Impact of Anthropogenic Nitrogen Deposition on Oceanic Nitrous Oxide

Nitrous oxide is a greenhouse gas and also plays a role in reducing ozone in the stratosphere. It is produced in the ocean through the action of bacteria which act on oceanic chemical species formed during the breakdown of organic matter. The burning of fossil fuels and the application of nitrogen based fertilisers releases reactive nitrogen species into the atmosphere. These reactive nitrogen species can be transported by air currents over the open ocean where they are deposited on the ocean surface. Here they provide a source of 'new' nitrogen nutrients for the oceanic micro-organisms which can then produce additional oceanic organic matter. The breakdown of this 'additional' organic matter acts to release an additional source of nitrous oxide from the ocean to the atmosphere, where it can affect the earth's radiation balance (through its role as a greenhouse gas), or travel to the stratosphere and increase ozone depletion. A recent study has attempted to quantify the potential increases in ocean fluxes of nitrous oxide caused by predicted increases of fossil fuel use and fertiliser application. The investigators lacked a realistic model of ocean biology which also represented the processes governing the formation of nitrous oxide, and so estimated the increased flux to the atmosphere with a simple linearised calculation. In this proposal we plan to build a realistic global model of the processes controlling oceanic nitrous oxide formation (and destruction); a model with the ability to represent the spatial and temporal variations of the interactions between nitrous oxide cycling processes, ocean biology and ocean circulation. This model will extend an existing ocean biology model (PlankTOM) at UEA. We will use the combined model to estimate the changes in nitrous oxide fluxes from the ocean to the atmosphere for future scenarios of increased reactive nitrogen addition to the ocean. Although this proposal focuses on the consequences for nitrous oxide, the modelling tool we plan to build can be applied to a broad range of problems involving the interactions of ocean biology, other greenhouse gases such as CO2, and climate.