Abstracts

Naomi S. Wells^1*, 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: naomi.wells@scu.edu.au

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.