Nitrogen circulation in a Mediterranean holm oak forest, La Castanya, Montseny, northeastern Spain

Abstract. The importance of storm frequency as well as the groundwater and hyporheic inputs on nitrate (NO₃-N) and ammonium (NH₄-N) levels in stream water were studied in a small perennial Mediterranean catchment, Riera Major, in northeast Spain. NO₃-N concentrations ranged from 0.15 to 1.9 mg l^-1. Discharge explained 47% of the annual NO₃-N concentration variance, but this percentage increased to 97% when single floods were analysed. The rate of change in nitrate concentration with respect to flow, ΔNO₃-N/ΔQ, ranged widely from 0 to 20 μg NO₃-N s l^-2. The ΔNO₃-N/ΔQ values fitted to a non linear model with respect to the storm flow magnitude (ΔQ) (r²=0.48, d.f.=22, P<0.01). High values of ΔNO₃-N/ΔQ occurred at intermediate ΔQ values, whereas low ΔNO₃-N/ΔQ values occurred during severe storms (ΔQ > 400 l s^-1). N₃-N concentrations exhibit anticlockwise hysteresis patterns with changing flow and the patterns observed for autumnal and winter storms indicated that groundwater was the main N₃-N source for stream and hyporheic water. At baseflow, NO₃-N concentration in groundwater was higher (t=4.75, d.f.=29, P>0.001) and co-varied with concentrations in the stream (r=0.91, d.f.=28, P<0.001). In contrast, NO₃-N concentration in hyporheic water was identical to that in stream water. The role of the hyporheic zone as source or sink for ammonium was studied hyporheic was studied comparing its concentrations in stream and hyporheic zone before and after a major storm occurred in October 1994 that removed particulate organic matter stored in sediments. Results showed high ammonium concentrations (75±28 s.d. μg NH₄-N l^-1) before the storm flow in the hyporheic zone. After the storm, the ammonium concentration in the hyporheic dropped by 80% (13.6±8 μg N₄-N l^-1) and approached to the level found in stream water (11±8 μg NH₄-N l^-1) indicating that indisturbed hyporheic sediments act as a source for ammonium. After the storm, the ammonium concentrations in the stream, hyporheic and groundwater zones were very similar suggesting that stream ammonium concentrations are sustained mainly by input from groundwater. The present study provides evidence that storm flow magnitude is an important source of variability of nitrate concentration and fluxes in Mediterranean streams subjected to an irregular precipitation regime with prolonged dry periods.

Keywords: nitrate, discharge regime, hyporheic zone, groundwater, Mediterranean, stream, Riera Major