Coastal wetlands have immense potential to store carbon (C) in vegetation and sediments, a vital component of the global C cycle. How C storage in coastal wetlands will be affected by accelerated sea level rise (SLR) as a result of climate change, however, is uncertain. It is hypothesized that shifts in stressors, such as salinity, can shift the soil C balance from a net C sink to a C source, stimulating peat collapse, which will, in turn, accelerate the effects of SLR. The objectives of this study are to investigate how simulated saltwater intrusion and increased inundation into brackish water wetlands will change net ecosystem productivity and soil C balance. Using mesocosm experiments with elevated (~20 ppt) and ambient (~10 ppt) salinity, we quantified changes in gross primary production, plant respiration (R), ecosystem R, microbial C processing, and net ecosystem exchange in brackish wetland peat soils. All statistics were run using a two-way repeated measures ANOVA with Tukey’s post-hoc test for within-treatment variations.
Results after one year reveal that ambient cores took up more C (-2045 g m-2 y-1) than elevated cores (-172 g m-2 y-1), showing that increased saltwater exposure significantly (ANOVA, p=0.002) decreases the ability of a brackish marsh to store C with level of inundation having no significant effect (Tukey’s, p=1.000). Although ecosystem CO2 R decreased by 28% in the elevated saltwater cores, gross ecosystem exchange (GEE), a proxy for plant productivity, decreased by 71%. Although soil R was different across treatments (ANOVA, p<0.001), this was a result of inundation level (Tukey’s, p=0.001) and not elevated salinity (Tukey’s, p=0.955). Decreased GEE with no change in soil R as a result of elevated salinity suggests that the biggest effect elevated salinity has on coastal marshes is the capacity to reduce plant productivity and therefore C inputs into the soil. These results are of critical importance when considering that coastal marshes need C inputs into their soil to accrete with rising seas. If the decrease in coastal marsh NEE as a result of saltwater intrusion becomes too great, peat collapse and loss of habitat to open water is a serious concern.