A Large-scale Warming Experiment Reveals Changes to Stream Flow and Solute Concentrations in a Northern Peatland Ecosystem

Peatlands are sources of water to downstream ecosystems. Therefore, changes to peatland hydrology and biogeochemistry can affect the quantity and quality of stream water that drains to downstream systems. Increased temperatures may reduce stream flow and alter the chemistry of peatland waters; however, the effects of warming on peatland hydrology and biogeochemistry have not been tested at an ecosystem scale. The Spruce and Peatland Responses Under Changing Environments (SPRUCE) experiment is examining the effects of warming and elevated CO₂ on peatland ecosystem dynamics. Ten, 12-m diameter, 7-m-tall enclosures were constructed within a northern Minnesota peatland to increase air and soil temperatures (+0, +2.25, +4.5, +6.75, +9 °C) at ambient or elevated (+500 ppm) CO₂ concentrations. A belowground corral and outflow system is used to measure water that passively and naturally drains from each enclosure (termed "outflow" which is analogous to stream flow at the entire-peatland scale), and flow-weighted water chemistry sampling is automated. After two years of warming, there were substantial decreases in outflow from the warmer enclosures. Total annual outflow was on average 44% lower from the +9 °C enclosures compared to the +0 °C enclosures in 2016, and 37% lower in 2017. This reduction in outflow likely reflects increased evapotranspiration in the warmer enclosures. In contrast, the response of water chemistry to warming was variable. Nutrient concentrations (i.e., ammonium, nitrate, phosphate, total nitrogen [N], total phosphorus [P]) were similar across warming treatments; however, total organic carbon (TOC) and cation (i.e., calcium, magnesium) concentrations were elevated. Higher TOC and cation concentrations may reflect increased mineralization rates. It is likely that N and P mineralization also increased with warming; however, elevated N and P concentrations may not have been measurable due to rapid uptake by biota in this nutrient-poor ecosystem. Overall, the first two years of experimental warming provide evidence that the quantity and quality of peatland drainage water will change with increased temperatures that are expected in the future. Continued measurements will assess the longer-term (i.e., 10 year) responses to warming.