Sulfur Redox Status in Peatland Soil and Outflow Waters Diverge with Climate Warming

Boreal peatlands are important continental reservoirs of carbon and other elements. Changes in climate, especially increasing temperatures and more variable precipitation, alter oxidation-reduction (redox) conditions and fluxes of atmospheric and aquatic pollutants from peatlands. Here, we measure the effect of warming and elevated carbon dioxide on the speciation of sulfur in boreal peatland soil and outflow water over three years. In whole ecosystem warming experiments, with temperature levels of +0 °C (control), +2.25 °C, +4.5 °C, +6.75 °C, and +9 °C above ambient, water table height was negatively correlated with warming. Warming was correlated with changes in the size of sulfur pools, specifically, sulfur content (weight%) decreased in soils and sulfate (SO 4 2- aq) concentrations increased in outflow. Reflecting the warmer and drier conditions, the percentage of oxidized sulfur in soil, as
measured by X-ray absorption near edge structure (XANES) spectroscopy, increased with warming. Sulfur speciation in soil showed increases in ester-sulfate (R-O-SO 3 - ) content at the expense of organic disulfide (R-S-S-R’) content. In contrast to the soil, the percentage of oxidized sulfur decreased in outflow with warming. The changes in sulfur speciation in outflow were characterized by increased organic monosulfide (R-S-R’, R-S-H) content at the
expense of ester-sulfate. Overall, the peatland sulfur pools are becoming more oxidized in the soil and more chemically reduced in the outflow water in response to soil and air warming. The connection between these opposite redox trends is likely due to enhanced microbial activity in porewaters and outflow with warming. Specifically, we observed that ester-sulfate partitions from soil to outflow waters during heavy rainfall periods (based on weekly
precipitation). We surmise that increases in ester-sulfate in outflow make it available for microbial sulfur reduction processes that are also enhanced at warmer temperatures. Our study indicates that the peatland response to climate warming is complex: oxidation of sulfur in soil and the chemical reduction of sulfur in the outflow water are both correlated with warming. Notably, no significant effect of elevated carbon dioxide on sulfur pools was detected. Our findings are consistent with a net export of organic sulfur from the peatland to receiving surface waters. Furthermore, the overall loss of sulfur from this peatland is consistent with enhanced decomposition and increased plant available nutrients reported previously for this whole ecosystem warming experiment. Warming-induced changes to sulfur pools in peatlands affect the fluxes of other constituents, such as organic carbon and the pollutant methyl-mercury, that have downstream consequences for climate and water quality.