Dissolved Inorganic Carbon Export Across the Soil/Stream Interface and Its Fate in a Boreal Headwater Stream

Abstract

The aim of this investigation was to determine the lateral exportof dissolved inorganic carbon (DIC) from soils of a Swedish boreal forest to a first order stream and to estimate the partitioning of this DIC into CO2 evasion from the stream surface and the DIC pool exported down through the catchment by streamwater. The groundwater entering the stream was supersaturated with CO2 with values as high as 17 times equilibrium with the atmosphere. Up to 90% of the estimated daily soil DIC export to the stream was emitted to the atmosphere as CO2 within 200 m of the water entering the stream. The annual DIC export from the soil to the stream was estimated to be 3.2 (+/- 0.1) g C m(-2) yr(-1) (normalized to catchment size). Ninety percent of the variation in soil DIC export could be explained by the variation in groundwater discharge and the DIC concentrations per se, were of minor importance. A significant correlation (R(l) = 0.74, P < 0.01) between soil DIC export and CO2 emission from the stream surface suggests that emission dynamics were primarily driven by the export of terrestrial DIC and that in-stream processes were less important. Our results reveal that current budget estimates of lateral DIC export from soils to aquatic conduits need to be revised because they do not account for conditions prevailing in headwater streams. Any quantification of lateral stream C export and CO2 emissions from freshwater systems must include headwater streams as well as the lower parts of the aquatic conduit.

Hydro-ecological controls on dissolved carbon dynamics in groundwater and export to streams in a temperate pine forest

“…4-5). As reported in other studies (Fiedler et al, 2006;Öquist et al, 2009), carbon export rates were mainly determined by discharge, the variations in carbon concentrations and exports being relatively small compared to the flow variation. However, for the whole sampling period, the mean weighted carbon export is almost the same for both DIC (0.9 ± 0.5 mmol m −2 day −1 ) and DOC (0.7 ± 0.7 mmol m −2 day −1 ) (Table 5), and the forest ecosystem exports in total 1.6 ± 0.9 mmol m −2 day −1 (equivalent to 7.0 ± 3.9 g C m −2 yr −1 ), 40 % as DOC and 60 % as DIC (Table 6).…”

“…Spatially, groundwater DOC was on average higher at the mesophyllous to wet Landes station (Bilos and piezometer 3), than at the dry Landes (piezometer 2) during periods with a low water table (Table 4). Indeed, several studies have reported decreasing DOC concentrations in groundwater in concurrence with increasing subsoil thickness and water table depth (Datry et al, 2004;Goldscheider et al, 2006;Pabich et al, 2001), with DOC concentrations at or close to zero reported in deep (> 1 km) and old groundwater (Pabich et al, 2001). At our study site, the fraction of groundwater DOC that predominates at a low water table was probably more recalcitrant, more stabilized and more aged than during high flow.…”

“…This terrestrial carbon leaching from groundwater to streams is of the same order of magnitude as carbon leaching from subsoils (11.9 ± 5.9 g C m −2 yr −1 ) in five temperate forest plots across Europe (Kindler et al, 2011), in a temperate Japanese deciduous forest from soils to streams (4.0 g C m −2 yr −1 ) (Shibata et al, 2005), or in European forests (9.6 ± 3.2 g C m −2 yr −1 ) (Luyssaert et al, 2010). As in groundwater, DOC and DIC concentrations in firstorder streams were significantly anticorrelated ( Table 2), suggesting that dissolved carbon dynamics in streams are mostly impacted by groundwater inputs (Kawasaki et al, 2005;Öquist et al, 2009). We could observe higher DOC concentrations in streams during early winter and high-flow periods than during growing season and late summer periods (Table 4).…”

“…These studies are generally restricted to submarine and coastal environments (Atkins et al, 2013;Sadat-Noori et al, 2016;Santos et al, 2012) and boreal lakes (Einarsdottir et al, 2017), but rarely streams. The few studies that estimate exports of carbon from forested landscapes to streams are generally (i) based on carbon observations in soil water (i.e., in the unsaturated zone of the soil) combined with a soil water model that simulates the volume of soil water leached to streams (Öquist et al, 2009;Kindler et al, 2011;Leith et al, 2015), (ii) based on carbon observations in stream combined with stream discharge (Billett et al, 2004;Dawson et al, 2002;Olefeldt et al, 2013;Shibata et al, 2001Shibata et al, , 2005, or (iii) as described by the active pipe concept (Cole et al, 2007), as the sum of the three major riverine carbon fluxes: CO 2 degassing, organic carbon burial in sediments and carbon export downstream (Jonsson et al, 2007). These studies do not provide a complete understanding of the link between carbon hydrological export and the physicochemical and biological processes occurring in soils and groundwater.…”

A fertile peatland forest does not constitute a major greenhouse gas sink

“…However, there was no consistent snow layer in Skogaryd in 2008, so that the impact of the spring melting period was probably negligible. Total inorganic C (TIC) export reported in the literature seems to be of minor importance (< 30 kg C ha −1 yr −1 ; Öquist et al, 2009;Rantakari et al, 2010;Wallin et al, 2010;Dyson et al, 2011). Overall, with reported maxima of 30 and 115 kg C ha −1 yr −1 , the contribution of discharge C losses to the total C budget at Skogaryd is probably low as literature The measured CH 4 flux was very low and always negative; i.e.…”