Fertilizer Nitrogen Transformations, Losses and Efficiency of Use in Sugarcane Cropping under Wet Tropical Environment of Australia
Praphan Prasertsak 1 Paul Saffigna 2 Brian Prove 3 and John Freney 4
Sugarcane is the dominant crop in the wet tropical zone of north Queensland, Australia. Similar to other crops, nitrogen is the main fertilizer applied to sugarcane. Urea is currently the main form of nitrogen fertilizer used in the industry because it is the cheapest form of fertililizer nitrogen available. The recent practice of cutting cane green and trash retention in the Australian sugarcane industry has been found to sustain the soil productivity by improving soil structure, increasing soil organic matter and nutrient levels. However, this practice has created problems for sugarcane growers since considerable losses of fertilizer nitrogen always occur when urea is applied on top of the sugarcane trash. Subsurface application of urea seems to be the most practical way to reduce nitrogen loss by ammonia volatilization. However, most of the cane growers still currently broadcast or band urea on the surface of sugarcane trash because of its simplicity and low cost of application. Recovery of fertilizer nitrogen in the above ground plant parts of sugarcane is commonly 20 % - 40 % of the applied nitrogen, which is lower than the recovery by most other field crops. The large losses of fertilizer nitrogen via a number of loss processes (volatilization, denitrification and leaching) is the main cause of the low nitrogen use efficiency in sugarcane crops. These losses can also create pollution of the environment including ground and surface waters and air. In order to improve the economics of production and to minimize environmental impact, it is essential to have more information on the magnitude of these processes. To achieve this goal, three field experiments were conducted between November 1993 and October 1994 at Innisfail, north Queenslan,. In the microplot experiment, surface and subsurface application of ammonium sulfate and urea were compared. The results showed that the rate of urea hydrolysis was higher when urea was surface applied than when it was applid below the surface. Nitrification rates were high in all treatments and nitrification was complete within one month after nitrogen application. The subsurface application of urea increased the total recovery of nitrogen substantially when compared with that in the surface application treatment. In the 15N macroplot experiment, the effect of placing the urea below the soil surface on nitrogen loss was studied. Subsurface application of urea significantly reduced the losses of nitrogen and also significantly increased the nitrogen use efficiency by 10 % (25 % and 35 % in the surface and subsurface application treatments, respectively). The effect of fertilizer placement on ammonia volatilization follwing application of urea wass studied using a micrometeorological method. The results showed that burying urea beneath the soil reduced nitrogen loss from 37 % to 6 %.
1 Suphan Buri Field Crops Research Center, U-Thong, Suphan Buri 72160, Thailand
2 Faculty of Environmental Science, Griffirth University, Nathan, Brisbane, Queensland 4111, Australia
3 QDPI, Center for Wet Tropics Agriculture, South Johnstone, Queensland 4859, Australia
4 Division of Plant Industry, CSIRO, PO Box 1600, Canberra, ACT 2601, Australia
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