The fate of nitrogen fixed in Wyoming streams

Nitrogen fixation is an under measured process that may be greatly contributing to N budgets of aquatic ecosystems.  Acetylene reduction is the most common method used for measuring nitrogen fixation in streams.   However, the acetylene reduction method measures the activity of the nitrogenase enzyme rather than measuring N₂ uptake directly.  We measured N₂ fixation using both ¹⁵N₂ uptake and the acetylene reduction method on three streams during summer 2007 in order to examine the theoretical assumed ratio of 3:1 moles ethylene produced to nitrogen fixed.  Our empirically measured ratio of 7.8 ± 0.4 moles ethylene produced to ¹⁵N fixed is more than two times greater than the theoretical ratio of 3:1.  To expand upon this research, we used ¹⁵N₂ as a tracer to measure the release of fixed N into the surrounding chamber water as ¹⁵N-NH₄⁺ and ¹⁵N-DON during summer 2008.  Nitrogen fixation is more than two orders of magnitude greater than biofilm N accumulation in Ditch Creek, WY throughout the summer season.  Thus, a large portion of the fixed N must be lost as dissolved N or due to biofilm sloughing.