Spatial and temporal variation of nitrous oxide and methane flux between subtropical mangrove sediments and the atmosphere

We quantified spatial and temporal variations of the fluxes of nitrous oxide (N2O) and methane (CH4) and associated abiotic sediment parameters across a subtropical river estuary sediment dominated by grey mangrove (Avicennia marina). N2O and CH4 fluxes from sediment were measured adjacent to the river ("fringe") and in the mangrove forest ("forest") at 3-h intervals throughout the day during autumn, winter and summer. N2O fluxes from sediment ranged from an average of -4 μg to 65 μg N2O m-2 h-1 representing N2O sink and emission. CH4 emissions varied by several orders of magnitude from 3 μg to 17.4 mg CH4 m-2 h-1. Fluxes of N2O and CH4 differed significantly between sampling seasons, as well as between fringe and forest positions. In addition, N2O flux differed significantly between time of day of sampling. Higher bulk density and total carbon content in sediment were significant contributors towards decreasing N2O emission; rates of N2O emission increased with less negative sediment redox potential (E(h)). Porewater profiles of nitrate plus nitrite (NOx-) suggest that denitrification was the major process of nitrogen transformation in the sediment and possible contributor to N2O production. A significant decrease in CH4 emission was observed with increasing E(h), but higher sediment temperature was the most significant variable contributing to CH4 emission. From April 2004 to July 2005, sediment levels of dissolved ammonium, nitrate, and total carbon content declined, most likely from decreased input of diffuse nutrient and carbon sources upstream from the study site; concomitantly average CH4 emissions decreased significantly. On the basis of their global warming potentials, N2O and CH4 fluxes, expressed as CO2-equivalent (CO2-e) emissions, showed that CH4 emissions dominated in summer and autumn seasons (82-98% CO2-e emissions), whereas N2O emissions dominated in winter (67-95% of CO2-e emissions) when overall CO2-e emissions were low. Our study highlights the importance of seasonal N2O contributions, particularly when conditions driving CH4 emissions may be less favourable. For the accurate upscaling of N2O and CH4 flux to annual rates, we need to assess relative contributions of individual trace gases to net CO2-e emissions, and the influence of elevated nutrient inputs and mitigation options across a number of mangrove sites or across regional scales. This requires a careful sampling design at site-level that captures the potentially considerable temporal and spatial variation of N2O and CH4 emissions.