Nitrous oxide and methane emissions from the restored mangrove ecosystem of the Ciénaga Grande de Santa Marta, Colombia

Abstract

Most studies on emission of the greenhouse gases nitrous oxide (N2O) and methane (CH4) have been carried out in temperate areas so there is generally a lack of data from subtropical and tropical climates. A large part of the subtropical and tropical coastal wetlands consists of mangrove ecosystems, which have potential to act as sources of N2O and CH4. We measured N2O and CH4 emissions during 11 months in the brackish lagoon system Ciénaga Grande de Santa Marta (CGSM) on the Caribbean coast of Colombia. The area has been seriously influenced by human disturbance which resulted in approximately 60% mangrove mortality but the ecosystem is now being rehabilitated. In addition to N2O and CH4 emissions at four sampling sites, we also measured temperature, salinity, pH, dissolved oxygen (DO), redox potential, nitrite (NO2−), nitrate (NO3−), ammonium (NH4+) and organic material in the water and/or the sediment.We found the mangrove sediments of CGSM to be a source of N2O and CH4 but there was large variability of the emissions on both temporal and spatial scale with N2O and CH4 fluxes in the range 35–1180 μg m−2 h−1 and 0–31570 μg m−2 h−1, respectively. Correlations between N2O fluxes and the water variables showed that salinity significantly contributed towards decreased emission of N2O (r = −0.38). By contrast, water concentrations of NO3− were correlated with increased emission of N2O (r = 0.54). Emission of CH4 was negatively correlated with DO in the water (r = −0.34) and there was a positive correlation between organic matter and CH4 emission (r = 0.75). It was found that untreated wastewater discharged into the Magdalena River and in turn, the lagoon system may have resulted in a substantially higher emission of both N2O and CH4 as the wastewater contributes substrate to the processes in the form of N and organic matter. With better treatment of wastewater, it is possible that emission of both gases would be lower, especially at the sites closest to Magdalena River.