Oxygenation of acid sulfate soils stimulates CO2 emission: Roles of acidic dissolution and hydroxyl radical oxidation

Emission of CO2 from acid sulfate soil regions has received great attention in recent years. Production of CO2 due to microbial respiration has been documented by previous studies, but the abiotic mechanisms are poorly understood. In this study, CO2 production was measured from oxygenation of 60 acid sulfate soil samples that were collected from a typical acid sulfate soil region. The soil samples were characterized by low pH (3.3–5.8) and high sulfate content (0.51–5.03 mg/g). The cumulative concentrations of CO2 were 0.76–54.54 μmol per g dry soil upon oxygenation for 24 h. When 0.1% HgCl2 was added into soil suspension to kill microbes, the yield of CO2 decreased by 8.7%–41.1%, which suggests that both microbial respiration and abiotic pathways contributed to CO2 production and abiotic pathway played a major role. Both acidic dissolution and hydroxyl radical (OH) oxidation were identified to be the abiotic mechanisms for CO2 production. For acidic dissolution, CO2 was produced from the decomposition of carbonate by H+ generated. Production of CO2 from acidic dissolution was dependent on the content of carbonate and the generation of H+. A combination of Fe speciation sequential extraction and kinetic model shows that H+ was mainly generated from the oxidation of both Fe2+ and pyrite by O2. For OH oxidation, CO2 was produced from the oxidation of organic carbon by OH produced and its relative importance for CO2 production was estimated to be 14.6%–27.6%. OH was mainly produced from exchangeable Fe(II) oxidation in sulfuric horizon but from pyrite oxidation in potential acid sulfate soils. This study presents the overlooked abiotic mechanisms, acidic dissolution and OH oxidation, for CO2 emission in acid sulfate soils when perturbed by O2. Our findings implicate that contribution of microbial respiration for CO2 production is not as large as previously thought to be in acid sulfate soils, and instead abiotic mechanisms contribute largely, particularly for oxygenation of potential acid sulfate soils.