Interacting Effects of Elevated CO2 and Warming on Coastal Wetland Carbon Dynamics

Abstract:

Coastal wetlands are dynamic terrestrial-aquatic interfaces where interactions between plants, microbes, and hydrogeomorphology regulate ecosystem responses to global change, and where plants are ecosystem engineers that regulate wetland soil carbon sequestration. Though multiple climate stressors are expected to simultaneously affect these ecosystems, many manipulative field experiments only address one at a time. The Salt Marsh Accretion Response to Temperature eXperiment (SMARTX) was established in a Chesapeake Bay tidal marsh in 2016 to understand the combined ecosystem-scale effects of simultaneous warming and elevated CO₂ (eCO₂). We actively manipulate whole-ecosystem temperature in both the plant canopy and soil (to a depth of 1.5 m) to cover a gradient from ambient to +5.1 °C. The two warming extremes are crossed with eCO₂ (+350 ppm).

Over the past five years, +5.1 °C of warming doubled CH₄ emissions, but warming crossed with eCO₂ caused emissions to decrease. eCO₂ + warming also reduced the average nominal oxidation state of carbon (NOSC) in the DOC pool and increased heterogeneity in the soil surface, compared to plots with just warming. We hypothesized that these opposing responses were due to the stimulation of plant-mediated transport of O₂ into otherwise anaerobic soils under eCO₂ conditions, but not under warming alone. To test this hypothesis, we installed an automated redox system in March 2020 that collects replicated measurements every 30 minutes. Redox potential was highly variable in early summer, but by early July eCO₂ + warming plots had consistently higher redox potential than warming only plots, indicating a more oxidizing environment. These real-time trends in redox potential also tracked the continuously measured water level and weekly plant phenology assessments over the year, illustrating the strong links between vegetation, biogeochemistry, and hydrology as drivers of carbon cycling in these ecosystems, especially under global change conditions.