Trace gas fluxes from tidal salt marsh soils: implications for carbon–sulfur biogeochemistry

Abstract

Abstract. Tidal salt marsh soils can be a dynamic source of greenhouse gases such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), as well as sulfur-based trace gases such as carbon disulfide (CS2) and dimethylsulfide (DMS) which play roles in global climate and carbon–sulfur biogeochemistry. Due to the difficulty in measuring trace gases in coastal ecosystems (e.g., flooding, salinity), our current understanding is based on snapshot instantaneous measurements (e.g., performed during daytime low tide) which complicates our ability to assess the role of these ecosystems for natural climate solutions. We performed continuous, automated measurements of soil trace gas fluxes throughout the growing season to obtain high-temporal frequency data and to provide insights into magnitudes and temporal variability across rapidly changing conditions such as tidal cycles. We found that soil CO2 fluxes did not show a consistent diel pattern, CH4, N2O, and CS2 fluxes were highly variable with frequent pulse emissions (> 2500 %, > 10 000 %, and > 4500 % change, respectively), and DMS fluxes only occurred midday with changes > 185 000 %. When we compared continuous measurements with discrete temporal measurements (during daytime, at low tide), discrete measurements of soil CO2 fluxes were comparable with those from continuous measurements but misrepresent the temporal variability and magnitudes of CH4, N2O, DMS, and CS2. Discrepancies between the continuous and discrete measurement data result in differences for calculating the sustained global warming potential (SGWP), mainly by an overestimation of CH4 fluxes when using discrete measurements. The high temporal variability of trace gas fluxes complicates the accurate calculation of budgets for use in blue carbon accounting and earth system models.

Physiochemical Controls on the Horizontal Exchange of Blue Carbon Across the Salt Marsh‐Tidal Channel Interface

“…In addition to understanding the physical and chemical controls on horizontal C transport, it is also essential to know how soil trace gas emissions change across tidal cycles (Capooci & Vargas, 2022b). Fluxes of soil greenhouse gases (e.g., CO 2 , CH 4 ) offset the global cooling effect of stored C in soil.…”

The paradox of assessing greenhouse gases from soils for nature-based solutions

“…Second, soil GHG fluxes could present hot moments, which are transient events with disproportionately high values that are often missed with an FTS approach (Vargas et al, 2018;Butterbach-Bahl et al, 2004). Third, cumulative sums and uncertainty ranges are biased or misleading when derived using an FTS approach (Tallec et al, 2019;Lucas-Moffat et al, 2018;Capooci and Vargas, 2022b). Our study demonstrates that an optimized approach consistently provided closer estimates for cumulative sums and uncertainty ranges when compared with automated measurements (Fig.…”

“…Automated measurements have revolutionized our understanding of the temporal patterns and magnitudes of soil GHG fluxes (Savage et al, 2014;Bond-Lamberty et al, 2020;Tang et al, 2006;Capooci and Vargas, 2022b). These measurements have limitations in representing spatial variability and have higher equipment costs that limit their broad applicability across study sites (Vargas et al, 2011).…”

“…We postulate that representing the variability in soil N 2 O fluxes is more sensitive to the FTS approach (> 170 % and > 30 % for cumulative sums and uncertainty ranges, respectively) than for soil CH 4 and CO 2 fluxes. Fourth, it is possible that if the information derived from an FTS approach is biased, then functional relationships could also be different from those derived from automated measurements (Capooci and Vargas, 2022a). It has been argued that hypothesis testing and our capability of forecasting responses of soil GHG fluxes to changing climate conditions is also biased with information from the FTS approach (Vicca et al, 2014).…”

Hyperspectral Reflectance for Measuring Canopy‐Level Nutrients and Photosynthesis in a Salt Marsh

“…We found that P, K, and Na appear to play an important role in increasing F A among all categories (i.e., ecosystem scale, S. alterniflora , and S. cynosuroides ), while an increase in Fe had a negative effect on F A . The influence of N and S on F A was more variable among vegetation categories (Figure 7), suggesting confounding effects with local sediment biogeochemistry that varies across the marsh (Capooci & Vargas, 2022b; Seyfferth et al., 2020). Other nutrients such as Cu and Si that are important for plant growth of Spartina spp.…”