Transport of dissolved organic matter from soils to surface water in agricultural areas: identifying and modeling the pathways and processes.

All natural waters contain organic matter in the form of Dissolved Organic Matter (DOM) and Particulate Organic Matter (POM). The organic matter in natural waters is either from terrestrial origin (allochthonous), is biologically produced in situ (autochthonous) or comes from domestic and industrial effluents. The dissolved (<0.45µm) rather than the particulate (>0.45 µm) fraction is of largest environmental significance in the aquatic system. The dissolved organic matter (DOM) affects a suite of processes in surface water and soils because it provides energy and nutrients for the biota and it affects the bioavailability of trace metals and organic pollutants. In addition, DOM plays a role as electron donor in denitrification in riparian zones, and leads to the formation of unwanted disinfection by-products in drinking water production. The stream flow of a catchment is generated by a combination of (1) baseflow (return flow from groundwater), (2) interflow (rapid subsurface flow through pipes, macropores, and seepage zones in the soil), and (3) overland flow from a saturated soil surface. The fate of DOM differs along these flow paths and, therefore, hydrology, topography, and geographical distribution of different DOM source areas and how they are connected through flow paths to the discharging river play a critical role for DOM effluxes from headwater catchments. DOM composition is similarly affected by the hydrology. High flow events also lead to preferential flow through the unsaturated zone reducing the interaction between the percolating fluid and the soil matrix and consequently reducing the sorption of the more strongly sorbing DOM compounds. Most of the research on DOM transport in soils and the geographical and topographical controls on DOM export to rivers has been carried out in forests, wetlands or peat areas whereas relatively little work has been done in agricultural areas. Moreover, efforts to model the DOM export are rare. Key missing information for developing such model is a quantitative description of the DOM source term and a model describing the fate of DOM during its passage through soil. The objectives of this research project are (i) to identify the source of DOM in soil and its transport pathways to surface water in agricultural areas, and (2) to develop a mechanistic model describing these processes at watershed scale. To assess the relative importance of flow paths, an observational and an experimental approach will be combined. The observations will be made in two areas with very distinct soilscapes: one set of measurements will be conducted in a 117-ha agricultural catchment in the erosion-prone silt-loam belt in central Belgium, characterized by deep silt-loam Luvisols and a smooth, undulating topography. This catchment is dominated by arable land. Another set of observations will be made in a small (10-50 ha) headwater catchment (Rollesbroich catchment) in the Eifel region near Aachen, Germany. This catchment is dominated by grassland. The experiments will assess the effects of land management practices on DOM fluxes by overland flow (rainfall simulation experiments) and by vertical migration in the field. The land management practices (treatments) are conventional tillage versus conservation tillage and impacts of the application of animal manure. Emphasis in our modeling work will be on using the model to test hypotheses on the transport pathways and mechanisms for the two contrasting catchments.

Keywords:catchment, transport pathways, modeling, disolved organic matter

Disciplines:Physical geography and environmental geoscience, Soil sciences, challenges and pollution, Agriculture, land and farm management