Modeling impacts of drought‐induced salinity intrusion on carbon dynamics in tidal freshwater forested wetlands

Abstract

Tidal freshwater forested wetlands (TFFW) provide critical ecosystem services including an essential habitat for a variety of wildlife species and significant carbon sinks for atmospheric carbon dioxide. However, large uncertainties remain concerning the impacts of climate change on the magnitude and variability of carbon fluxes and storage across a range of TFFW. In this study, we developed a process‐driven Tidal Freshwater Wetlands DeNitrification‐DeComposition model (TFW‐DNDC) that has integrated new features, such as soil salinity effects on plant productivity and soil organic matter decomposition to explore carbon dynamics in the TFFW in response to drought‐induced saltwater intrusion. Eight sites along the floodplains of the Waccamaw River (USA) and the Savannah River (USA) were selected to represent the TFFW transition from healthy to moderately and highly salt‐impacted forests, and eventually to oligohaline marshes. The TFW‐DNDC was calibrated and validated using field observed annual litterfall, stem growth, root growth, soil heterotrophic respiration, and soil organic carbon storage. Analyses indicate that plant productivity and soil carbon sequestration in TFFW could change substantially in response to increased soil pore water salinity and reduced soil water table due to drought, but in interactive ways dependent on the river simulated. These responses are variable due to nonlinear relationships between carbon cycling processes and environmental drivers. Plant productivity, plant respiration, soil organic carbon sequestration rate, and storage in the highly salt‐impacted forest sites decreased significantly under drought conditions compared with normal conditions. Considering the high likelihood of healthy and moderately salt‐impacted forests becoming highly salt‐impacted forests under future climate change and sea‐level rise, it is very likely that the TFFW will lose their capacity as carbon sinks without up‐slope migration.

Modeling impacts of saltwater intrusion on methane and nitrous oxide emissions in tidal forested wetlands

“…There were significant “river by site by scenario” ( p < 0.0001) and “site by scenario” ( p < 0.05) interactions for simulated CH 4 and N 2 O fluxes (Appendix S1: Tables S1 and S2, simulation results are available in Wang, Dai, Krauss, et al, 2022), indicating that drought‐induced saltwater intrusion significantly affected the emissions of CH 4 and N 2 O, and the responses of CH 4 and N 2 O emissions to drought‐induced saltwater intrusion varied significantly by river system (the Savannah and Waccamaw rivers) and by TFFW study site location along rivers (upper, middle, lower, and marsh). One‐way ANOVAs (Appendix S1: Table S3) revealed that the increase in soil salinity and reduction in soil water level due to drought and drought‐induced saltwater intrusion tended to greatly affect CH 4 and N 2 O emissions in TFFW, and the impacts of drought‐induced saltwater intrusion would vary with the river systems and site geomorphologic settings to determine the magnitude and direction of the biogeochemical processes in responses to drought‐induced saltwater intrusion.…”

“…The large increase in N 2 O emission in Savannah marsh sites under the drought condition may also be related to the lower nitrate loading rate at the oligohaline marsh site (18 μmol N m −2 day −1 ) compared to other sites (44–251 μmol N m −2 day −1 ) (Noe et al, 2013). Under the drought condition, SOM decomposition increased significantly (Wang, Dai, Trettin, et al, 2022), leading to increase in available NO 3 ‐N thus an increase in dentification in this N‐limited environment (Korol & Noe, 2020). Available NO 3 ‐N is often used over N 2 O as a terminal electron acceptor by denitrifying microorganisms in N‐limited ecosystems, and complete denitrification to N 2 is less likely (Megonigal & Neubauer, 2009).…”

“…TFW‐DNDC is a process‐driven biogeochemistry model for tidal freshwater wetlands that can be used to predict impacts of increasing soil salinity due to climate change and sea level rise; specifically, it simulates the dynamics of C and N in TFFW ecosystems and assesses the impacts of increasing salinity on plant productivity, plant and soil respiration, and SOC sequestration rate and storage (Wang, Dai, Trettin, et al, 2022). TFW‐DNDC consists of five sub‐models: hydrology, plant growth, soil organic matter, methanogenesis/methanotroph, and nitrification/denitrification (Figure 2).…”

“…TFW‐DNDC was previously calibrated and validated against field observations for both Savannah River and Waccamaw River sites using local ecological drivers for assessing annual litterfall, wood growth, root growth, plant respiration, and soil organic carbon storage (Wang, Dai, Trettin, et al, 2022). To ensure that TFW‐DNDC is capable of simulating temporal (daily) variations in CH 4 and N 2 O emissions across the various TFFW sites, model validation was also conducted by comparing simulated daily CH 4 and N 2 O emissions with observed values.…”

“…Soil water level and porewater salinity are critical factors that affect CH 4 and N 2 O emissions in TFFW through their interactions on plant growth, mortality, soil and plant respiration, soil organic matter decomposition, methanogenesis, methanotrophy, N/P mineralization, nitrification, denitrification, and other processes (Dai et al, 2018a; Krauss et al, 2012; Liu et al, 2017; Megonigal & Schlesinger, 2002; Noe et al, 2013; Wang et al, 2017; Wang, Dai, Trettin, et al, 2022). With saltwater intrusion, microbial pathways of anaerobic organic matter mineralization shift from methanogenesis to iron reduction and sulfate reduction (Weston et al, 2006), which could reduce relative CH 4 flux with progressive salinization (Holm et al, 2016; Poffenbarger et al, 2011).…”