At face value farmers looking to cut greenhouse gas emissions could be forgiven for looking to cut their use of nitrogen (N) fertiliser, which has a high carbon footprint to produce due to the energy intensive manufacturing process.
However, agricultural scientists have found in whole of lifecycle research that in many applications Australian farmers need to consider higher rates of nitrogen application in order to help bolster their soil organic matter and in turn their soil carbon levels.
The results are clear - greenhouse gas (GHG) accounting has found that the loss of soil carbon as a result of inadequate soil nutrition can increase carbon footprints.
CSIRO researcher John Kirkegaard, who has authored several reports into N management in farm systems, recently worked on a paper prepared by lead author Maartje Sevenster on rethinking N-fertiliser and greenhouse gas balances in cropping systems, using data from Dr Kirkegaard's long-term agronomic trial run for 30 years at Harden in southern NSW.
Dr Kirkegaard said the research had found that low rates of N application normally thought to reduce carbon footprints can also lead to "N-mining" linked to a considerable loss of soil carbon, meaning a rethink of nitrogen management strategies in regards to emissions was required.
The research showed that increasing nitrogen application in dryland cropping systems could result in a significant decrease in net greenhouse gas (GHG) emissions intensity linked to higher soil carbon levels.
It follows on from a similar finding from a 40-year trial in southern Queensland in 2015 conducted by Weijin Wang and Ram Dalal, with higher nitrogen application resulting in considerable decrease in soil carbon losses in experiments.
"It may sound a little counterintuitive in terms of a carbon management strategy to suggest putting out more synthetic fertiliser but in order to minimise emissions, continuous croppers in most environments in Australia are going to have to up their nitrogen inputs (from N fertiliser or by growing legumes) to ensure they are not losing soil carbon," he said.
"You have to understand that the fertiliser situation in Australia is very different from the US or Europe where much of the debate about nitrogen and the carbon footprint of fertilisers is coming from."
"In those places, with heavy subsidies, farmers can afford to put on quite high applications of N, it is not a major cost within their system."
"This is in contrast to Australia, where it is one of the average farmer's biggest input costs and our nitrogen budgets are carefully matched out to mirror target yields."
"What we have found in our research is that for long-term soil fertility you have to look beyond just allocating fertiliser for the crops, you have to consider the soil as well."
"This certainly doesn't mean just slopping out excessive rates of urea but to carefully increase rates to match what the soil organic matter needs as well."
Dr Kirkegaard said there was a close and clearly understood relationship between soil organic matter (SOM) and nutrients such as nitrogen, phosphorus and sulfur, found in clearly defined ratios in soil organic matter.
The crux of the matter is that to create more organic matter, and by extension soil carbon, soil microbes require nutrients contained in the other elements to grow, before dying and creating more organic matter and carbon.
"Soil organic matter is always thought to bring better fertility, but you also need fertility, via those nutrients ,to build it in the first place."
Previous work conducted by veteran soil scientist Clive Kirkby found there was a clear recipe to bolster carbon levels, irrespective of soil type or existing carbon levels.
Dr Kirkby's work showed that irrespective of the soil type or amount of carbon in the soil, each tonne per hectare of soil carbon that is sequestered in soil organic matter required 85 kilograms of nitrogen, 20kg of phosphorus and 14kg of sulfur to grow the microbes necessary to create more SOM.
If those nutrients were not available in the soil even when farmers retained carbon-rich stubble residue, the microbes could not grow and carbon levels would not rise.
Dr Kirkegaard said results for the higher-nutrient treatment in trials with rates calculated to maximise carbon sequestered from retained crop residue showed a decreased carbon footprint.
"The additional nitrous oxide and other estimated emissions due to higher nitrogen application were more than offset by the carbon dioxide removed through carbon sequestration in the soil."
The Sevenster paper concluded that it was important to recognize the role of nitrogen in certain environments in enhancing soil natural capital and in lowering net GHG emissions.
It found that improved fertiliser application can result in a decrease in GHG intensity per tonne grain of up to 20pc, by minimising the amount of nitrogen that volatilised and contributed to nitrous oxide emissions.
Dr Kirkegaard said he was now conducting further research on the matter as part of a Grains Research and Development Corporation (GRDC) funded project, running from 2023 to 2028, which has eight sites around Australia testing ten different fertiliser and stubble management strategies.
The Carbon Series was produced in collaboration with the Australian Science Media Centre with support from the META Public Interest Journalism Fund administered by the Walkley Foundation.