Soil CO2 concentrations and efflux dynamics of a tree island in the Pantanal wetland

The Pantanal is the largest tropical wetland on the planet, and yet little information is available on the biome's carbon cycle. We used an automatic station to measure soil CO2 concentrations and oxidation‐reduction potential over the 2014 and 2015 flood cycles of a tree island in the Pantanal that is immune to inundation during the wetland's annual flooding. The soil CO2 concentration profile was then used to estimate soil CO2 efflux over the two periods. In 2014, subsurface soil saturation at 0.30 m depth created conditions in that layer that led to CO2 buildup close to 200,000 ppm and soil oxidation‐reduction potential below −300 mV, conditions that were not repeated in 2015 due to annual variability in soil saturation at the site. Mean CO2 efflux over the 2015 flood cycle was 0.023 ± 0.103 mg CO2‐C m−2 s−1 representing a total annual efflux of 593 ± 2690 mg CO2‐C m−2 y−1. Unlike a nearby tree island site that experiences full inundation during the wet season, here the soil dried quickly following repeated rain events throughout the year, which led to the release of CO2 pulses from the soil. This study highlights not only the complexity and heterogeneity in the Pantanal's carbon balance based on differences in topography, flood cycles, and vegetation but also the challenges of applying the gradient method in the Pantanal due to deviations from steady state conditions. Plain Language Summary The Pantanal is the largest wetland in the world whose carbon balance is expected to change with seasonal flooding, especially in tree islands that can be immune to flooding. We provide a half‐hourly time series of soil CO2 concentrations, efflux, and oxidation‐reduction potential at a tree island site in the Pantanal. Soil CO2 concentration at 0.30 m depth was as high as 200,000 ppm, while our 2015 average soil efflux estimate using the gradient method was 0.023 mg CO2‐C m−2 s−1. Our results allow for a spatial assessment of soil CO2 effluxes in the Pantanal, revealing complex biogeochemical processes in the tropical wetland with controls more heavily influenced by topography and vegetation. This research also highlights instrumental challenges in applying the gradient method in the tropical wetland. Key Points Tree island soil CO2 concentrations were as high as 200,000 ppm (0.30 m depth) Mean 2015 soil CO2 efflux at the site was 0.023 ± 0.103 mg CO2‐C m−2 s−1 The gradient method presents challenges for application in the Pantanal