Resolution of the nitrogen (N) cycle in the marine environment requires an accurate assessment of dinitrogen (N_(2)) fixation. We present here an update on progress in conducting field measurements of acetylene reduction (AR) and ~(15)N_(2) tracer assimilation in the oligotrophic North Pacific Subtropical Gyre (NPSG). The AR assay was conducted on discrete seawater samples using a headspace analysis system, followed by quantification of ethylene (C_(2)H_(4)) with a reducing compound photodetector. The rates of C_(2)H_(4) production were measurable for nonconcentrated seawater samples after an incubation period of 3 to 4 h. The ~(15)N_(2) tracer measurements compared the addition of ~(15)N_(2) as a gas bubble and dissolved as ~(15)N_(2) enriched seawater. On all sampling occasions and at all depths, a 2- to 6-fold increase in the rate of ~(15)N_(2) assimilation was measured when ~(15)N_(2)-enriched seawater was added to the seawater sample compared to the addition of ~(15)N_(2) as a gas bubble. In addition, we show that the ~(15)N_(2)-enriched seawater can be prepared prior to its use with no detectable loss (<1.7%) of dissolved ~(15)N_(2) during 4 weeks of storage, facilitating its use in the field. The ratio of C_(2)H_(4) production to ~(15)N_(2) assimilation varied from 7 to 27 when measured simultaneously in surface seawater samples. Collectively, the modifications to the AR assay and the ~(15)N_(2) assimilation technique present opportunities for more accurate and high frequency measurements (e.g., diel scale) of N_(2) fixation, providing further insight into the contribution of different groups of diazotrophs to the input of N in the global oceans.
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