Laser spectroscopy is an emerging technology for measuring nitrous oxide (N 2 O) dynamics in the environment, but most studies have focused on atmospheric applications. We have coupled a commercially available cavity ring-down spectroscope (CRDS) (Picarro G5101-I isotopic N 2 O analyzer) to an air/water gas equilibration device to collect continuous in situ dissolved N 2 O molar concentration and bulk nitrogen isotopic (d 15 N-N 2 O) data. The d 15 N-N 2 O values measured by the CRDS unit were found to be significantly affected by changes in the mixing ratios of O 2 , CO, CH 4 , and CO 2 . There was also an effect of N 2 O mixing ratio on d 15 N-N 2 O. A series of equations was developed to correct for the matrix effect of O 2 and the spectral interference by CH 4 . Chemical traps effectively prevented interferences by CO and CO 2 . The maximum corrections required for N 2 O mixing ratio and O 2 matrix effects, were 1& (at a mixing ratio of 1.2 ppmv), and 11& (at 0% O 2 content), respectively. The CH 4 correction only became important at mixing ratios greater than 500 ppmv (>0.5&). Measurements of N 2 O molar concentration and d 15 N-N 2 O from the CRDS isotopic N 2 O analyzer were similar to those measured with isotope ratio mass spectrometry. We demonstrated the utility of the laser-based system with field deployments in three estuarine tidal creeks in subtropical Australia. Future work in this field should focus on the application of the laser-based system to the measurement of N 2 O isotopologues in aquatic habitats, allowing for further constraints to be placed on the pathways of N 2 O cycling in aquatic system.

Floods frequently produce deoxygenation and acidification in waters of artificially drained coastal acid sulfate soil (CASS) wetlands. These conditions are ideal for carbon dioxide and methane production. We investigated CO 2 and CH 4 dynamics and quantified carbon loss within an artificially drained CASS wetland during and after a flood. We separated the system into wetland soils (inundated soil during flood and exposed soil during post flood period), drain water, and creek water and performed measurements of free CO 2 ([CO 2 *]), CH 4 , dissolved inorganic and organic carbon (DIC and DOC), stable carbon isotopes, and radon ( 222 Rn: natural tracer for groundwater discharge) to determine aquatic carbon loss pathways. [CO 2 *] and CH 4 values in the creek reached 721 and 81 μM, respectively, 2 weeks following a flood during a severe deoxygenation phase (dissolved oxygen~0% saturation). CO 2 and CH 4 emissions from the floodplain to the atmosphere were 17-fold and 170-fold higher during the flooded period compared to the post-flood period, respectively. CO 2 emissions accounted for about 90% of total floodplain mass carbon losses during both the flooded and post-flood periods. Assuming a 20 and 100 year global warming potential (GWP) for CH 4 of 105 and 27 CO 2 -equivalents, CH 4 emission contributed to 85% and 60% of total floodplain CO 2-equivalent emissions, respectively. Stable carbon isotopes (δ 13 C in dissolved CO 2 and CH 4 ) and 222 Rn indicated that carbon dynamics within the creek were more likely driven by drainage of surface floodwaters from the CASS wetland rather than groundwater seepage. This study demonstrated that >90% of CO 2 and CH 4 emissions from the wetland system occurred during the flood period and that the inundated wetland was responsible for~95% of CO 2 -equivalent emissions over the floodplain.

No abstract

No abstract