Seawater is more than just salt: Disentangling the effects of salinization and pH on coastal soil carbon cycling

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Emily Ury, Justin P. Wright and Emily Bernhardt, Biology, Duke University, Durham, NC, Marcelo Ardon, Forestry and Environmental Resources, North Carolina State University, Raleigh, NC

Background/Question/Methods
Climate change is causing sea level rise and saltwater intrusion, with significant impacts on coastal wetlands and their ecological functions. Salinization of freshwater ecosystems is impacting carbon cycling, but experimental work to understand these impacts reveals widely divergent outcomes on soil carbon processes. Prior studies show both salt suppression and salt enhancement of soil respiration and soil carbon solubility. Refining our understanding of the controls of carbon cycling in wetlands in the face sea level rise has important implications for global systems modeling and decision making for coastal conservation and restoration. Here, we performed a salt addition experiment on two series of wetland soils with similar site history but variable soil pH and base saturation status. We independently manipulated salt concentrations and solution pH to tease apart the effect of these seawater components on soil carbon cycling. We measured soil respiration over a 21-day incubation period. We also measured dissolved organic content and aromaticity after the initial treatment rinse and final water extraction at the end of the incubation period.
Results/Conclusions
Microbial respiration and dissolved organic carbon solubility were depressed by marine salts in both soils, while pH manipulation alone had no effect. Salinity treatments had a far greater effect on soil pH than the pH manipulations and there was a strong interaction between salt treatments and soil type that affected the magnitude of soil carbon responses. Site soils varied significantly in pH and base saturation, suggesting that an interaction between salinity and edaphic factors is mediating soil carbon processes. The degree of salinization and the effective pH shift following seawater exposure may vary widely based on initial soil conditions; this may explain much of the variation in reported salt effects on soil carbon dynamics. We suggest that these edaphic factors may help to explain inconsistencies between carbon cycle responses to experimental salinization reported in the literature.