Progress in quantifying coastal N2O emissions in order to close the (terrestrial) biogenic nitrogen budget

Naomi S. Wells1*, Damien Maher1, Dirk Erler1, Vera Sandel1, Badin Gibbes2, Matt Hipsey3, James Udy4, Bradley Eyre1

1Centre for Coastal Biogeochemistry, Southern Cross University, Lismore, NSW, Australia

2School of Civil Engineering, University of Queensland, Brisbane, QLD, Australia

3School of Earth and the Environment, University of Western Australia, Crawley, WA, Australia

4Healthy Waterways, Brisbane, QLD, Australia

*Corresponding author: [email protected]

Abstract

Aquatic nitrous oxide (N2O) emissions are both a poorly constrained component of the global greenhouse gas budget and a rarely quantified loss pathway during transport of reactive nitrogen (N) from land to sea. Quantification of N2O losses from coastal environments are particularly vital, as these regions are both biogeochemical hotspots and subject to dramatic increases in N loading from urbanisation and upstream agricultural intensification. This study aimed to link spatial intensive measurements of water-atmosphere N2O fluxes with biogeochemical controls across a land-use intensity gradient. We used recently developed cavity enhanced laser absorption spectroscopy to obtain quasi continuous (1 sec-1) measurements of dissolved N2O across the salinity gradient in eight sub-tropical estuaries subjected to varying land-use intensities. Land use had a dramatic effect: N2O fluxes from estuaries surrounded by >60% woody vegetation were an order of magnitude lower than from those surrounded by <30% woody vegetation, and the estuary mouth created a net N2O sink only in the four least impacted systems. The fact that N2O fluxes, but not nitrate concentrations, peaked at the freshwater-saltwater interface (1-5 psu) in seven of eight surveyed estuaries suggested that benthic processes, not point source pollution, controlled N2O emissions. The fact that groundwater infiltration did not drive N2O peaks supports the idea that benthic biology, rather than hydrology, regulates estuarine N2O losses. As N2O did not track the spatial patterns of the commonly measured N species (ammonium, nitrate), an accurate catchment N balance could only be achieved via directly measuring estuarine N2O emissions.