Spatial and Temporal Variation in Dissolved Organic Matter Source and Export from a Disturbed Sub-Tropical Peatland

Abstract

Ombrotrophic forested peatlands such as the Great Dismal Swamp (GDS) accrue large amounts of organic carbon in saturated soils, where slowed decomposition due to anoxic conditions historically made these systems major carbon sinks. However, following centuries of disturbance from drainage, many peatlands are now carbon sources, as oxidative decom position processes cause larger outfluxes of carbon than are replenished within the system. Estimates of peatland carbon loss primarily account for atmospheric fluxes; however, the construction of drainage networks within these systems facilitates surface water connections that potentially allow for substantial export of dissolved organic matter (DOM). In this study, we describe the spatial and temporal trends of DOM across a catchment within the GDS, compare the qualities of the DOM pool to water soluble organic matter extracted from plant and soil sources, and calculate an areal dissolved organic carbon (DOC) export flux. DOC concentrations varied across the catchment, from low concentrations with little sea sonal variation at the catchment inflow (18.6 ± 3.00 mg/L), to much higher concentrations at the outflow and larger variance across seasons (85.5 ± 23.3 mg/L). Analysis of DOM quality suggests that the DOM pool across much of the refuge is dominated by humic and fulvic-like compounds throughout the year, with variation due to seasonal variations in organic matter input, temperature, and flow. DOM that is leaving the refuge is largely recalcitrant with aromatic qualities and presents signs of heavy microbial processing similar to groundwater and heavily humified lower peat horizons. DOC export estimates (via daily sensor-derived DOC concentrations and ditch flows) exhibit seasonal variation in magnitude associated with flow and organic matter inputs. Indeed, DOC exits the refuge in quantities comparable to measured atmospheric emissions from decomposing deadwood (0.22 tC/ha/yr, or roughly one-third dead wood emissions) and may shift the source-sink behavior of the refuge. Refuge management and restoration strategies, however, have significantly reduced flows from ditch networks and thus associated DOC export. Peatland rewetting has been shown to have variable effects on total carbon emissions; however, this work suggests that reducing outflow volume has the potential to limit the amount of DOM export and its downstream impacts.