Controls of vegetation, hydrology, and climate on DOC production in Alaskan peatlands

Northern peatlands are thought to serve as a net sink for atmospheric CO2, and store ~30% of the world’s soil C pool. Due to their high organic matter content, they also represent an important source of dissolved organic carbon (DOC) to aquatic ecosystems. In general, high latitude ecosystems are experiencing rapid climate change, which in peatlands can lead to soil moisture deficits with increased evapotranspiration, or to surface flooding where permafrost thaw causes ground subsidence (thermokarst). Because both the production and transport of DOC is controlled by hydrology, it is unclear how these hydrologic disturbances will affect DOC production, its quality, and its fate in northern peatlands. The Alaskan Peatland Experiment (http://www.uoguelph.ca/Apex), located near the Bonanza Creek Experimental Forest outside Fairbanks, examines the effects of altered hydrology on peatland C cycling through water table manipulations in a rich fen (including drought and flooding treatments initiated in 2005), and by studying a gradient of permafrost thaw in a forested bog (established in 2008). The objectives of this study were to quantify the effects of peatland type, vegetation, seasonal ice, and hydrologic disturbance on seasonal DOC production across the APEX sites. Results show that the thermokarst bog on average contained more DOC than the rich fen (average growing season concentrations in the bog: 65.94 ± 1.11 mg/L; fen: 53.22 ± 2.47 mg/L), and that DOC from the bog was more labile with lower specific ultraviolet absorbance (SUVA) values. In both sites, DOC concentrations and SUVA increased through the growing season, likely as a combined result of increased decomposition with warmer soils, loss of seasonal ice, and increased plant productivity. In the fen, experimental drought increased DOC concentrations in most study years. Seasonal water table fluctuations were more minimal in the bog, and DOC concentration did not vary with water table position or soil temperature. Previous studies have found that permafrost thaw stimulates plant productivity. In the thermokarst bog, but not the rich fen, we found strong positive relationships between rates of gross primary production (quantified as CO2 uptake through static chambers) on DOC concentrations (R2=0.74; p<0.001) and SUVA (R2=0.19; p=0.07). Together, these results show that biological controls on DOC production and quality vary among northern peatland types. Fluctuations in soil climate and hydrology and their effects on microbial activity appear to be dominant controls on DOC in the rich fen, while DOC in the thermokarst bog appears to be more strongly coupled with primary production. We suggest that information on hydrogeologic setting, plant community structure and productivity, and soil organic matter quality is required to accurately model DOC production and quality in northern peatlands.