Mitigation - soil carbon storage and capture in agriculture There are three types of carbon in soil. These include:
- Particulate organic carbon – plant and animal material between 0.05 and 2mm.
- Humus – decomposed material less than 0.05mm that are dominated by molecules attached to soil minerals.
- Recalcitrant organic carbon – this is biological stable,typically in the form of charcoal.
During the consultation process for the CRP, there was general consensus that agricultural practices that result in sequestration of carbon and improve productivity, should be encouraged where it has no significant impact on the environment or profitability. The challenge is to understand, measure and apply the techniques that deliver the maximum benefits. This can only be achieved through research, demonstration trials, experimentation and extension.
Incorporating organic matter
Increased soil organic matter can increase the availability of nitrogen, phosphorous and sulphur for plants. It can also increase cationic exchange capacity and the water holding capacity of soil. Where appropriate, adding organic material can result in improved productivity of soils. It can be seen as both a mitigation activity and an adaptation activity as it can increase the levels of carbon in soil and in the right situations improve the productivity of soil. It should be noted that in agricultural soils, using organic matter with a high carbon to nitrogen ratio can cause problems. To complete the nitrogen cycle and continue decomposition the microbial cells will draw any available soil nitrogen, in the proper proportion, to make use of available carbon. This can reduce available nitrogen as a fertiliser for growing plants until some later season when it is no longer being used in the life-cycles of soil bacteria. When the energy source, carbon, is less than that required for converting available nitrogen into protein, organisms make full use of the available carbon and get rid of the excess nitrogen as ammonia. This release of ammonia to the atmosphere produces a loss of nitrogen from the compost pile and should be kept to a minimum. It is extremely important for land managers to understand what ratio best suits their situation when considering using compost.
There have been a number of trials and experiments undertaken that have attempted to incorporate organic matter, for example, manure top spreading and application at depth to improve soil structure and deal with subsurface soil constraints. These activities have the potential to impact on the environment, particularly groundwater and surface water if not managed correctly. When conducting these activities proponents should:
- Incorporate appropriate setbacks from rivers, streams and wetlands, for example 50 metres.
- Monitor water quality of nearby waterways.
- Consult the Environmental Protection Authority (EPA).
To date, many of the demonstration trials that have been conducted have had variable results. It is unsure if the practice is only suited to certain soil types and seasonal conditions. There is also uncertainty if it is more profitable to incorporate material into soil or to top-dress. These are questions that require further investigation.
Many demonstrations have shown that the practice is cost prohibitive unless landholders can access a cheap source of compost close to their property. Given the amount of residue from crops, techniques and management practices to fully maximise the use of this resource could be further investigated. This may be an opportunity for further research and demonstration techniques to increase the soils carbon cycling capacity. Priority demonstration trials will be based on assessing suitability and efficacy of techniques of retaining farm crop residues to improve productivity, at relevant scale in the Wimmera agricultural environment.
Mitigation – greenhouse gas soil emissions in agriculture
Emissions of carbon dioxide and nitrous oxide from soil are considered greenhouse gases. The loss of carbon and nitrogen via these compounds can have negative impacts on soil fertility. It is important to understand these processes if we are to understand how farm management can influence farm productivity, reduce emissions and mitigate climate change. There are currently demonstration trials in the Wimmera investigating how different farming techniques and conditions influence rates of nitrous oxide and carbon dioxide loss. These demonstrations are relatively short term (three years) and there is consensus that we will gain a better understanding if these trials were maintained over longer periods where the influence of seasonality could be understood and loss rate calculated with more certainty.
Nitrogen use efficiency
Different farming practices can influence nitrogen use inefficiency and emissions of nitrous oxide to the atmosphere. This can pose a cost or benefit to a farming system. Nitrogen use efficiency refers to the ability to convert soil nitrate into plant material nitrate, for plant growth. The nitrate-N comes from fertiliser, crop residues, manures and soil organic matter, but it is the efficiency of conversion of fertiliser into grain or plant growth that is generally of greatest concern to growers. Efficiency is reduced by seasonal conditions, crop diseases, losses of N from the soil as gases, N leaching or immobilisation of N into organic forms.
- Maintain carbon and nitrous oxide emission trials for as long as possible.
- Support the promotion of practices that result in nitrogen use efficiency.