Abstract
A method is described for isolating the total pool of dissolved organic nitrogen (DON) from oligotrophic lake water for subsequent isotopic analysis of the15N/14N ratio in tracer experiments. The procedure involves reducing an acidified filtered sample with elemental zinc to convert nitrate + nitrite to ammonium and then analyzing and sparging with N2 to remove ammonia. The residue (DON) is evaporated to dryness, rehydrated, spotted on a filter, and vacuum ampulated for micro-Dumas conversion to N2 and isotopic analysis by optical emission spectrometry. We have used the method to accurately estimate 15N enrichments of < 1 atom-% in DON collected from 150-mL samples of [15N]nitrate-enriched mesocosms in Castle Lake, California.
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