Ecosystem Carbon Exchange And Its Response To Tidal Flooding In A Salt Marsh In The Yellow River Delta

The salt marsh is an important component in the blue carbon ecosystem,and its carbon cycle is also a crucial part of the global carbon cycle.Periodic tidal flooding is the basic hydrological feature of the salt marsh,and it is also a key influencing factor for ecosystem carbon exchange in the salt marsh.Tidal flooding can directly affect the ecosystem CO₂ and CH₄ exchange in the salt marsh,and can also change the response of ecosystem carbon exchange to other environmental factors.In recent years,caused by global warming,sea-level rise impacts directly upon the tidal hydrological conditions of the salt marsh,which will further affect its carbon cycle.Therefore,the carbon sink strength of the blue carbon ecosystem is being seriously threatened.Thus,clarifying the response of carbon exchange in the salt marsh to tidal flooding can provide a reliable scientific basis for the global“blue carbon sink”formation process and its response mechanism to future climate change.Based on this background,using a combination of long-term location monitoring and in-situ control experiments,this study aims to clarify the dynamic variations of ecosystem CO₂ exchange,reveal the effects of tidal flooding on ecosystem CO₂ exchange at multiple timescales,and analyze the response of ecosystem CO₂ and CH₄ exchange to different stages of the tidal flooding process in a salt marsh in the Yellow River Delta.The main results could be summarized as the following:（1）During the growing seasons of 2018 and 2019,the salt marsh in the study area was a carbon sink.The daily average carbon uptake rate was-0.59 g C m^-2 d^-1,and the cumulative carbon uptake is-210 g C m^-2.Net ecosystem CO₂ exchange（NEE）had significant diurnal variations and seasonal variations.Diurnal variations of NEE showed a"U"shaped curve during the growing season,and these curves varied in amplitude between different months.During the study period,the maximum rate of ecosystem CO₂ uptake was-3.90μmol m^-2 s^-1,and the maximum rate of nighttime CO₂emission was 0.45μmol m^-2 s^-1.Moreover,the seasonal variation of NEE was mainly caused by the difference in NEE dynamics between the early and mid-growing season and the terminal of the growing season.Variations of NEE were also influenced by the combined effects of various environmental factors.The daytime NEE(NEE_daytime)was mainly affected by the light,but the light-response curve varied with months during the growing season.In addition,A_max varies from 3.00±0.14μmol m^-2 s^-1 to 4.49±0.27μmol m^-2 s^-1.NEE_nighttime showed a statistically significant relationship with temperature（P<0.001）.However,the relationship was also affected by tidal flooding,which made the R² of the temperature response curve was only 0.0282.（2）At multiple timescales,wavelet analysis showed that tidal flooding significantly influenced NEE at the multiday scale（8-16 days）and the seasonal scale（64-128 days）.In addition,tidal flooding also changed the response of NEE to light and temperature.Tidal flooding inhibited NEE_nighttime as well as decreased temperature sensitivity(Q₁₀)of NEE_nighttime from 1.37 to 1.16,which means that tidal flooding weakens the response of NEE_nighttime to temperature.In contrast,the response of NEE_daytime to tidal flooding was more complicated,as the NEE_daytime reacted differently with different months.Overall,A_max decreased from 4.76±0.20μmol m^-2 s^-1 to 4.45±0.19μmol m^-2 s^-1 due to tidal effects.（3）For the tidal flooding process,ecosystem CO₂ and CH₄ fluxes fluctuated notably with different tidal stages in the salt marsh,and the water level and soil salinity were the main factors controlling carbon flux.During the tidal inundation period,the increasing water levels significantly inhibited the rate of CO₂ uptake in the salt marsh.Notably,ecosystem CO₂ uptake was completely inhibited when tidal water submerged the plant(rising tide stage:0.54±0.08μmol m^-2 s^-1;tidal flooding-3 h:0.46±0.10μmol m^-2 s^-1;tidal flooding-22 h:0.44±0.05μmol m^-2 s^-1).Higher soil salinity also significantly reduced the rate of CO₂ uptake at the before flooding stage and the after ebbing stage（P<0.05,R²=0.70）.In contrast,ecosystem CH₄ exchange was not significantly related to soil salinity and water level,which were because ecosystem CH₄fluxes was more sensitive to changes in soil redox environment in the salt marsh.In this study,CH₄ emissions of the after ebbing stage was higher than that of the before flooding stage,and this difference was significant of low water level（LWL）and middle water level（MWL）treatments(LWL:0.56±0.12 vs 0.38±0.09 nmol m^-2 s^-1;MWL:0.79±0.13 vs 0.40±0.09 nmol m^-2 s^-1).Besides,the rate of CH₄ emissions of LWL,MWL,and HWL（high water level）treatments increased with a prolonged inundation time.In summary,this study highlighted the critical effects of tidal flooding on the carbon exchange process in the salt marsh and was valuable for a better understanding of the“blue carbon sink”role of the salt marsh under global climate change.