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

Abstract

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 ecosystem production (GEP) and ecosystem respiration (ER) response to precipitation changes. 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. Our results demonstrated that acclimation of vegetation to salinization leads to the asymmetric response of ecosystem carbon exchange along the precipitation gradient, while seasonal flooding may amplify the positive asymmetric response by affecting vegetation community composition in saline wetlands. The findings underscore the importance of seasonal flooding in modulating wetland ecosystem responses within global change manipulation experiments.