Flushing of nitrate from a forested watershed: An insight into hydrological nitrate mobilization mechanisms through seasonal high-frequency stream nitrate dynamics

The variability of hydrologically induced mobilization of nitrate during sequences of hydrological events in different seasons was studied through high-frequency measurements of streamwater nitrate concentrations accompanied by a detailed hydrometeorological monitoring system. The study was carried out in 2006, within the 42 km2 forested Padež watershed in the southwestern part of Slovenia, which is characterized by distinctive flushing, an almost torrential hydrological regime influenced by impermeable flysch geological settings. More than 15 recorded hydrographs which, in the hydrological and biogeochemical sense, differed substantially, disclosed a highly variable but at the same time a strong linkage between hydrological and biogeochemical controls of nitrate exports from the spatial perspective of a watershed. During most of the hydrographs, with the exception of early spring rainfall events, a positive relationship between the nitrate concentration and discharge was observed with peak nitrate concentrations having a time delay in the order of a few hours after the hydrograph peaks. Peak nitrate concentrations in periods of rainfall events span from 3.5 mg/l-N in late spring to 14 mg/l-N in the case of the autumn hydrograph. However, the dilution effect as a consequence of high event water contributions was observed in certain events. The role of specific hydrological events on the nitrate mobilization proved to be important as the size of the accumulated nitrate pool available for mobilization was large throughout most of the hydrographs. The biogeochemical environment of the forest soils presumably drastically alters the size of the available nitrate pool at the studied watershed and, together with the specific hydrological conditions, shifts the watershed from the flush-limited state in the late spring, summer and autumn to the source-limited state in winter and early spring.