The influence of landscape organized heterogeneity on riverine nitrate dynamics

Landscape organized (or structured) heterogeneity is often assumed to influence hydrological and biogeochemical patterns across space and time. In this study, we quantified landscape organized heterogeneity with two indices describing the spatial configuration of nitrogen sources or sinks regarding 1) their hydrological distance to the nearest stream (i.e. upslope/downslope heterogeneity: in the lateral dimension) and 2) their hydrological distance to the outlet in the river network (i.e. upstream/downstream heterogeneity: in the longitudinal dimension). The nitrogen sources considered are agricultural fields, defined from interpretation of satellite images, and the sinks are riparian wetland, defined from a topoclimatic index. Using public nitrate concentration and discharge data from 180 catchments in western France (5-150km²), we tested whether landscape organized heterogeneity influenced riverine nitrate concentration and dynamics. The metrics computed to characterize nitrate concentration and dynamics were the flow-weighted concentration (FWNO3), the slope of the log(C)-log(Q) relationship (slope b) and the ratio of the coefficients of variation of concentration and discharge (CVratio). Results showed a high positive correlation between slope b and the CVratio, but no correlation between the later and FWNO3. 43% of the catchment exhibited a positive b slope, indicating maximum nitrate during the winter high flow period and 17% exhibited a negative b slope, indicating maximum nitrate during the summer/fall low flow period; the remaining 40% exhibited a near-zero slope. Landscape organized heterogeneity was larger in the lateral dimension for both nitrogen source and sinks than in the longitudinal dimension. In the lateral dimension, nitrogen sources were primarily located upslope and nitrate sinks downslope. In the longitudinal dimension, no general trend was observed for nitrogen sources and nitrate sinks were rather located upstream. Heterogeneity in the lateral dimension was highly variable among catchments for the smaller catchments and less variable for the larger ones. Heterogeneity in the longitudinal dimension did not exhibit a visible relationship with catchment size. No relationship was found between indices of landscape heterogeneity and FWNO3, arguably because other primary factors (such as the nitrogen surplus or runoff) control most of the regional variability in FWNO3. We found non-linear relationships between our indices of nitrogen sink organization and the b-slope or the CVratio, both in the lateral and longitudinal dimensions. The catchments with a negative b-slope (maximum nitrate during low-flow season) had their wetlands located more upstream and/or more upslope than the average. The relationship with nitrogen sources were opposite by construction (agricultural fields are often located outside wetland areas) but less clear. Further work is ongoing to explore the influence of landscape spatial organization on phosphorus concentration and dynamics.