Research looks to arrest escaping nitrogen
- Trials in Western Australia have shown that farm practices that increase organic carbon levels will also lead to a small increase in nitrous oxide (N2O) emissions from nitrogen-fertilised sandy soil.
- However, the same research also showed increased soil organic carbon levels led to higher grain yield and quality.
- N2O emissions measured in WA were less than 0.1 per cent of the nitrogen fertiliser applied – much less than the international benchmark of one per cent.
- In Victoria’s high-rainfall zone, where starting soil organic carbon levels were high (due to years of grass-legume pastures that were cultivated before cropping), high N2O emissions were produced.
- Nationally, research done in WA, Queensland and Victoria confirmed N2O emissions in Australia are no more than 0.3 per cent of the nitrogen fertiliser applied.
Farm practices that increase soil organic carbon levels can improve grain yield and quality with only small increases in nitrous oxide emissions on sandy soils
One of the largest nitrous oxide (N2O) research projects yet undertaken in Australia has yielded some positive news for grain growers and the community generally.
Scientists working on the national research project, supported by the GRDC in partnership with the Australian Government’s Department of Agriculture, investigated the impact of increased soil organic carbon levels on N2O emissions, grain yield and grain quality in Western Australia, Victoria and Queensland.
The amount of N2O emitted from the soil is of interest to researchers and the Australian Government because this greenhouse gas is 300 times more potent than carbon dioxide in its contribution to global warming and climate change.
Dr Louise Barton, a senior research fellow at the University of WA (UWA), says N2O emissions are produced when soil
microbes go about their everyday business: “N2O can be emitted from all soils, whether they grow wheat, garden plants or lawn, in fact anywhere soil microbes are actively converting organic forms of nitrogen to ammonium and nitrate,” she says.
“When soil microbes convert organic nitrogen to nitrate, the process can produce N2O. The nitrate is also at risk of being denitrified if the soil becomes particularly wet and N2O is emitted.”
The question raised by Dr Barton and the subject of her study was what would happen to N2O emissions when soil organic carbon levels were lifted in a bid to use the soil to sequester more carbon?
To find out, she collaborated with the Liebe Group, a grower-led research organisation based in Buntine, WA, 300 kilometres north-east of Perth, whose members had established a long-term trial in 2003 to investigate different ways of increasing soil carbon.
The trial was located in a Mediterranean-type climate that generally records 285 millimetres of annual rain (mainly winter), and average maximum and minimum temperatures of 26.2°C and 11.6°C, respectively.
At the trial, Dr Barton’s research team gathered results from an experiment that comprised replicated plots that had or had not been amended with organic matter. One treatment was tilled soil (tillage) and the other treatment was organic matter tilled into the soil (OM + tillage).
Also tested at the trial was the impact of two nitrogen fertiliser rates (nil and 100 kilograms per hectare per year) on N2O emissions from the different OM treatments.
Every three years since 2003, the Liebe Group has added 20 tonnes/ha of OM to the trial site and incorporated it. A total of 80t/ha has been applied to date. The soil was also tilled annually.
Crops at the site for the duration of Dr Barton’s trial comprised canola in 2012 and barley in 2013.
At the start of the trial, organic carbon in the top 100 millimetres of soil was 0.5 per cent in the tillage treatment and 1.2 per cent in the OM + tillage treatment.
The soil was free draining sand with a pH (calcium chloride) of 6.2 and a bulk density of 1.4 grams per centimetre.
After two years, the results demonstrated that increasing the soil organic carbon level increased N2O emissions from nitrogen-fertilised soil. It also improved wheat yield (Figure 1) and quality.
While Dr Barton’s automated measuring devices showed that adding more soil organic carbon increased annual N2O emissions, the emissions were low and less than 0.1 per cent of the nitrogen fertiliser applied.
Dr Barton says the results are good news for growers and the wider Australian community because they confirm N2O emissions are low here – much less than the international standard. This might be one of the few benefits of Australian soils’ naturally low fertility and microbial activity.
This could benefit Australian grain growers if the European Union or any other country was to introduce mandatory N2O accreditation standards for suppliers.
The outcomes of Dr Barton’s WA research, and associated work done in Victoria and Queensland, have been communicated to the Intergovernmental Panel on Climate Change, the leading international body for the assessment of climate change.
Across the Nullarbor
In the northern grains region, University of Queensland chair in tropical agronomy Professor Mike Bell, from the Queensland Alliance for Agriculture and Food Innovation (QAAFI), says trials in Queensland demonstrated N2O emissions were higher than in WA but were still less than one per cent of the nitrogen fertiliser applied.
However, while N2O emissions were comparatively low, research in summer sorghum crops in the north demonstrated that 20 to 40 per cent of applied nitrogen can still be lost to the atmosphere.
Victorian Department of Economic Development, Jobs, Transport and Resources (DEDJTR) senior researcher Dr Roger Armstrong says trials in a medium-rainfall area near Horsham, Victoria, found 20 to 45 per cent of applied nitrogen was lost to the atmosphere.
Further, at Hamilton, Victoria, where annual rainfall is 750mm and background soil nitrogen and carbon levels are high, project team member Dr Rob Harris measured losses of up to 85 per cent of nitrogen (applied at sowing as urea) from the soil-crop system.
“The results show there are big financial implications for grain growers in these areas,” Dr Armstrong says.
Professor Bell says gaseous nitrogen losses can be high when there is a coincidence of wet soils, an abundance of residue or carbon in the soil to support a high level of microbial activity and plenty of residual soil nitrogen.
“Nitrogen losses can be spectacular on soils coming out of pastures where organic carbon levels are high in high-rainfall areas,” he says.
“The biggest risk for losses in the north is where growers are double-cropping because there is typically plenty of stubble, the season is reasonably wet and nitrogen fertiliser rates are high to compensate for the depleted soil reserves from the initial crop.”
Dr Armstrong agrees, adding that losses not only come from fertiliser-derived nitrogen but also from nitrogen derived from legumes.
He points to research by Dr Oxana Belyaeva, working on the project near Hamilton, Victoria, on paddocks coming out of grass-legume mixed pastures to be sown to crops.
“The cultivation of grassy legume pastures that have high background levels of organic nitrogen and labile (available) carbon can result in rapid rates of nitrogen mineralisation and the occurrence of high nitrate nitrogen concentrations before the crop is even sown,” Dr Armstrong says.
The case for testing
“It is the mineralisation of that nitrate by soil microbes in advance of having a crop there to utilise it that increases the risks of the release of N2O to the atmosphere through denitrification. Not only do these losses have carbon accounting implications, but they also represent a waste of soil nitrogen reserves that could be used to grow future grain crops.”
For southern growers in these situations, as well as those parts of the Wimmera that have sodic clay soils that are prone to temporary waterlogging, Dr Armstrong says soil nitrogen testing is a must.
“There’s no point adding large amounts of fertiliser if you already have high background soil levels because you may lose a large proportion of it,” he says. “Sure soil testing costs money, but there is a bigger cost that will be borne if nitrogen is applied when crops don’t need it.”
In the north, Professor Bell agrees that soil testing is important and adds that he would like to see more legumes grown.
He says this would reduce the spend on synthetic fertilisers, with research results showing it would also deliver less N2O emissions per tonne of grain produced.
For WA, Dr Barton encourages growers to apply only the amount of fertiliser that would meet the needs of a crop on a seasonal basis.
“Don’t be scared about increasing your soil carbon levels because your emissions are likely to be very low and that’s great because it means the carbon footprint of your grain production is very low too by world standards,” she says.
To lower the carbon footprint further, Dr Barton was investigating whether increasing the organic carbon content of soils would mean growers could actually decrease their nitrogen fertiliser inputs over time.
So far, she has not been able to detect a response to added nitrogen due to dry growing seasons and high starting soil nitrogen levels at the Liebe Group site during the past two years.
“In WA when it rains in summer, soil microbes convert the nitrogen found in organic matter into plant-available forms of soil nitrogen (ammonium and nitrate) where it remains waiting for the next crop to use it, provided it doesn’t leach beyond the root zone,” she says.
“In some summers we’ve had enough rain that by seeding, there’s sufficient nitrogen available in the soil to meet the entire needs of the crop.”
However, Dr Barton warns that a lot can change between when growers test their soil in January to February and when sowing kicks off from April through until June.
She says this means growers and their advisers need to take extra care when determining their nitrogen fertiliser inputs to ensure that starting soil nitrogen reserves are estimated correctly based on the best available knowledge and modelling data to ensure the inputs applied are not wasted.
Further GRDC-supported investigations will be undertaken this year as part of a one-year research project to better understand how nitrogen fertiliser decisions are made, the fate of nitrogen applied to crops and any knowledge gaps for ongoing research.
More information:Dr Louise Barton, UWA,
08 6488 2543,
Professor Mike Bell, QAAFI,
07 54601 140,
03 5362 2336,
Footage of Dr Louise Barton, Professor Mike Bell and Dr Roger Armstrong discussing the results of these research outcomes is available at the eXtensionAUS Crop Nutrition website.
GRDC Project Code UWA00156, UQ00066, DAV00125, UQ00079
Region National, North, Overseas, West