interaction between different climate change factors and providing data support and a theoretical basis for ecosystem models to simulate and predict the carbon sink capacity.