Factors controlling nitrogen release from two forested catchments with contrasting hydrochemical responses

Quantifying biogeochemical cycles of nitrogen (N) and the associated fluxes to surface waters remains challenging, given the need to deal wish spatial and temporal variability and to characterize complex and heterogeneous landscapes. We focused our study on catchments S14 and S15 located in the Adirondack Mountains of New York, USA, which have similar topographic and hydrologic characteristics but Contrasting stream nitrate (NO3-) concentrations. We characterized the mechanisms by which NO3- reaches the streams during hydrological events in these catchments, aiming to reconcile our field data with our conceptual model of factors that regulate nutrient exports from forested catchments. Combined hydrometric, chemical and isotopic (delta O-18-H2O) data showed that the relative contributions of both soil and ground water sources were similar between the two catchments. Temporal patterns of stream chemistry were markedly different between S14 and S15, however, because the water sources in the two catchments have different solute concentrations. During late summer/fall, the largest source of NO3- in S14 was till groundwater. whereas shallow soil was the largest NO3- source in S15. NO3- concentrations in surface water decreased in S14. whereas they increased in S15 because an increasing proportion of stream flow was derived from shallow soil sources. During snowmelt, the largest sources of NO3- were in the near-surface soil in both catchments. Concentrations of NO3- increased as stream discharge increased and usually peaked before peak discharge, when shallow soil water sources made the largest contribution to stream discharge. The timing of peaks in stream NO3- concentrations was affected by antecedent moisture conditions. By elucidating the factors that affect sources and transport of N, including differences in the soil nutrient cycling and hydrological characteristics of S14 and S15, this study contributes to the overall conceptualization of NO3- release front temperate forested catchments. Copyright (C) 2007 John Wiley & Sons, Ltd.