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This paper examines the combined effects of catchment complexity in terms of physiography, land use patterns, and lithology on the export of dissolved inorganic nitrogen and dissolved silica from heterogeneous nested catchments in the Grand-Duchy of Luxembourg. Using results from water quality monitoring at 24 sampling sites, we determined the first-order controls on these fluxes. Land cover with four classes (forest, agriculture, grassland and urban), dominant lithology (schist, marls, sandstone, limestone, alluvial sediments), physiographic indices (elevation, flow path length, ratio of flow path length to flow path gradient, topographic wetness index, and other), and a suite of hydrological indices (baseflow, flashiness index and runoff coefficient), were calculated and used as potential controls. Given the high co-dependence of the predictors, Partial Least Square Regression was used to shed light on the linkages between export fluxes and the metrics composed of the 19 selected catchment characteristics. The first-order controls were determined by calculating the Variable Influence on Projection (VIP). These values revealed that the overall dissolved inorganic nitrogen fluxes are controlled primarily by lithology, land cover, topographic wetness index and flow path length. The first-order controls of dissolved silica fluxes are runoff coefficient, average topographic slope and land cover. Fluxes of dissolved inorganic nitrogen and dissolved silica did not show any strong relation to catchment scale. Apart from the widely accepted effect of land cover, our results indicate that catchment topography has an essential impact on the fluxes of dissolved inorganic nitrogen. The ratio of flow path length and flow path gradient, previously suggested as a proxy of mean water transit time, exerted a relatively strong control on dissolved inorganic nitrogen fluxes (VIP>1) with a negative relation to dissolved inorganic nitrogen fluxes, suggesting that dissolved inorganic nitrogen fluxes decrease with an increasing flow path gradient.
