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TECHNICAL REPORTS

The Effects of Season and Agriculture on Nitrous Oxide Production in Headwater Streams

^a Dep. Biological Sciences, Univ. of Notre Dame, 191 Galvin, Notre Dame, IN 46556
^b current address: USEPA, 26 W. Martin Luther King Dr., ML 498, Cincinnati, OH 45268
^c current address: Dep. of Geography and Land Studies, Central Washington Univ., Ellensburg, WA 98926

^* Corresponding author (beaulieu.jake{at}epa.gov).

Received for publication January 3, 2008. Streams and rivers are a globally significant source of nitrous oxide (N₂O), a potent greenhouse gas. However, there remains much uncertainty in the magnitude of N₂O emissions from these sources, partly due to an incomplete understanding of the factors that control microbial N₂O production in lotic sediments. During 2004–2005 we measured sediment N₂O production in 12 headwater streams across an agricultural land use gradient. Stream water nitrate (NO₃^–) concentrations were positively related to the proportion of agricultural land use in the basin and frequently exceeded 20 mg N L^–1 in the stream draining the most agricultural basin. Stream sediments were nearly always a net source of N₂O, and production rates were positively related to stream water NO₃^– concentrations and sediment carbon content. There were no seasonal patterns in N₂O production rates during 2004, but stream water NO₃^– and N₂O production both peaked during the winter of 2005. The spike in NO₃^– concentrations likely resulted from winter rain and snowmelt that flushed NO₃^– from the soils following a dry summer and fall. In turn, the elevated stream water NO₃^– concentrations stimulated in-stream N₂O production rates. Overall, we were only able to explain 29% of the variation in N₂O production rates on a log scale. The unexplained variation may be due to differences in the fraction of denitrified NO₃^– that is converted to N₂O among the study sites, or that our measures of substrate availability in the water column were not reflective of substrate availability in the porewater used by denitrifiers.

Abbreviations: N₂O, nitrous oxide • NO₃^–, nitrate • OM, organic matter