Seasonal variation in nitrous oxide and methane emissions from subtropical estuary and coastal mangrove sediments, Australia

Mangrove sediments can act as sources of the greenhouse trace gases, nitrous oxide (N2O) and methane (CH4). Confident reporting of trace gas emissions from mangrove sediments at local levels is important for regional emissions inventories, since small changes in N2O and CH4 fluxes greatly influence greenhouse gas budgets due to their high global warming potentials. It is also important to identify the drivers of trace gas emission, to prioritise management for minimising emissions. We measured N2O and CH4 fluxes and abiotic sediment parameters at midday low tide in winter and summer seasons, at four sites (27 degrees 33'S, 152 degrees 59'E) ranging from estuary to ocean sub-tropical mangrove sediments, having varied anthropogenic impacts. At all sites, sediment N2O and CH4 emissions were significantly lower during winter (7-26 mu g N2O m-2 center dot h-1; 47-466 mu g CH4 m-2 center dot h-1) compared to summer (28-202 mu g N2O m-2 center dot h-1; 247-1570 mu g CH4 m-2 center dot h-1). Sediment temperature, ranging from 18 to 33 degrees C, strongly influenced N2O and CH4 emissions. Highest emissions (202 mu g N2O m-2 center dot h-1, 1570 mu g CH4 m-2 center dot h-1) were detected at human-impacted estuary sites, which generally had higher total carbon (< 8%) and total nitrogen (< 0.4%) in sediments and reduced salinity (< 16 dS center dot m-1). Large between-site variation highlights the need for regular monitoring of sub-tropical mangroves to capture short-lived, episodic N2O and CH4 flux events that are affected by sediment biophysico-chemical conditions at site level. This is important, particularly at sites receiving anthropogenic nutrients, and that have variable freshwater inputs and tidal hydrology.