Hydrological Controls on Nitrogen and DOC Transport at the Plot, Hillslope and Catchment Scale, HJ Andrews Experimental Forest.

While the flushing of nutrients at the catchment scale has been described in many forested environments during the last decade, the flushing mechanisms, flowpaths and geographic sources of different N species (DON, NO3- and NH4+) and DOC are still poorly resolved, especially during different storm size and antecedent wetness conditions. We characterized flowpaths of N and DOC at the hillslope scale during and between storm events in WS10, H.J. Andrews, Oregon, USA, for five storms over the period Fall 2004 until Spring 2005. This catchment is dominated by hillslopes with negligible riparian water storage due to 1986 and 1996 debris flows that evacuated the valley bottom. This enabled us to study the hydrological and biogeochemical coupling between the hillslope and catchment in a way unimpeded by riparian zone groundwater dynamics.Through a combination of hydrometric and chemistry data from groundwater wells, tension and zero tension lysimeters at different depths, tensiometers, soil moisture probes and hillslope runoff from a 10 meter wide trench at the hillslope, we were able to resolve the dominant flowpaths. Fluorescence (a proxy for DOC) of hillslope and catchment runoff was monitored continuously with a fluorometer during storms. Preliminary data analysis showed a significant relationship between DOC concentrations and fluorescence values suggesting that fluorescence can be used to characterize DOC dynamics at small time scales. Our high frequency DOC characterization showed a clockwise hystersis pattern of DOC and total N against discharge for both hillslope and catchment runoff. This suggests flushing of nutrients in near and/ or in stream zones during the initial part of the storm. Total N and DOC concentrations in groundwater wells and lysimeters at shallow soil depths were high compared to other potential sources during storms. Our interpretation is that vertical preferential flow of high concentration water drives the groundwater contribution to hillslope- and catchment observed concentrations. Tensiometer data indicates that transient saturation at subtle changes in hydraulic conductivity within the soil profile storm might drive rapid lateral delivery of nutrients to the stream during the storm peak. Overall, our results suggest that increased nutrient concentrations during storms are caused by vertical and lateral preferential flow, and that thresholds at the hillslope scale modulate the connect from plot to the catchment scale.