Diel and seasonal patterns of soil CO2 efflux in a temperate tidal marsh

Abstract

Tidal marshes store large amounts of carbon; however, little is known about the patterns, magnitudes, and biophysical drivers that regulate CO₂ efflux from these ecosystems. Due to harsh environmental conditions (e.g., flooding), it is difficult to measure continuous soil CO₂ efflux in tidal marshes. These data are necessary to inform empirical and process-based models and to better quantify carbon budgets. We performed automated (30 min) and manual (bi-monthly) soil CO₂ efflux measurements, for ~20 months, at two sites in a temperate tidal marsh: tall Spartina (TS; dominated by S. cynosuroides) and short Spartina (SS; dominated by S. alterniflora). These measurements were coupled with water quality, canopy spectral reflectance, and meteorological measurements. There were no consistent diel patterns of soil CO₂ efflux, suggesting a decoupling of soil CO₂ efflux with diel variations in temperature and tides (i.e., water level) showing a hysteresis effect. Mean soil CO₂ efflux was significantly higher at SS (2.15 ± 1.60 μmol CO₂ m^-2 s^-1) than at TS (0.55 ± 0.80 μmol CO₂ m^-2 s^-1), highlighting distinct biogeochemical spatial variability. At the annual scale, air temperature explained >50% of the variability in soil CO₂ efflux at both sites; and water level and salinity were secondary drivers of soil CO₂ efflux at SS and TS, respectively. Annual soil CO₂ efflux varied from 287-876 to 153-211 g C m^-2 y^-1 at SS and TS, respectively, but manual measurements underestimated the annual flux by <67% at SS and <23% at TS. These results suggest that measuring and modeling diel soil CO₂ efflux variability in tidal marshes may be more challenging than previously expected and highlight large discrepancies between manual and automated soil CO₂ efflux measurements. New technical approaches are needed to implement long-term automated measurements of soil CO₂ efflux across wetlands to properly estimate the carbon balance of these ecosystems.

Keywords: Aquatic-terrestrial interface; Blue carbon; Carbon dioxide; Hysteresis; Soil respiration; Wetland.