Charlie Walker

Prospects of improving efficiency of fertiliser nitrogen in Australian agriculture: a review of enhanced efficiency fertilisers

Fertiliser nitrogen use in Australia has increased from 35 Gg N in 1961 to 972 Gg N in 2002, and most of the nitrogen is used for growing cereals. However, the nitrogen is not used efficiently, and wheat plants, for example, assimilated only 41% of the nitrogen applied. This review confirms that the efficiency of fertiliser nitrogen can be improved through management practices which increase the crops ability to compete with loss processes. However, the results of the review suggest that management practices alone will not prevent all losses (e.g. by denitrification), and it may be necessary to use enhanced efficiency fertilisers, such as controlled release products, and urease and nitrification inhibitors, to obtain a marked improvement in efficiency. Some of these products (e.g. nitrification inhibitors) when used in Australian agriculture have increased yield or reduced nitrogen loss in irrigated wheat, maize and cotton, and flooded rice, but most of the information concerning the use of enhanced efficiency fertilisers to reduce nitrogen loss to the environment has come from other countries. The potential role of enhanced efficiency fertilisers to increase yield in the various agricultural industries and prevent contamination of the environment in Australia is discussed.

Influence of temperature and soil type on inhibition of urea hydrolysis by N-(n-butyl) thiophosphoric triamide in wheat and pasture soils in south-eastern Australia

Incubation experiments were conducted to assess the effectiveness of N-(n-butyl) thiophosphoric triamide (NBPT) for inhibiting hydrolysis of urea in three wheat-growing soils and one pasture soil in south-eastern Australia, under a range of temperatures (5, 15, 25°C). The effectiveness of NBPT decreased with increasing temperature and with increasing urease activity. In the acidic pasture soil with high urease activity (186 μg N/g soil.h) and high organic carbon content (11%), NBPT (0.1% w/w urea) had little impact on urea hydrolysis rates over all temperatures, with 55% urea remaining). At 25°C in the wheat soils, NBPT slowed the rate of urea hydrolysis, but by Days 14 and 15, <2% of the urea remained. NBPT applied at a rate of 0.1% urea would be an effective tool for slowing urea hydrolysis in the wheat-cropping soils under cool-climate conditions. The delay in urea hydrolysis in the pasture soil still provides the opportunity for increased flexibility in farm management, such as irrigation scheduling.

Influence of enhanced efficiency fertilisation techniques on nitrous oxide emissions and productivity response from urea in a temperate Australian ryegrass pasture

The effect of a nitrification inhibitor on nitrous oxide (N2O) emissions across seasons, the effect of a urease inhibitor and a fine particle spray (both targeting ammonia (NH3) loss) on N2O emissions, and the potential for productivity benefits and efficiencies by using these enhanced efficiency fertilisers (EEFs) were investigated in temperate pastures. The study compared three treatments over an eight month period (April to December 2010): (1) urea (U), (2) urea with a nitrification inhibitor (3,4-dimethylpyrazole phosphate) (DMPP), and (3) urea with a urease inhibitor (N-(n-butyl) thiophosphoric triamide (NBTPT)) (GU). In autumn, when NH3 loss was predicted to be high, the effect of urea applied as a fine particle spray (containing urea, NBTPT and gibberellic acid (10g ha–1)) (FPA) on N2O emissions and productivity was determined. N2O emissions from urea applied to pastures were low, and were larger in spring than autumn due to soil moisture and temperature. DMPP was an effective tool for mitigating N2O emissions, decreasing fertiliser-induced N2O emissions relative to urea by 76% over eight months. However, the urease inhibitor (NBTPT) (GU) increased N2O emissions from urea by 153% over eight months. FPA had no impact on N2O, but was only examined during periods of low emission (autumn). No significant biomass productivity, agronomic efficiency benefits, or improvements in apparent fertiliser recovery were observed with the DMPP and GU treatments. A significant biomass productivity benefit was observed with the FPA treatment 55 days after fertiliser was applied, most likely because of the gibberellic acid. The outcomes highlight that although DMPP effectively decreased N2O emissions it had no impact on biomass productivity compared with urea. The use of the GU increased N2O emissions by preserving NH3 in the soil. To avoid this a lower rate of N should be applied with the urease inhibitor.

Using urease and nitrification inhibitors to decrease ammonia and nitrous oxide emissions and improve productivity in a subtropical pasture

Urease and nitrification inhibitors are designed to mitigate ammonia (NH3) volatilization and nitrous oxide (N2O) emission, but uncertainties on the agronomic and economic benefits of these inhibitors prevent their widespread adoption in pasture systems, particularly in subtropical regions where no such information is available. Here we report a field experiment that was conducted in a subtropical pasture in Queensland, Australia to examine whether the use of the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT, applied as Green UreaNV®) and the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP, applied as Urea with ENTEC®) is environmentally, agronomically and economically viable. We found that Green UreaNV® and Urea with ENTEC® decreased NH3 volatilization and N2O emission by 44 and 15%, respectively, compared to granular urea. Pasture biomass and nitrogen (N) uptake were increased by 22-36% and 23-32%, respectively, with application of the inhibitors compared to granular urea. A simple economic assessment indicates that the fertilizer cost for pasture production was 5.4, 4.4 and 6.0 Australian cents per kg dry matter for urea, Green UreaNV® and Urea with ENTEC®, respectively, during the experimental period. The mitigation of N loss using the inhibitors can reduce the environmental cost associated with pasture production. These results suggest that the use of these inhibitors can provide environmental, agronomic and economic benefits to a subtropical pasture.