Ammonia volatilisation from nitrogen fertilisers surface-applied to bare fallows, wheat crops and perennial-grass-based pastures on Vertosols

Farmers on Vertosols in the northern grains region of Australia are increasingly using pre-crop broadcasting and in-crop topdressing of nitrogen (N) fertilisers. Surface application risks gaseous loss via ammonia volatilisation, but the magnitude of N loss is unknown. Because both soil properties and environmental conditions influence ammonia volatilisation, measurements need to be field-based and non-intrusive, e.g. micrometeorological. We used an integrated horizontal flux technique to measure ammonia volatilised from neutral to alkaline Vertosols for a month after the application of several fertiliser products in 10 bare-fallow paddocks, seven mid-tillering wheat crops, and two perennial-grass-based pastures. Ammonia loss from urea averaged 11% (5.4-19%) when applied to fallow paddocks, 4.8% (3.1-7.6%) when applied to wheat, and 27% when applied to pasture. Volatilisation from urea applied to pastures was high, because there was little rain after spreading. Losses from ammonium sulfate applied to pastures were >60% less than from urea. Nitrogen losses from ammonium sulfate were high (18.6-33.8%) from soils with >10 g 100 g(-1) of calcium carbonate (CaCO3), but were 52% less than from urea at five of eight fallow paddocks on non-calcareous soils, and 76% less than from urea at the two pasture paddocks. Coating urea with N-(n-butyl)thiophosphoric triamide reduced ammonia loss at just two of eight fallow paddocks and one of three in-crop paddocks. Ammonia volatilisation from aqueous solutions of urea, urea ammonium nitrate, and ammonium nitrate were either less than or no different from losses from granulated urea, but not consistent. With the exception of ammonium sulfate applied to soils with >10 g 100 g(-1) of CaCO3, surface application of N fertiliser during autumn-winter on cropped Vertosols in the Australian northern grains region does not lead to major N loss via ammonia volatilisation.