Effect of deep peat warming on porewater chemistry in a northern peatland ecosystem

Background/Question/Methods

Peatlands have accumulated organic carbon in peat soils over thousands of years.  Even though these ecosystems only cover 3% of the land surface, they store approximately 30% of terrestrial carbon.  Climate change is projected to have a disproportionate effect at higher latitudes where many peatlands are located.  Experiments evaluating the effects of warming on the fate of peatland carbon stocks and associated biogeochemical cycles are needed.  The Spruce and Peatland Responses Under Climatic and Environmental change (SPRUCE) project is an ecosystem-scale experiment examining the responses of an ombrotrophic black spruce-Sphagnum bog in northern Minnesota to elevated temperature (+0, +2.25, +4.5, +6.75, and +9°C) and CO₂ concentrations (800-900 ppm) using 12-m diameter, open-topped chambers.  The treatments will run for 10 years beginning in summer 2015.  In June 2014, belowground warming was initiated to evaluate the responses of deep peat soils (warming targeted at 2 m depth; 4000-7000 year old peat).  We sampled depth-specific piezometers (0-3 m depth) biweekly to examine the responses of peat porewater chemistry to warming.  We predicted that mineralization of peat at 2 m depth would increase total organic carbon (TOC) concentration, with potential effects on coupled biogeochemical cycles (i.e., increased ammonium concentrations).

Results/Conclusions

The 10 experimental plots reached their target temperatures between July and mid-September 2014.  There were no significant changes in porewater TOC concentrations at 2 m across all treatments during the July to mid-September period, and TOC concentrations averaged 35 mg/L.  However, by the end of the sampling season (October/November 2014), TOC concentrations increased in the warmest plots.  On average, TOC increased by 2 mg/L in the +4.5°C plots, and by 5 mg/L in the +6.75°C and +9°C plots.  In the +0°C and +2.25°C plots, TOC concentration changes were less than 1 mg/L.  These changes were only observed at 2 m depth where the deep peat heating was targeted.  Deep peat heating did not affect porewater pH or specific conductivity at 2 m depth.  Porewater samples are currently being analyzed for nutrients (ammonium, nitrate, phosphate), anions, and cations, and responses of the full suite of solutes will be investigated.