Rural Industries Research & Development Corporation

Research Compendium 1993 - 1994

Research Project

THE ROLE OF LEGUME-BASED PASTURE IN GREENHOUSE GAS EMISSIONS FROM AUSTRALIA

Objectives

• To quantify the rate of emission of the greenhouse gas, nitrous oxide from legume-based pastures in south-eastern Australia.
• To examine the role of liming as a possible management strategy for reducing these emissions.

Background

One of the current environmental issues facing the Australian rural industries is the possibility of climate change due to the enhanced greenhouse effect, particularly the contribution of the rural industries to anthropogenic gas emissions. In the National Greenhouse Inventory 1988 and 1990 for Australia, the major emissions of trace gases from the agricultural sector were listed as methane and nitrous oxide, with nitrous oxide making up about one-fifth of the combined emissions.

Research

Research was carried out to establish the relationships between the rate of nitrous oxide emission from legume-based pasture and measurable controlling factors: (1) the proportion of legume cover in the pasture, (2) dry matter production and its nutrient content, (3) organic carbon and nitrogen levels in the soil, (4) nitrogen fixation and denitrification rates (determined by isotopic methods), (5) animal grazing via urine deposition and (6) weather conditions. The influence of liming on emission rates was also measured.

Outcomes

The annual average emission of nitrous oxide from the legume pasture was about 0.35 kg of nitrogen per hectare, the highest emission rate observed so far on an annual basis from Australian agricultural or natural ecosystems. The annual emission rate of nitrous oxide measured was 0.12 kg of nitrogen/hec from a native forest in Gippsland in Victoria.

Legume pasture in Australia emitted nitrous oxide at a rate of 5-50% of most grazed grasslands in the northern hemisphere. This is presumably because the soils in Australia are much poorer and, while nitrogen fertilisers are used on pastures in the northern hemisphere, they are generally not used on legume pastures in Australia.

Adding lime to legume pasture did not significantly affect emission of nitrous oxide.

The transformation of fixed nitrogen into free nitrogen and its return to soil via urine and faeces results in an extremely non-uniform increase in gaseous nitrogen losses. Between 0.1% and 0.2% of urine nitrogen was found to be released as nitrous oxide. For a stocking rate of 11 sheep per hectare, an annual emission of nitrous oxide of 0.2 kg of nitrogen per hectare was calculated.

Implications

This study has provided the current best estimate of the combined methane and nitrous oxide greenhouse gas emissions from improved grazed pasture to be nearly 20% less than that previously estimated in the national inventory. The study indicates that the estimate of nitrous oxide emissions from grazed legume pasture in the national greenhouse gas inventory should be substantially reduced.

RIRDC Project No: CSD-47A

RESEARCHER: Mr I.E. Galbally

ORGANISATION: CSIRO Division of Atmospheric Research

PHONE: 03 586 7684

PUBLICATIONS:

Galbally, I. 1985. 'The emission of nitrogen into the remote atmosphere' in The Biogeochemical Cycling of Sulphur and Nitrogen in The Remote Atmosphere. eds J. Galloway et al. Reidel.

Galbally, I., Fraser, P.J., Meyer, C.P and Griffith, D.W.T. 1992. 'Biosphere/atmosphere exchange of trace gases over Australia' pp. 117-149, in Australia's Renewable Resources: Sustainability and Global Change, eds. R.M. Gifford and M.M. Barson, Bureau of Rural Resources Proceedings, No. 14, Canberra.

Last updated: 10 October 1996
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