Watershed nitrogen input and riverine export on the west coast of the US
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- Schaefer, S.C., Hollibaugh, J.T. & Alber, M. Biogeochemistry (2009) 93: 219. doi:10.1007/s10533-009-9299-7
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This study evaluated the sources, sinks, and factors controlling net export of nitrogen (N) from watersheds on the west coast of the US. We calculated input of new N to 22 watersheds for 1992 and 2002. 1992 inputs ranged from 541 to 11,644 kg N km−2 year−1, with an overall area-weighted average of 1,870 kg N km−2 year−1. In 2002, the range of inputs was 490–10,875 kg N km−2 year−1, averaging 2,158 kg N km−2 year−1. Fertilizer was the most important source of new N, averaging 956 (1992) and 1,073 kg N km−2 year−1 (2002). Atmospheric deposition was the next most important input, averaging 833 (1992) and 717 kg N km−2 year−1 (2002), followed by biological N fixation in agricultural lands. Riverine N export, calculated based on measurements taken at the furthest downstream USGS water quality monitoring station, averaged 165 (1992) and 196 kg N km−2 year−1 (2002), although data were available for only 7 watersheds at the latter time point. Downstream riverine N export was correlated with variations in streamflow (export = 0.94 × streamflow − 5.65, R2 = 0.66), with N inputs explaining an additional 16% of the variance (export = 1.06 × streamflow + 0.06 × input − 227.78, R2 = 0.82). The percentage of N input that is exported averaged 12%. Percent export was also related to streamflow (%export = 0.05 × streamflow − 2.61, R2 = 0.60). The correlations with streamflow are likely a result of its large dynamic range in these systems. However, the processes that control watershed N export are not yet completely understood.