Abstract
Nitrous oxide (N2O) is an important greenhouse gas, but large uncertainties remain in global budgets. Mangroves are thought to be a source of N2O to the atmosphere in spite of the limited available data. Here we report high resolution time series observations in pristine Australian mangroves along a broad latitudinal gradient to assess the potential role of mangroves in global N2O budgets. Surprisingly, five out of six creeks were under-saturated in dissolved N2O, demonstrating mangrove creek waters were a sink for atmospheric N2O. Air-water flux estimates showed an uptake of 1.52 ± 0.17 μmol m−2 d−1, while an independent mass balance revealed an average sink of 1.05 ± 0.59 μmol m−2 d−1. If these results can be upscaled to the global mangrove area, the N2O sink (~2.0 × 108 mol yr−1) would offset ~6% of the estimated global riverine N2O source. Our observations contrast previous estimates based on soil fluxes or mangrove waters influenced by upstream freshwater inputs. We suggest that the lack of available nitrogen in pristine mangroves favours N2O consumption. Widespread and growing coastal eutrophication may change mangrove waters from a sink to a source of N2O to the atmosphere, representing a positive feedback to climate change.
Highlights
Nitrous oxide (N2O) is an important greenhouse gas, but large uncertainties remain in global budgets
The largest portion of global denitrification from natural sources is thought to occur within coastal waters (~45%)[3], with these areas considered globally significant N2O sources[4]
This study was undertaken over two 24-h periods in a mangrove tidal creek influenced by upstream freshwater inputs
Summary
Denitrification[13,14,15]. With salinity ranging from 0 to 28 in the Andaman Island study, the higher N2O concentrations observed may have been driven by freshwater nitrogen loads, rather than mangrove related processes. Mangroves may conserve nitrogen through nitrate reduction via dissimilatory nitrate reduction to ammonium (DNRA) rather than denitrification[22]. This may enhance the N2O sink capacity of mangroves via two ways. Using an independent mass balance approach, mangroves were estimated to have an atmospheric flux of −3.20 to 0.03 μmol m−2 d−1 (mean −1.05 ± 0.59 μmol m−2 d−1). Overall, these two independent approaches are consistent in that 5 out of the 6 systems were a sink for atmospheric N2O, giving confidence in our observations.
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