Greenhouse Bases in Agriculture Workshop

Alan Niscioli, Technical Officer

Greenhouse gas emissions, fertiliser management strategies and nitrogen use efficiency were the focus of discussion at the Kununurra Greenhouse Gases in Agriculture Workshop, held on 14 March at the Frank Wise Institute in Kununurra WA. The workshop was hosted by Living Farm, in partnership with the Northern Territory Department of Primary Industry and Fisheries (DPIF), Queensland University of Technology (QUT) and the Department of Agriculture and Food Western Australia (DAFWA). Guest speakers included Dr Andrew Wherrett from Living Farm, Dr David Rowlings from QUT and Dr Mila Bristow from DPIF.

Guest speakers presented findings from their various Action on The Ground (AOTG) projects being run across the north Australia which use national methods to quantify soil greenhouse gas emissions.

Andrew discussed findings from his Kununurra trials where he examined the effects of fertiliser application on soil nitrous oxide (N2O) emissions from flood irrigated systems. N2O is a powerful greenhouse gas 300 times more potent than carbon dioxide. Soil emissions are an indicator of nitrogen (N) losses after fertiliser application. Nitrogen can be lost in many ways, including leaching through the soil, through runoff and erosion, and as gases lost to the atmosphere. The latter can account for over 50% of N loss and occurs through volatilisation of ammonia or through the breakdown of nitrate by microbes into other forms such as N2O.

Rate of soil N2O emission are accelerated in the warm, wet soils of northern Australia, particularly in regions of intensive farming around Kununurra and horticultural regions around Darwin and Katherine. Although some N2O emissions are unavoidable, farmers can reduce them.

In one of his trials Dr Wherrett demonstrated the effectiveness of a nitrification inhibitor called eNtrench™ in reducing N2O emissions. The product is used in conjunction with N based fertilisers and like other nitrification inhibitors it works by reducing the bacterial conversion of ammonium nitrogen to a nitrate form. In his trials he also demonstrated that in-furrow injection of fertiliser at seeding reduced volatilisation of fertiliser which can be high in surface and side dressed applications, both of which are common practices in the Ord.

In a three-year research trial in southern Queensland Dr Rowlings and his team examined the fate of fertiliser by using N labelled with short-lived radioactive isotopes and then tracking them through soil and sorghum plants during the growing season. They found that 57% of the fertiliser N was recovered by the crop and from this recovered N they found that 75% was removed by the grain during harvest. This represents a fertiliser use efficiency (applied N removed in grain) of 44–45%. They were also able to determine that 16% of the N fertiliser was left in the soil while 27–28% of N was lost to the atmosphere.

Dr Mila Bristow presented outcomes from the NT Action on the Ground program. From 2012 to 2015 DPIF conducted experiments examining the use of Wet season cover crops to improve N management between crops and assessed the effectiveness of cover crops in mitigating N2O emissions during the Wet Season. Findings from the experiments suggest that soil greenhouse gas emissions are driven by rainfall and management events such as cultivation, rather than by the type of cover crop grown. Research found soils with cover crops store far greater whole plant and soil N than do bare soils, preventing losses through leaching. However, cover crops had little impact on residual soil N remaining after the Dry Season.

Over the three years the NT research team also monitored emissions and N use in a range of Dry Season vegetable and melon crops. It was found that majority of NT farmers apply fertilisers at recommended rates and their emissions currently rank in the lower range of emissions reported for other irrigated crops worldwide. However, there were some instances where emissions were high and could be improved.

The NT team also evaluated the effectiveness of enhanced efficiency fertilisers on pastures in the Katherine region. Results from these trials showed that greenhouse gas emissions from hay crop soils occur immediately after the application of fertilisers. They also showed that by switching from general products such as urea to an enhanced efficiency product such as EntecTM, soil N2O emissions were reduced by 25–60% with no reduction in hay quality.

The NT Action on The Ground project has now shifted focus to mangoes and bananas where trials have begun in the Darwin region and the Ord region of WA. The team hopes to gain a better understanding of soil N2O emissions to improve N management in both mango and banana cropping systems. To complete this work the team will use automated monitoring equipment which will provide greater accuracy and provide a deeper understanding of the processes involved.

Last updated: 07 August 2019