Comparing Fluxes of Dissolved Organic Carbon During Throughfall and Stream Events in a Forested Catchment

Abstract:

Constraining estimates of terrestrial to aquatic carbon flux remains a fundamental challenge for carbon cycle studies. Terrestrial-aquatic carbon transport is generally estimated by characterizing dissolved organic carbon (DOC) dynamics in streams and groundwater with little consideration of the interactions between vegetation and rainfall. Two hydrological pathways deliver rainfall from the forest canopy to the forest floor: throughfall and stemflow. Both carry tree-derived DOC (tree-DOC) from the vegetation to the forest floor and potentially beyond. We assessed DOC in throughfall and stemflow in the Sleepers River Research Watershed in Vermont. Nearly all precipitation fell to the forest floor as throughfall with <1% arriving as stemflow. Tree-DOC concentrations ranged from 1 – 50 and 6 – 195 mg-C/L in throughfall and stemflow, respectively. Throughfall event DOC concentrations decreased exponentially with increasing storm size indicating tree-DOC transport is source-limited. Event DOC concentrations for 76 throughfall events were estimated using an exponential decay model prior to calculating throughfall event DOC flux during 2017 and 2018. Throughfall DOC fluxes were paired with DOC flux in stream water modeled from grab samples using the rloadest/loadflex package in R (Appling et al., 2015). Throughfall DOC fluxes exceeded stream export in all paired events. Total throughfall DOC flux from the canopy between May and November was ~3 kg C (7.5g C m^-2) compared to the total stream DOC flux of ~0.3 kg C (0.75 g C m^-2) during the same period. Excluding winter tree-DOC flux, annual tree-DOC flux was ~5 times the annual stream DOC flux. These results indicate canopy DOC export is a significant carbon flux warranting further investigation in forested ecosystems and the streams that drain them.