Opposing Effects of Temperature and Elevated CO2 on Tidal Wetland Methane Emissions and Soil Carbon Sequestration

Elevated carbon dioxide and warming perturb ecosystem carbon cycling, causing both positive and negative feedbacks on greenhouse gas concentrations. We began a field experiment in a tidal marsh to investigate the mechanisms by which warming and elevated CO₂ (eCO₂) perturb carbon gain via plant-driven sequestration in soils, and carbon loss primarily via CH₄ emissions. The Salt Marsh Accretion Response to Temperature eXperiment (SMARTX) is an active, whole-ecosystem experiment with four levels of warming encompassing the plant canopy and soils to a depth of 1.5 m. The two warming extremes (+0, +5.1 °C) are crossed with eCO₂ (+350 ppm above ambient). Warming by 5.1 °C doubled CH₄ emissions for three possible reasons: (i) more carbon substrates for methanogenesis, (ii) a decline in SO₄ reduction, and (iii) a decrease in the proportion of CH₄ consumed by methane oxidation. By contrast, +5.1 °C warming crossed with eCO₂ caused CH₄ emissions to decrease. We hypothesize that the opposing of temperature and eCO₂ are due to differences in root production coupled with an increase in plant-mediated transport of O₂ into anaerobic soils. Supporting this hypothesis is the observation that the +5.1 °C x eCO₂ treatment had the most roots, the lowest CH₄ emissions, and FTICR-MS evidence of accelerated decomposition of dissolved organic matter. FTICR-MS data showed that eCO₂ decreased the nominal oxidation state of dissolved organic carbon compounds, primarily through the loss of lignin-like structures. Evidence that eCO₂ also accelerated decomposition of solid-phase soil organic matter is the observation that soil elevation gain (a proxy for soil carbon sequestration) increased with +5.1 °C of warming alone, but not when +5.1 °C and eCO₂ were crossed. Our results suggest that temperature and eCO₂ will have opposing effects on future carbon sequestration in tidal wetlands, with higher plant production offset by higher soil organic matter decomposition, and higher CH₄ production offset by higher CH₄ oxidation.