Abstract

Concern has mounted over recent decades regarding the emission of nitrous oxide (N₂O) to the atmosphere through human activities. Modern agriculture has contributed to this with elevated use of nitrogenous fertilizers and irrigation. Irrigated cotton grown on alkaline heavy clay soils often uses nitrogen fertiliser inefficiently, due largely to N loss (commonly 50–100 kg N/ha) through denitrification. However, the amount of denitrified N emitted as N₂O has rarely been measured. This paper derives estimates of the quantities of N₂O emitted from N fertiliser applied to alkaline grey clays.

A negative exponential function between the N₂O/N₂ mole fraction and soil pH was derived from a search of laboratory and field studies published by numerous authors using a wide range of soil types. A greater proportion of N₂O relative to N₂ is emitted from acid soils; approximately equivalent amounts of each gas are emitted from soil of pH 6.0. For the alkaline grey clays (pH 8.3–8.5), the N₂O/N₂ mole fraction was about 0.024.

The quantities of N₂O emitted from alkaline grey clays during the growth of a cotton crop were estimated by applying this relationship to ¹⁵N balance studies where N fertiliser losses had been measured. Using this approach, about 2 kg N/ha (~1.1% of the N applied) was calculated to be lost as N₂O during the cotton-growing season. This is similar to the value of 1.25% commonly used to estimate N₂O emissions from N fertiliser, but this estimation should only be applied to alkaline soils; a larger percentage of the fertiliser N denitrified from acid soils should be emitted as N₂O-N. These estimates of N₂O emissions require validation with field experimentation.

The low (negligible) values for N₂O emission from flooded fields compared with laboratory observations are discussed. It is possible that high N₂O emissions observed under laboratory conditions result from the shallow depth of soil, reducing the opportunity for N₂O to be further reduced as it diffuses through the soil profile. Management strategies that have the potential to reduce N₂O emissions are discussed.