The role of hillslope hydrology in controlling nutrient loss

Abstract

Hydrological controls on DOC and N transport at the catchment scale were studied for five storm events from the fall of 2004 through the spring of 2005 in WS10, H.J. Andrews Experimental Forest in the western Cascade Mountains of Oregon. This catchment is devoid of any riparian zone and characterized by hillslopes that issue directly into the stream. This enabled us to compare a trenched hillslope response to the stream response without the influence of riparian zone mixing. DOC and N concentrations and dissolved organic matter (DOM) quality (specific UV-absorbance (SUVA) and C:N of DOM) were investigated at the plot scale, in lateral subsurface flow from the trenched hillslope and stream water at the catchment outlet at the annual and seasonal scale (transition vs. wet period) during baseflow and stormflow conditions. DON was the dominant form of total dissolved nitrogen (TDN) in all sampled solutions, except in transient groundwater, where DIN was the dominant form. Organic horizon leachate and transient groundwater were characterized by high SUVA, and high DOC and total N concentrations, while SUVA and DOC and DON concentrations in lysimeters decreased with depth in the soil profile. This suggests vertical preferential flow without much soil matrix interaction occurred at the site. Deep groundwater (from a spring at the base of the hillslope) was characterized by low SUVA and low DOC and N concentrations. SUVA was always lower in lateral subsurface flow than in stream water at the seasonal scale, even during the wet period when other solutes were similar between lateral subsurface flow and stream water. This suggested mixing of deep groundwater and shallow transient groundwater was different at the hillslope scale compared to the catchment scale. DOC and DON sources were finite (production of DOC and DON from the hillslope soils appeared to be limited) at the seasonal scale since DOC and DON concentrations were significantly lower during the wet period compared to the transition period during stormflow conditions. This was also reflected in the DOC and DON peak and flow weighted storm event concentrations and antecedent soil moisture relationship where drier conditions (less prior flushing) resulted in the highest DOC and DON peak and flow weighted storm event concentrations. Results from this study showed the importance of the hillslope component in DOC and N transport at the catchment scale and underscore the importance of sampling solutes below the root zone (transient groundwater) and the value of using SUVA to fingerprint DOC sources.