『Abstract
　Globally, fertilizers are identified as principle sources of nitrate in waters of intensely cultivated areas. Here this general concept is appraised on a seasonal basis over a two year period, under temperate climatic conditions. Water (δ²H and δ¹⁸O) and nitrate (δ¹⁵N and δ¹⁸O) isotopes in surface water and groundwater suggest that freshwater is acting as a transport vector conducting nitrate from agricultural soils to groundwater and ultimately to surface water. Measured nitrate isotopes of organic and inorganic fertilizers and of nitrate in groundwater are used to constrain a conceptual apportionment model quantifying the relative seasonal N contributions in an area of intense potato production. source inputs differ strongly between the growing (summer and fall) and non-growing (winter and spring) periods. Chemical fertilizers and soil organic matter equally dominate and contribute to the growing period load, whereas soil organic matter dominates the non-growing period load, and accounts for over half of the overall annual nitrogen charge. These findings reveal the magnitude of nitrogen cycling by soil organic matter, and point to the benefits of controlling the timing of its nitrate release from this organic material. We conclude that strategies to attenuate contamination by nitrate in waters of temperate climate row-cropping regions must consider nitrogen cycling by soil organic matter, including the crucial role of crop residues throughout both the growing and non-growing seasons.

Keywords: Winter nitrification; Groundwater; Seasonal sources; Soil organic matter; Agricultural impacts』

Introduction
Regional context and the Wilmot watershed/aquifer system
Approach and methodology
　Sampling
　Analytical methods
　Calculations for source apportionment of nitrate
Results and interpretation
　Water and nitrate characteristics
　Delineation of isotopic fields for potential sources of nitrate
　Nitrate source apportionment in the Wilmot watershed
Discussion
　Sensitivity tests of source apportionment model
　Source apportionment of nitrate from the mass balance agronomic method
　Implications for assessing N cycling in agricultural lands of temperate regions
Conclusions
Acknowledgements
References