Spatial and temporal variation of nitrous oxide and methane flux between subtropical mangrove sediments and the atmosphere [An article from: Soil Biology and Biochemistry]

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Description:
We quantified spatial and temporal variations of the fluxes of nitrous oxide (N"2O) and methane (CH"4) and associated abiotic sediment parameters across a subtropical river estuary sediment dominated by grey mangrove (Avicennia marina). N"2O and CH"4 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. N"2O fluxes from sediment ranged from an average of -4@mg to 65@mgN"2Om^-^2h^-^1 representing N"2O sink and emission. CH"4 emissions varied by several orders of magnitude from 3@mg to 17.4mgCH"4m^-^2h^-^1. Fluxes of N"2O and CH"4 differed significantly between sampling seasons, as well as between fringe and forest positions. In addition, N"2O flux differed significantly between time of day of sampling. Higher bulk density and total carbon content in sediment were significant contributors towards decreasing N"2O emission; rates of N"2O emission increased with less negative sediment redox potential (E"h). Porewater profiles of nitrate plus nitrite (NO"x^-) suggest that denitrification was the major process of nitrogen transformation in the sediment and possible contributor to N"2O production. A significant decrease in CH"4 emission was observed with increasing E"h, but higher sediment temperature was the most significant variable contributing to CH"4 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 CH"4 emissions decreased significantly. On the basis of their global warming potentials, N"2O and CH"4 fluxes, expressed as CO"2-equivalent (CO"2-e) emissions, showed that CH"4 emissions dominated in summer and autumn seasons (82-98% CO"2-e emissions), whereas N"2O emissions dominated in winter (67-95% of CO"2-e emissions) when overall CO"2-e emissions were low. Our study highlights the importance of seasonal N"2O contributions, particularly when conditions driving CH"4 emissions may be less favourable. For the accurate upscaling of N"2O and CH"4 flux to annual rates, we need to assess relative contributions of individual trace gases to net CO"2-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 N"2O and CH"4 emissions.