Soil carbon has become a hot subject, with benefits in terms of climate mitigation as well as supporting the resilience and sustainability of agri-food production systems.
CSIRO scientist Lynne Macdonald spoke on the topic at a Mallee Sustainable Farming event last Thursday, at Loxton, where she discussed how drier areas of pastoral and cropping regions typically had lower carbon stocks, compared with wetter regions.
"Carbon and nitrogen cycling go hand in hand when it comes to managing our soils," she said.
"As carbon content changes we are also changing the ability to supply nitrogen through the season.
"It is important for farmers to begin to recognise the value of carbon flowing through the sandy soils of the low rainfall Mallee region."
Sandy soils naturally have lower carbon stocks, compared with other soil types, and ensuring ground cover and continued inputs from crop biomasses offered practical approaches support to support system resilience, she said.
Dr MacDonald says another aspect to consider is the balance between building soil carbon and using it for nutrient provision and biological functions, which naturally require its decomposition or loss from the soil.
If we know the yield gap, we can estimate the opportunity to increase carbon inputs and flows through the soil system
- LYNNE MACDONALD
"It is not a one-way street where carbon can be continuously built, but a balancing act between phases of building and phases of using it to support productivity," she said.
"Managing the crop biomass will assist in maximising carbon inputs to soils.
"Ensuring ground cover to minimise erosion and carbon losses is important in Mallee systems."
She said growers needed to consider how to maximise carbon inputs, and to understand what was constraining plant productivity.
Dr MacDonald says it is important to recognise arable crops allocate about 50 per cent of the carbon they capture below ground.
RELATED: Support key to halting spread
"Root derived carbon is a lot more intimately associated with soil, clay particles, and the microbial community, compared to above ground plant residues," she said.
"Particulate organic matter - recognisable plant material - tends to cycle relatively quickly, with a portion of it becoming more decomposed humus-like material.
"It is no longer recognisable as plant material, due to being processed by the microbial community, and is closely associated with the mineral component."
Dr MacDonald says it helps to slow the break down because the microbial community cannot get to it quite as easily.
"It sticks around for longer and typically the material is in the system for about five-years, some of it is flowing through to the next pool referred to as resistant organic carbon, which is really long-term stable," she said.
"As material becomes more decomposed within the soil system, we have a reduction in the carbon to nitrogen ratio, which means carbon dioxide is being lost as it's broken down and the nutrient component is becoming relatively enriched.
"Carbon and nitrogen are strongly linked in soils. A reduction in carbon, also means a reduction in the inherent ability of the soil to provide nitrogen through the season through mineralisation."
Dr Macdonald said CSIRO had been working on tools to better estimate what carbon stock changes were possible in different environments.
"If we know the yield gap, we can estimate the opportunity to increase carbon inputs and flows through the soil system" she said.
"It will change depending on soil type but 30pc (retention factor) is not a bad assumption to begin with and then essentially we can calculate the carbon that is flowing underground, likely to stay within the system for years to come."
Dr Macdonald said ultimately when looking for opportunities to manage soil carbon content, growers needed to understand if the system was already achieving its potential, or if increased carbon inputs could be achieved.
"If the yield gap is small then the opportunity for change may be limited, but if there are productivity constraints within the system, and there are economically viable options to overcome them, then the potential for increasing carbon flow through the system will be greater " she said.
Research within CSIRO is aimed at providing tools to support grower understanding of soil carbon and where in the landscape there are opportunities or risks associated with carbon.
During the session, Dr Macdonald was asked, for those farmers already at the roof of carbon sustainability, where could they go and what did the future of low rainfall zones look like.
Attendees feared some growers who were managing their properties very well had the littlest rate of change available and smallest potential of income.
Dr Macdonald acknowledged it was an interesting question on how sustainable production systems were valued.
"There is a lot of interest in capturing the sustainability of our agricultural systems, which goes beyond carbon alone and takes into account other factors such as water and nutrient use and system biodiversity," she said.
"Where farmers are at the upper level of sustainability performance, then we need to capture these stories and the co-benefits that come along with it.
"As our carbon and sustainability markets mature, then those at the top of the game should be receiving a premium for their products."
- Start the day with all the big news in agriculture. Sign up here to receive our daily Stock Journal newsletter.
