Temporal Variability of N2O and CH4 in Coastal Sediments: Assessing the Impact of Organic Matter Inputs

N₂O and CH₄ contribute respectively, about 5-6% and 15% to the anthropogenic greenhouse effect. Coastal areas are potential sources of those gases, which can be produced in sediments from organic matter (OM) degradation. Oxygen microprofiles, sediment organic nitrogen and carbon content, and pore-water concentrations of N₂O, CH₄ and nutrients were measured in sediment from three marine systems ending in Cadiz Bay (SW Spain) in order to identify the OM diagenetic pathways. Hydrodynamic characteristics of the studied environments (Guadalete River estuary, Sancti Petri Channel and San Pedro Creek) are different, even though they all are shallow and tidally affected. The area holds a high population density receiving large amount of OM from both, urban sources and primary sector activities. Significant differences (p<0.05) were found between the seasonal samplings. In general, bottom waters and surface of sediments were well oxygenated, although oxygen never penetrated deeper than 3 cm. In contrast, CH₄ concentrations increased with depth as a result of the anaerobic degradation of OM. N₂O profiles showed maximum concentration peaks below 5 cm depth, most likely because of denitrification/nitrate-reduction processes. N₂O and CH₄ concentrations were notable higher at stations placed close to the OM inputs. For instance, estimated diffusive fluxes of CH₄ at the sediment-water interface in the Guadalete Estuary were 3 orders of magnitude higher at the OM contaminated station (reaching 8.3 mmol m^-2 d^-1) than at the station located farther away (fluxes varied between 1.2 and 3.4 μmol m^-2 d^-1). Sediments in the Estuary were also a source of N₂O to the water column (range: 2.1 – 111.2 μmol m^-2 d^-1), however sediments of the two other marine systems were N₂O sinks or weak sources (range: -2.5 – 0.2 μmol m^-2 d^-1). Results highlight the complexity of the OM diagenesis under different physical and biogeochemical conditions that consequently affects the sediment-water fluxes.