『Abstract
　Human activities have greatly increased the input of biologically available nitrogen (N) from land-based sources to aquatic ecosystems; yet few studies have examined how human actions influence N export in regions with a strong seasonality in water availability. In this study, we quantified N inputs and outputs for 23 California watersheds and examined how climate, hydrology, and land use practices influenced watershed N export. N inputs ranged from 581 to 11,234 kg N km^-2 year^-1 among watersheds, with 80％ of total input for the region originating from agriculture (inorganic fertilizer, manure, and legumes). Of the potential N sources examined, mean annual concentration of dissolved organic N and dissolved inorganic N in study rivers correlated most strongly with manure N input (r² = 0.54 and 0.53, respectively). Seasonal N export varied by basin and was correlated with climate, anthropogenic N inputs, and reservoir releases. Fractional export of watershed N inputs by study rivers annually was small (median of 8％) and scaled exponentially with runoff (r = 0.66). Collectively, our results show that anthropogenic activities have altered both the magnitude and timing of watershed N export in California and suggest that targeted management in specific locations and times of the year could reduce N export to downstream systems in the region.

Keywords: Agriculture; California; Denitrification; Nitrogen; Nutrient budget; Watershed; Mediterranean climate』

Introduction
Site descriptions
Methods
　Watershed characteristics
　Nitrogen input
　Atmospheric deposition
　N-fixation
　Inorganic fertilizer and manure application
　Sewage input
　Anthropogenic N removal
　Total net watershed N input
　Sensitivity analysis of of N inputs
　Watershed N export
　Seasonal pattern of N export
Results and discussion
　N inputs in the CV
　N export
　Seasonal N export
　Factors influencing annual N export
　Summary and implications
Acknowledgments
References