The effects of season and agriculture on nitrous oxide production in headwater streams

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.

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