Effect of Soil Fertility and Land Use Change On Emission of Nitrous Oxide and Nitric Oxide From Grazed Pasture Soils.

The use of superphosphate to correct phosphorus deficiency and the introduction of legumes into large areas have increased the quality and quantity of forage for grazing animals, increased soil organic matter, soil nitrogen (N) and soil fertility. After a number of years the pasture is ploughed in to mineralize organic N and a wheat crop is planted. The effects of changes in fertility of pastures, and the change from pasture to a wheat crop on the emission of the greenhouse gas nitrous oxide and the indirect greenhouse gas nitric oxide from soil, were measured concurrently and continuously for 21 months. The results showed that more NO and N₂O was emitted from each of the high fertility treatments than was emitted from the corresponding low fertility treatments. For example, in 2008/09 the emissions of NO and N₂O from the high fertility mixed pasture and wheat were 2.37 and 0.67 kg N ha^-1, respectively, while those from the low fertility mixed pasture and wheat were 0.84 and 0.36 kg N ha^-1, respectively. Changing the land use from a mixed clover grass pasture to a wheat crop resulted in increased emissions of NO and N₂O in the two seasons; the respective emissions from the pasture were 0.69 and 0.66 kg N ha^-1, and for the wheat were 3.27 and 3.24 kg N ha^-1. The results of the current study show that similar amounts of NO and N₂O (6.82 and 6.46 kg N ha^-1, respectively) were lost from the combined treatments. The importance of soil water content to nitrous oxide flux highlights the critical importance of rainfall to annual flux, and demonstrates the difficulty in estimating fluxes from both short term data and single year data.