Flooding and Water Chemistry Drive Soil Biogeochemistry and GHG fluxes in a Coastal Forest

Coastal ecosystems are highly vulnerable to climate change due to sea level rise, increased storm frequency and intensity, and changes in precipitation patterns. These hydrological disturbances affect soil biogeochemical processes in coastal forests, potentially transforming these upland habitats into wetlands and changing ecological functions. However, the initial impacts on belowground processes and the mechanisms driving GHG fluxes during this transition remain poorly understood. This study investigates how flooding events with different water chemistries influence the production and consumption of GHGs in coastal forest soils under controlled laboratory conditions. Intact soil cores were collected from a temperate deciduous coastal forest in Maryland, USA. Freshwater (FW) and brackish water (BW) pulses were applied to simulate intense rainfall and storm surge events, respectively. Continuous CO₂, CH₄, and N₂O emissions were measured and coupled with air isotopic sampling (δ¹³C-CO₂, δ¹³C-CH₄) and porewater chemistry analyses (DOC, S^2-, Fe²⁺, Fe³⁺, Mn²⁺, NH₃, NO₃^-+NO₂^-, ORP, pH) to identify potential changes in metabolic pathways and characterize the biogeochemical responses. The results underscore the impact of water chemistry on biogeochemical processes, particularly in the BW treatment, which exhibited strong reducing conditions and active microbial metabolism. The elevated salinity and sulfate concentrations were associated with increased emissions of CH₄ and N₂O. The δ¹³C-CH₄ signature and elevated S^2- in porewater indicated the co-occurrence of methylotrophic methanogenesis and sulfate reduction. Elevated NH₃ concentrations and NO₃^-+NO₂^- production suggested the potential occurrence of dissimilatory nitrate reduction to ammonium (DNRA) and incomplete denitrification. These findings highlight the potential vulnerability of upland coastal forest soils to hydrologic disturbances and the complex interactions involved in the response of these ecosystems to inundation stressors.