Ground Cover Supplement : GC Supplement - More profit from crop nutrition II
6 Nitrogen The want and waste of the nitrogen economy Fertcare® accredited adviser Ian Crosthwaite, from BGA AgriServices in Kingaroy, Queensland, draws on the findings of NANORP research in his recommendations to growers, to help reduce applied nitrogen losses and, in turn, reduce greenhouse gas emissions. By Wayne Pluske NITROGEN MANAGEMENT PRACTICES in crops such as sorghum could be wasting $20 to $70 of nitrogen per hectare because 20 to 40 per cent of applied urea is being lost into the atmosphere as nitrous oxide (N2O). This is one of the key findings drawn from research led by the University of Queensland's Professor Mike Bell as part of trials in Queensland and New South Wales under the Australian Government's National Agricultural Nitrous Oxide Research Program (NANORP), which was coordinated by the GRDC and funded by the Australian Government's Department of Agriculture as part of their 'Filling the Research Gap' initiative. NANORP is looking at ways to increase nitrogen use efficiency (NUE) on farms to improve the economics of nutrient management and reduce damage to the environment when applied nitrogen is lost from the soil into the atmosphere as N2O emissions. N2O is a greenhouse gas about 300 times more potent than carbon dioxide. Fertcare® accredited adviser Ian Crosthwaite, from BGA AgriServices in Kingaroy, Queensland, and David Harbison, from D R Agriculture in Orange, NSW, are now incorporating the findings of NANORP research into their recommendations for grower clients. Their recommended nitrogen fertiliser rates are based on a crop's likely yield and demand for nitrogen and the contribution of nitrogen fixed in the soil by legumes in meeting that demand, plus the timing and placement of urea applications to help minimise soil nitrogen losses. NANORP trials in 2013-14 and 2014- 15 showed only small annual losses of soil nitrogen (about 1 to 2 kilograms of nitrogen (N)/ha) as N2O emissions. But Professor Bell says these emissions are an indicator of much larger applied nitrogen losses as dinitrogen gas (N2). He estimates this dinitrogen loss could be as high as 30kg N/ha/year. While such applied nitrogen waste is not good news for growers, the effects of these gaseous nitrogen losses on climate change are not as bad as previously thought because dinitrogen is not a damaging greenhouse gas, in contrast with N2O, which remains in the atmosphere for 114 years. N2O emissions result when microorganisms in the soil convert nitrate into nitrogen gases in a process called denitrification. Professor Bell says denitrification is most likely to occur when there is a combination of high soil nitrate nitrogen, an abundance of labile carbon (the labile pool of carbon includes easily decomposable organic materials that stay in the soil for fairly short periods, for example, where there is a heavy stubble load) and wet soil conditions. Denitrification is also more likely to occur early in the season when nitrogen fertiliser has been applied and plants are small, so plant uptake of soil nitrogen is low. Being an anaerobic process, denitrification is influenced by the amount of oxygen in the soil under wet conditions. Professor Bell says soils do not need to be waterlogged for soil oxygen to be low enough for denitrification to occur; they only need to be at, or slightly above, field water capacity in parts of the soil profile to run short of oxygen. The number of these microsites will vary depending on soil texture and structure. Better-structured and aerated soils, such as ferrosols and sands, tend to have higher oxygen levels when wet and are less prone to denitrification than heavy clays, such as vertosols. Heavy rain also reduces the amount of soil oxygen and increases the likelihood of denitrification, and this risk is further increased in soils with high labile carbon content, which promotes microbial activity. Professor Bell's NANORP research in sorghum suggests over-application of nitrogen fertiliser could be PHOTO: BGA AGRISERVICES GROWER TIP Better matching fertiliser nitrogen rates to likely yield targets is generally an effective strategy for reducing applied nitrogen losses and improving NUE in cropping systems.
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