Using Mid-IR Cavity Ring-Down Spectrometry to Simultaneously Measure N2o, CO2, and CH4 Fluxes: Responses to Ammonium Nitrate Additions in Salt Marshes

Abstract:

Greenhouse gas emissions from salt marshes, especially of nitrous oxide (N₂O), are a central interest because anthropogenic nutrient loads may substantially alter net climatic forcing of these globally significant ecosystems. In a series of lab and field experiments, a new cavity ring down spectrometer (CRDS, Picarro G2508) that uses mid-infrared (mid-IR) frequencies to measure N₂O was compared to a near-IR gas analyzer (LGR N₂O/CO analyzer). The Picarro G2508 reports N₂O as well as CO₂ and CH₄ concentrations roughly every second at the parts per billion level. Responses of N₂O fluxes to experimental ammonium nitrate additions in marsh mesocosms and marsh plots in situ were compared among these analyzers, along with minimum detectable N₂O fluxes. At fluxes above 150 µmol N₂O m^-2 d^-1, the Picarro G2508 and LGR analyzers performed similarly in both mesocosm and field plots that had been enriched with ammonium nitrate, however there were significantly lower minimum detectable N₂O fluxes (about 1 order of magnitude) for the LGR than for the Picarro. A gas chromatograph (Shimadzu GC 2014) was also used to test calibration of the G2508. These experiments suggest that mid-IR CRDS technology offers a new tool for simultaneous analyses of N₂O along with CO₂ and CH₄, which fills an important need for quantifying the net climatic forcing of ecosystems. However based on relatively high minimum N₂O detection levels of the CRDS, it may work best in highly eutrophic environments.