Transport of dissolved organic carbon from soil to surface water: Identification and modeling of pathways and controlling factors

Dissolved organic carbon (DOC) in the surface water is an important component of the global carbon cycle. Due to its active and mobile nature, it affects aquatic ecosystems in several ways and plays a significant role in the cycling and distribution of carbon within and between ecosystems. Although the estimated annual flux of DOC from the land to the oceans is ±0.4 Pg C/year, almost half the current net terrestrial uptake of ±0.9 Pg C/year, the factors controlling the transport of DOC from the soil to the surface water are not clear. Previous research on the transport of DOC in the soil and on the factors controlling DOC export towards the river system has mostly been carried out in forest and wetland areas. However, agricultural land use can lead to significant surface runoff and thereby enable a surface runoff pathway for the transport of DOC. Little information is available on the transport of DOC through surface runoff from agricultural fields and on the factors controlling this transport. At the catchment scale, previous work has mainly focused on either regular sampling during baseflow conditions or high-frequency monitoring during a limited number of events, leading only to a partial understanding of the factors controlling the DOC transport. Although experimental data on DOC export from catchments are available, efforts to model the DOC export are scarce. Therefore, the general aim of this work was to determine the factors controlling the transport of dissolved organic carbon from the soil to the surface water and to identify and model the transport pathways. Field experiments were conducted at different temporal and spatial scales. At the plot scale, rainfall simulations were carried out to identify the effect of soil properties, field characteristics and hydrological conditions on DOC export by surface runoff from agricultural fields. Additionally, the temporal evolution of DOC concentrations and specific UV absorbance (SUVA) values in runoff water during a rainfall event was observed. SUVA is an indicator of the aromaticity and the recalcitrant nature of DOM, whereby higher SUVA values are measured when the DOM is more aromatic. Four small headwater catchments contrasting in land use and hydrogeology were monitored to study the transport pathways delivering DOC towards the surface water at the catchment scale. Stream water was sampled on a regular base during dry weather conditions and at high frequency during rainfall events, to observe the temporal variation of DOC concentrations both seasonally and at the time scale of a rain event. Stream water, groundwater, soil pore water, precipitation/throughfall and riparian zone water samples were additionally analyzed for silica and major cations, allowing an end-member mixing analysis that gained insight into the contributing pathways delivering DOC at the catchment outlet during different flow regimes. For the Blégny grassland catchment, stream water DOC concentrations were modeled as a simple mixture of DOC from the different transport pathways delivering water at the catchment outlet. Therefore, discharge measured at the catchment outlet was modeled as a combination of discharge from different components using the FLEX hydrological model and the WETSPRO model. Results from the rainfall simulations showed that the antecedent rainfall conditions are the most important control on DOC concentrations and quality in surface runoff water from agricultural fields. Lower amounts of rainfall before the experiment, or lower initial moisture content of the soil lead to high concentrations of low aromatic DOC in the runoff water. Soil and field characteristics only have limited effect on DOC concentrations in runoff. Overall, DOC concentrations in runoff water are highest and SUVA values are lowest at the start of a rainfall event. In the stream water from small headwater catchments, DOC concentrations and SUVA values were higher in forest catchments than in pasture catchments. In our study catchments, no seasonal variation in baseflow stream DOC concentrations was observed. During rainfall events in all catchments however, both DOC concentrations and SUVA values increased with discharge, reached a maximum and decreased again as discharge returned to pre-event baseflow values. Overall, the majority of the total annual export of DOC from the study catchments was transported to the catchment outlet during times when discharge was elevated in response to a rainfall event. The changes in concentrations and quality of DOC during discharge events could be attributed to a change in contributions of transport pathways of water to the stream. In the forested catchments with deep groundwater tables and thick unsaturated zones, the main contributions to the stream water during baseflow was via the groundwater. Rising stream DOC concentrations during rainfall events were attributed to additional throughfall and riparian zone transport pathways. In the grassland catchments with shallow groundwater tables, stream flow mainly originated from shallow groundwater discharged at seeps. During rain events, an additional transport pathway through the organic rich top soil layer and water from the riparian zone caused DOC concentration to rise. The importance of the contributing pathways in the grassland catchment changed seasonally and depended on the degree of saturation of the catchment soils. The WETSPRO model calculating DOC concentrations in the stream water as a mixture of DOC from the different transport pathways, was able to reproduce DOC concentrations observed in the Blégny catchment. As the FLEX hydrological model however did not succeed in an accurate prediction of discharge measured in the field, modeled subflows of the FLEX model did not allow a good prediction of the observed variation in stream water DOC concentrations. Overall, this research showed that on agricultural soils that generate considerable amounts of surface runoff during a rainfall event, surface runoff is an important pathway for the transport of DOC from the soil to the surface water. In future research, this transport pathway thus deserves our increased interest. At the catchment scale, the majority of the total annual export of DOC for the study catchments was transported to the catchment outlet during times when discharge was elevated in response to a rainfall event. This proves the value of our work, whereby regular sampling during dry weather baseflow conditions was combined with high-frequency monitoring during a great number of rainfall events. Our results also show that hydrological modeling of observed discharge as the sum of discharges from different transport components, combined with chemical analysis of the contributing sources waters, successfully allows the modeling of DOC concentrations observed in the stream water.nrpages: 97status: publishe