Seasonal flooding amplifies the positive asymmetric response of ecosystem carbon exchange along the precipitation gradient in saline wetlands

Intensified precipitation variability profoundly affects saline wetland hydrological dynamics, potentially interfering with key carbon processes within ecosystems. However, the mechanisms underlying how precipitation changes affect ecosystems carbon processes in saline wetland remain unclear. Moreover, whether seasonal flooding influences wetland ecosystems' carbon processes response to precipitation changes is still poorly understood. Based on a six-year field precipitation experiment including five precipitation levels (-60%, -40%, +0%, +40%, and +60% of ambient precipitation) in the Yellow River Delta wetlands, we examined how seasonal flooding regulated ecosystem CO2 exchange (NEE), gross primary production (GPP) and ecosystem respiration (ER) response to precipitation changes. From 2018 to 2020, we acquired the aforementioned datasets concerning ecosystem carbon fluxes and environmental factors. Through the integration and statistical analysis of these datasets, we have summarised the following research findings. Over three years, ecosystem carbon fluxes exhibited a positive asymmetric response along the precipitation gradient, with the increment under wet treatments exceeding the reduction under dry treatments. Compared with the control, the -60% treatment significantly reduced GEP and ER by 9.7% and 9.3% respectively; whereas the +40% and +60% treatments significantly increased GEP, ER, and NEE by 18.0%, 23.5%, 15.4%, 19.4%, 22.1%, and 29.0% respectively. Moreover, the observed positive asymmetry in ecosystem carbon flux arose because, under reduced precipitation, vegetation coverage and total biomass were less affected by high soil salinity. Additionally, plants coverage responded differently to flooding under precipitation treatments: saline plants were most affected under reduced precipitation, while gramineous plants showed no significant difference.

降水变率加剧会显著影响盐沼湿地（saline wetland）的水文动态，进而可能干扰生态系统内的关键碳循环过程。然而，降水变化如何影响盐沼湿地的生态系统碳循环过程，其背后的机制仍不明晰。此外，季节性淹水是否会调控湿地生态系统碳循环过程对降水变化的响应，目前仍未得到充分解析。本研究基于黄河三角洲湿地开展的为期6年的野外降水控制实验，设置5个降水梯度（分别为自然降水（ambient precipitation）的-60%、-40%、0%、+40%与+60%），探究了季节性淹水如何调控生态系统CO₂交换（净生态系统交换，NEE）、总初级生产力（Gross Primary Production, GPP）与生态系统呼吸（Ecosystem Respiration, ER）对降水变化的响应。2018年至2020年间，本研究获取了上述涵盖生态系统碳通量与环境因子的数据集。通过对这些数据集的整合与统计分析，本研究总结得到如下研究结果。三年间，生态系统碳通量沿降水梯度呈现出正向不对称响应：增水处理下的通量增幅高于减水处理下的通量降幅。与对照组相比，-60%处理组分别使GPP与ER显著降低9.7%与9.3%；而+40%与+60%处理组则分别使GPP、ER与NEE提升18.0%、23.5%、15.4%、19.4%、22.1%与29.0%。此外，本研究观测到的生态系统碳通量正向不对称性，其产生原因在于：降水减少条件下，植被覆盖度与总生物量受土壤高盐度的影响程度更低。此外，不同降水处理下的植被覆盖度对淹水的响应存在差异：盐生植物在降水减少条件下受淹水的影响最为显著，而禾本科植物则无显著差异。