Carbon Cycling is Surprisingly Resistant to Large-Scale Salt Addition in a Forested Freshwater Wetland

Abstract:

Saltwater intrusion in coastal landscapes is increasing globally due to climate change driven sea level rise and extreme events. To understand the effect of salinization on ecosystem processes in freshwater forested wetlands, we conducted a large-scale salt addition experiment in coastal North Carolina. We ask: how are wetland carbon pools and fluxes affected by salt addition in a coastal plain forested wetland? We monitored responses to both vegetation and soil carbon stocks, as well as soil carbon form and fluxes directly from the field experiment and through a series of soil incubation experiments conducted in the lab. We examined the responses to the experimental treatment after four years of salt additions with a particular emphasis on carbon cycling and storage which are important functions of wetlands globally.

After four years of salt addition treatments, soil salinity was 3-10 times greater than control plots. Despite experimental salt additions, we saw no significant change in above ground tree growth, root biomass, leaf reflectance, or leaf C to N ratios. Soil carbon fluxes were inconsistent across treatment blocks, which we attribute to the highly heterogeneous nature of the soil matrix across sites. Using a linear mixed effects modeling approach, we found a significant effect of salinity on the pool of dissolved organic carbon, which is reduced at elevated salinity. Variable selection by omission of the mixed effects model suggests that pH is the most important variable controlling the concentration of soil extracted dissolved organic carbon. The effect of the salt treatment on soil carbon pools may have indirect effects on soil carbon fluxes and contribute to the variation observed across replicated experimental sites. Soil incubation assays conducted in the lab suggest that microbial communities from the field salt addition treatments are becoming resistant to further salt exposure. Our results emphasize the importance of long-term field experiments for testing predictions based on lab and greenhouse studies and elucidates the complexities of salinization as a multi-faceted chemical change further complicated by heterogeneous soil matrix effects.