Greenhouse Gas Emissions from Bear Creek in Central Iowa: The Importance of Direct Measurements on Low Ordered Agricultural Streams.

Indirect nitrous oxide (N₂O) emissions from agricultural streams have not been widely measured and are estimated with established uncertainty. The Intergovernmental Panel on Climate Change (IPCC) estimates N₂O emissions from streams and rivers by approximating an indirect emissions factor (EF5_r). Indirect emissions are then assumed to be the product of nitrogen leached from an ecosystem and the EF5_r. Although there is large uncertainty associated with the IPCCs EF5_r model, it remains our best estimate due to the absence of direct measurements of N₂O in the literature. The majority of Literature to date focuses on streams and rivers of high Strahler order. Nitrate (NO₃) concentrations are often greatest in first and second ordered agricultural streams. Therefore, low ordered streams have a greater potential for large N₂O fluxes. The objective of our study was to quantify and compare greenhouse gas emissions (CO_2, CH_4, and N₂O) from four sites located on Bear Creek in Central Iowa. Greenhouse gas fluxes were measured weekly using static floating chambers. Two sites were categorized as first order streams and two sites were categorized as second order streams. Nitrous oxide emission rates exceeded the IPCCs estimation for indirect emissions from agricultural streams. Emissions for both N₂O and CH₄ were greater at the first ordered sites, but not significantly different from the second ordered sites. Results found in this study highlight the importance of additional direct measurements of greenhouse gases from low ordered agricultural streams.