1

News From Agriculture And Life Sciences Division, Lincoln University

Good news – there’s less gas!

Countries participating in the Kyoto Protocol are required to either reduce or take responsibility for their excess greenhouse gas (GHG) emissions relative to 1990 levels.

In order to do this the signatories must first create an inventory of their anthropogenic (man-made) GHG emissions on an annual basis, using guidelines prescribed by the Intergovernmental Panel on Climate Change (IPCC).

Included in this inventory are the emissions of nitrous oxide (N2O). This gas has a global warming potential 296 times that of carbon dioxide, so even small emissions are significant.

Nitrous oxide arises from agricultural sources, and there can be both direct and indirect emissions. Indirect emissions include nitrous oxide from groundwater, surface drainage and rivers.

The method used to asses the indirect sources uses the mass of fertiliser and manure nitrogen lost through leaching and runoff per year multiplied by a series of default emission factors that are set by the IPCC.

However, very few studies have tried to measure these emission factors directly so data is sparse and there is uncertainty as to what the correct emission factors should be.

Researchers from Lincoln University’s Soil and Physical Sciences Group, Dr Tim Clough, Associate Professor Rob Sherlock and Landcare Research’s Dr Frank Kelliher have been studying emissions of nitrous oxide from a local river for several years and have discovered large discrepancies between the IPCC default factors pertaining to nitrous oxide emissions from rivers and actual measured values.

Over recent years two river-based projects measuring nitrous oxide emissions from a spring-fed river have been carried out by Lincoln University honours student, Janet Bertram, and summer scholarship student, Laura Buckthought

Janet and Laura both floated chambers on the surface of the river in order to accumulate and then sample gas above the surface of the water at up to four different sites and at different times of the day. They analysed the gas samples for nitrous oxide using a specially modified gas chromatograph. Measurements were also made of wind speeds, air and water temperatures, and the concentration of nitrate and dissolved nitrous oxide in the river.

The researchers then calculated the nitrous oxide emissions (flux) at the river surface using the difference between the chamber nitrous oxide concentration in air and the gas collection time. This flux, along with the river’s nitrate loading, was used to calculate an indirect emission factor for the river, which was then compared with the IPCC method and assumptions.

For both studies (one using measurements from four different sites and the other measuring one site over 72 hours) the results for the measured emission factor were significantly less than the baseline figures calculated using the IPCC’s method.

“These results show that the contribution from indirect nitrous oxide to New Zealand’s greenhouse gas budget is much less than previously thought, at least for the river studied,” said Dr Clough, who led the project.

“When incorporated in a revised greenhouse gas inventory the lower total emissions of nitrous oxide will reduce New Zealand’s liability when accounting for its greenhouse gas emissions.

“Our results are very important because direct measurements of nitrous oxide emissions in aquatic river environments are sparse and more data are required to determine the role that rivers play in both the national and global nitrous oxide budgets. The studies have now been published in international science journals.

“It may even be beneficial for the IPCC to set guidelines for nitrous oxide emissions for each country based on different river parameters, such as river geometry, water temperatures, turbidity and residence times, since these factors can influence the potential for nitrous oxide to be produced,” said Dr Clough.

ENDS

© Scoop Media