LINKING TRANSIENT CHANGES IN DISSOLVED ORGANIC CHEMISTRY TO WETLAND CARBON EMISSIONS

Abstract

Wetlands form a massive reservoir of soil carbon (C) that is disproportionately impacted by climate change and displays extreme variability in C cycling over different spatiotemporal scales. The biogeochemical pathways underpinning this variability remain a critical knowledge gap due in part to the oversimplification of soil organic matter (SOM) transformations in the wetland rhizosphere. Tidal marshes bordered by forest are of unique concern due to natural gradients in SOM chemistry along the wetland-to-forest transect. In addition to sea level rise, SOM chemistry is regulated by transient shifts in vegetation growth, organic molecular characteristics, and redox conditions. To better understand how these factors influence the correspondence between wetland C emissions and SOM transformations, we incubated soils collected from a tidal marsh dominated by the plant Phragmites australis. This plant is globally distributed and is recognized as a driver of soil C cycling and soil redox perturbations. We measured CO2 and CH4 emissions and characterized SOM transformations with high-resolution mass spectrometry to assess the effects of seasonality (relevant to plant growth in the field), local soil characteristics (based on transect position, relevant to sea level), and soil redox perturbation (analogous to plant radial oxygen loss) on C emissions. Our results suggest that redox modulation by Phragmites, as well as other wetland plants, has the potential to increase C emissions in the field (depending on transect location and seasonality) by stimulating the degradation of high-molecular-weight compounds in SOM. We find that the molecular mass and nominal oxidation state of C (NOSC) in the dissolved fraction are key metrics for understanding general trends in C chemistry and emissions, but variability in CH4 uniquely corresponds to transient changes in carboxylic-rich alicyclic molecules (CRAM). Considering that sea level rise will likely extend Phragmites into new soil zones, our work allows for predictions of how it could affect C cycling in newly invaded soils. This provides deeper mechanistic insight into the C cycle of wetland soils and underscores a need for further investigations of the wetland-to-forest ecotone to help improve landscape and global C models while informing new climate mitigation policies.