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Nitrous Oxide Emission through Plants

Research Centre, Agriculture and Agri-Food Canada, P.O. Box 3000, Lethbridge, AB, Canada T1J 4B1

C. M. Cho

Dep. of Soil Science, Univ. of Manitoba, Winnipeg, MB, Canada R3T 2N2

*Corresponding author (chang{at}em.agr.ca).

ABSTRACT

Agricultural soils are a major source of N₂O that has been linked to an enhanced greenhouse effect and the depletion of stratospheric ozone. The N₂O produced in the soil is usually assumed to enter the atmosphere by diffusion. We postulated that significant amounts of N₂O may also be emitted via plant transpiration, and conducted a controlled environment study to test this hypothesis. The shoots of canola (Brassica napus L.) were enclosed in chambers, and N₂O emission from the foliage was quantified by measuring changes in N₂O concentration following various soil treatments. At field capacity, a treatment not conducive to denitrification, there was no measurable emission of N₂O from the plant foliage. At saturation, however, N₂O concentration within the shoot chamber increased consistently with time. By the end of the sampling period, the N₂O concentration in the shoot chamber was 0.45 L L^–1, well above that in the soil head space (0.35 L L^–1), demonstrating that N₂O was released from the shoots and not the soil. Watering the soil with a N₂O solution with a concentration of approximately 8 mL L^–1 resulted in almost immediate release of N₂O from the shoots, indicating that the N₂O was not generated by the plants directly, but merely conveyed to the atmosphere by the transpiration stream. Experiments using barley (Hordeum vulgare L.) produced results similar to those with canola, suggesting that emission of N₂O from plants may be a common phenomenon. These results demonstrate a mechanism for N₂O emission from upland crops, and imply that measurement of N₂O release solely from the soil surface may underestimate actual fluxes from agroecosystems.

Received for publication December 12, 1996.

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