Peat porewater dissolved organic carbon concentration and lability increase with warming: a field temperature manipulation experiment in a poor-fen

Studies conducted across northern Europe and North America have shown increases in dissolved organic carbon (DOC) in aquatic systems in recent decades. While there is little consensus as to the exact mechanisms for the increases in DOC, hypotheses converge on such climate change factors as warming, increased precipitation variability, and changes in atmospheric deposition. In this study, we tested the effects of warming on peat porewater composition by actively warming a peatland with infrared lamps mounted 1.24 m above the peat surface for 3 years. Mean growing season peat temperatures in the warmed plots (n = 5) were 1.9 ± 0.4 °C warmer than the control plots at 5 cm depth (t statistic = 5.03, p = 0.007). Mean porewater DOC concentrations measured throughout the growing season were 15 % higher in the warmed plots (73.4 ± 3.2 mg L^?1) than in the control plots (63.7 ± 2.1 mg L^?1) at 25 cm (t = 4.69, p < 0.001). Furthermore, DOC from the warmed plots decayed nearly twice as fast as control plot DOC in laboratory incubations, and exhibited lower aromaticity than control plot porewater (reduction in SUVA₂₅₄ in heated plots compared with control plots). Dissolved organic nitrogen (DON) concentrations tracked DOC patterns as expected, but the amount of dissolved N per unit C decreased with warming. Previous work has shown that warming increased net primary production at this site, and together with measured increases in the activities of chitinases and glucosidases we suggest that the increased DOC concentrations observed with warming were derived in part from microbial-plant interactions in the rhizosphere. We also detected more nitrogen containing compounds with higher double bond equivalents (DBE) unique to the warmed plots, within the pool of biomolecules able to deprotonate (16 % of all compounds identified using ultrahigh resolution ion electrospray mass spectrometry); we suggest these compounds could be the products of increased plant, microbial, and enzyme activity occurring with warming. With continued warming in peatlands, an increase in relatively labile DOC concentrations could contribute to dissolved exports of DOC in runoff, and would likely contribute to the pool of efficient electron donors (and acceptors) in the production of CO₂ and CH₄ in terrestrial and aquatic environments.