Sources and fate of terrestrial dissolved organic carbon in lakes of a Boreal Plains region recently affected by wildfire

[摘要] Downstream mineralization and sedimentation of terrestrial dissolved organiccarbon (DOC) render lakes important for landscape carbon cycling in theboreal region. The chemical composition of terrestrial DOC, the downstreamdelivery of terrestrial DOC and its processing within aquatic ecosystem mayall be influenced by climate change, including increased occurrence ofwildfire. Here, we assessed composition and lability (during both dark- andUV incubations) of DOC from peatland groundwater and mineral soil groundwater,and from shallow lakes within a peatland-rich region on the Boreal Plains of western Canada that was recently affected by wildfire. Wildfire was found to increase aromaticity of DOC inpeat pore water above the water table, but had no effect on theconcentrations or composition of peatland groundwater DOCor mineral soil DOC. Using a mixing model we estimated that onaverage 98 and 78% of terrestrial DOC inputs to the lakes in fine- andcoarse-textured settings, respectively, originated from peatland groundwatersources. Accordingly, lake DOC composition reflected primarily a mixing ofpeatland and mineral soil groundwater sources, with no detectable influenceof the recent wildfire. Lake and peatland DOC had low biodegradability,lower than that of mineral soil DOC. However, both mineralization andsedimentation of peatland DOC increased substantially during UV incubationsthrough selective removal of aromatic humic and fulvic acids. Similar shiftsin DOC composition as observed during the UV incubations were also observedacross lakes with longer water residence times. The mixing model estimatedthat on average 54% (95% confidence interval: 36–64%) ofterrestrial DOC had been removed in lakes as a result of mineralization andsedimentation. Meanwhile, the reduction in absorbance at 254 nm was 71% (58–76%), which suggests selective removal of aromatic DOC. Hence, incubationresults, patterns of DOC composition among lakes and mixing model resultswere consistent with significant within-lake removal of terrestrial DOCthrough UV-mediated processes. Selective removal of highly aromatic DOCthrough UV-mediated processes implies that organic sources that areconsidered stabile in terrestrial ecosystems can be readily mineralized onceentering aquatic ecosystems. Together, our results suggest that regionalcharacteristics (climate, surface geology and lake morphometry) can preventwildfire from causing pulse perturbations to the linkages betweenterrestrial and aquatic C cycling and also regulate the processes thatdominate within-lake removal of terrestrial DOC.

[学科分类] 地球化学与岩石