Relating nitrogen export patterns from a mixed land use catchment in NW Spain with rainfall and streamflow.

The temporal variability in nitrogen (N) transport in the Corbeira agroforestry catchment (NW Spain) was analysed from October 2004 to September 2008. Nitrate (NO₃-N) and total Kjeldahl nitrogen (TKN) loads and concentrations were determined at various timescales (annual, seasonal and event). The results revealed a strong intra-annual and inter-annual variability in N transport influenced by weather patterns and consequently by the hydrological regime. Mean annual export of total N in the catchment was 5.5 kg ha⁻¹ year⁻¹, with NO₃-N being the dominant form. Runoff events comprised 10% of the study period but contributed 40 and 61% of the total NO₃-N and TKN loads, respectively. The NO₃-N and TKN concentrations were higher during runoff events than under baseflow conditions, pointing to diffuse sources of N. The mobilization of TKN during runoff events was attributed to surface runoff, while NO₃-N might be related to subsurface and groundwater flow. Runoff events were characterized by high variability in N loads and concentrations. Higher variability was observed in N loads than in N concentrations, indicating that event magnitude plays an important role in N transport in this catchment; event magnitude explained approximately 96% of the NO₃-N load. However, a combination of variables related to runoff event intensity (rainfall, discharge increase and kinetic energy) explained only 66% of the TKN load. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]