Carbon Dioxide and Methane Emissions from a California Salt Marsh

Wetland carbon sequestration is offset by carbon dioxide (CO2) and methane (CH4) emissions for which the magnitudes remain coarsely constrained. To better understand the spatial and temporal variations of gaseous carbon fluxes from marsh soils in a Mediterranean climate, I collected air and soil samples over the course of 10 months at Carpinteria Salt Marsh Reserve (CSMR) located in the County of Santa Barbara, California. The CSMR consists of four zones characterized by differences in elevation, tidal regime, soil properties, and vegetation. Twelve static chambers were deployed among two lower marsh zones, a mudflat, and a marsh-upland transition zone for fortnightly flux measurements from September 2015 to May 2016. In August 2015 and June 2016, soil cores up to 50 cm deep were extracted near the chambers, segmented by depth, and analyzed for soil moisture, bulk density, particle size distribution, electrical conductivity, pH, organic/inorganic carbon, and total nitrogen content. Averaged over the 9-month study period, the marsh-upland transition zone had the highest CO2 fluxes at 5.3 ± 0.7 g CO2 m-2 d-1, followed closely by the lower marsh zones (3.8 ± 0.6 g CO2 m-2 d-1 and 2.8 ± 0.7 g CO2 m-2 d-1), which were one order of magnitude higher than the CO2 fluxes from the mudflat (0.4 ± 0.1 g CO2 m-2 d-1). The CO2 fluxes varied significantly on a seasonal scale but were not consistently correlated with environmental variables measured. The CH4 fluxes had no clear seasonal patterns, but overall CH4 flux rates from the lower marsh zones (2.2 ± 1.5 mg CH4 m-2 d-1 and 1.9 ± 0.2 mg CH4 m-2 d-1) surpassed those from the mudflat (0.2 ± 0.06 mg CH4 m-2 d-1) by an order of magnitude, and the marsh-upland transition zone was a net methane sink (-0.07 ± 0.1 mg CH4 m-2 d-1). The CH4 fluxes correlated well with most soil properties by zone. Our results show that soil gaseous carbon fluxes from a coastal salt marsh vary by salt marsh zone