Carbon Farming Fact Sheet

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carbon farming fact sheet

2009

Store carbon for healthy soils and better yields Maintaining and increasing the level of organic carbon in the soil will have benefits in terms of maximising water-holding capacity and crop productivity but farmers should not expect large returns from carbon credits.

Role of carbon in the soil Increasing organic matter will maintain KEY POINTS or improve soil structure, soil water Soil organic carbon (SOC) contributes ■ balance and the productivity of soils increase soil organic carbon to a variety of soil functions, which over time. (SOC) by retaining crop residues, interact to influence crop production green manuring or cover cropping (see Figure 1). For example, soil How is carbon stored in the soil? and reducing stocking rates on biota obtains energy for growth and cropping paddocks. reproduction from the break down of The amount of carbon in a soil results ■ changes in SOC are often slow, carbon bonds in organic matter, which from the balance between inputs but continual productivity growth in turn contributes to improved soil (plant residues) and losses (microbial and retention of residue carbon structure. SOC also contributes to decomposition and mineralisation). should contribute to increased cation exchange, nutrient turnover and Organic carbon exists in the soil in SOC levels over time. pH buffering. fractions or ‘pools’. Some of these are ■ at current price estimates for A decline in soil organic carbon often readily available and turn over rapidly sequestered carbon, changing has a negative effect on soil structure (that is, are more labile), while others management solely to sell carbon and fertility, increasing run-off and are more stable and break down credits would be hard to justify. reducing yields. slowly. The higher the labile component

The key to maintaining and increasing soil organic carbon is to grow heavy crops and retain crop residues evenly across the paddock.

Figure 1 Functions performed by organic matter At this stage, carbon credits should present in soils be considered as a secondary benefit that may be realised while attempting Biological functions to enhance soil productivity by building soil carbon content.  provides energy to biological processes  provides nutrients (N, P and S) Biochar  contributes to the resilience Biochar is a fine-grained charcoal high in organic carbon. Applied to soil, most types of biochar take hundreds of years to degrade and release carbon. It is produced by heating Functions of Soil organic matter, such as crop waste, Organic Matter wood chips or manure in the absence of oxygen – a process known as pyrolysis. Biochar differs from ordinary charcoal in the way it is produced – in Physical functions Chemical functions an oxygen-reduced, very controlled environment, with greater carbon  improves the structural stability of soils  contributes to cation exchange capacity capture and co-production of biofuel  influences water retention properties  enhances pH buffering that can be used for energy. Whereas  alters soil thermal properties  complexes cations charcoal is only derived from plant material, biochar can be derived from a number of materials, including per year. Avoid excessive tillage at the Reduced tillage trials have shown a animal manure. end of the pasture phase or much of gain of 0.1 to 0.2t carbon per hectare the added soil organic matter will be per year may be possible in the In terms of agriculture, biochar is rapidly depleted. northern grain belt. attracting attention due to the apparent benefits it offers to soil quality, crop Savings from carbon – At current estimates of $10 per yield and as a method of sequestering fact or fiction? tonne of sequestered carbon and the carbon in the soil. Some of the slow potential rates of SOC change, reported benefits include enhancing Plant photosynthesis and biomass it will be hard to justify modifying soil fertility and increasing microbial production are key processes involved management practices solely for the activity but as much of the research in removing carbon dioxide from the purpose of selling carbon credits. is preliminary, more research is atmosphere and potentially capturing it needed under Australian conditions to within the soil. If carbon offsets were valued at determine any real benefits. $20/tonne of carbon, then the Australian soils could potentially store payments would be $2-3/ha and likely Biochar added to soil contributes more SOC but achieving this goal while to be less than the cost of monitoring to the resistant organic matter maintaining an economically viable and administering a carbon credit pool, which is important for carbon farm enterprise remains a challenge. program. sequestration. Adding biochar to soil may benefit crop production 

Removal of crop residues by burning or baling results in a decline in soil carbon. PAGE 3

Table 1 Four organic matter fractions have been identified. While any build up in SOC is beneficial, increasing resource use efficiency These have a range of functions and greater profitability of the farm Organic carbon fraction Key functions business should be the main aim. These provide energy to soil biological processes Organic matter levels can be increased Crop residues – this is material greater than and are readily broken down providing soil or stabilised by growing high yielding, 2mm found in the soil and on the soil surface conditions (moisture, temperature, and oxygen) that favour soil biology. high biomass or a high frequency of crops in rotation. Well-managed In suitable conditions this is broken down Particulate organic matter (POM) – again found relatively quickly but more slowly than crop pastures are also an effective way of in the soil and on the soil surface but this residues and is important for soil structure, increasing soil carbon. material has a size range of 0.05–2mm energy for biological processes and provision of nutrients. Management that eliminates residue It plays a role in all key soil functions but burning or removal, soil erosion, fertility Humus – organic matter of unrecognisable is particularly important in the provision of decline, over-grazing and low biomass structure (molecules) associated with particles nutrients. For example the majority of available crops will help to build SOC levels. <0.05mm soil nitrogen is found in the humus fraction. This decomposes very slowly and, if present Best practice for in large enough quantities, can contribute building soil carbon Recalcitrant organic matter – this is biologically significantly to soil cation exchange capacity stable and typically in the form of charcoal in soils with less than 50% clay, as well as in One hundred per cent ground cover, controlling soil temperature. 100 per cent of the time is the carbon farmer’s goal. To reduce the risk of of the total soil organic matter pool, the Impact of farming erosion, a minimum of 50 per cent higher the biological fertility status of ground cover should be maintained Soil fertility and SOC in Australian the soil (Table 1). in most soils. Other practices for soils have declined over decades increasing SOC include: Most organic matter pools (apart from of cropping. In many cropping soils charcoal) decompose over years or organic carbon is still in decline. increase soil moisture capture – to decades and need to be replaced grow bigger crops; In cropping systems the potential to by fresh litter to maintain soil carbon build SOC depends on the capacity introduce well managed pasture levels. to produce large quantities of crop phases or increase cropping frequency; As SOC decomposes the size of the biomass and to return and retain particles decreases, residues become carbon in the soil. Practices such as produce high biomass crops and more nutrient rich and turn-over minimising tillage, retaining not burning pasture; time increases from hours to days to or baling stubble and managing soil reduce or eliminate tillage; erosion all help to return or retain decades to hundreds of years. retain stubble and crop residues; and carbon in the soil. The amount of organic carbon results consider organic amendments. from the balance between inputs (plant Maintaining or increasing SOC is and animal residues and off-farm good farming practice. Higher soil A grass-legume pasture can build soil inputs) and losses (mineralisation of organic carbon has benefits for nutrient carbon levels by more than one tonne SOM resulting in evolution of carbon turnover and improved soil structure, per hectare per year, which could lift dioxide, erosion and product removal). as well as other key soil functions. soil organic carbon by 0.05 per cent

The potential amount of organic carbon the soil can hold varies with factors such as clay content, soil depth and climate. For example, sandy soils can hold less SOC than clay soils (see Figure 2) and there is less in lower rainfall areas.

To build up SOC to its potential (determined by soil type and climate) under a managed cropping or grazing system, high levels of production need to be maintained and the removal of plant residues minimised. The speed of crop residue breakdown Changes in soil carbon content are is determined by slow and measurements over several moisture, temperature decades may be needed to define accurately the effects of particular and oxygen. management treatments on SOC Crop residues provide content. Measurement of more labile the carbon energy for pools can provide an earlier indication biological processes of potential longer-term change. and contribute to the liable carbon pool. PAGE 4

through its interaction with soil cation- FIGURE 2 EFFECT OF SOIL ORGANIC MATTER COMPONENTS ON DETERMINING exchange capacity, nutrient retention SOIL CHARACTERISTICS IN RELATION TO CLAY CONTENT and release and the biological components of soil. Clay content (%) Clays 100 Biochar properties vary significantly, 80 due mainly to the different types of 60 feedstock (the material it is made from) 40 and processing conditions. Some soil 20 types are very receptive to biochar Sands 0 application and have shown increases CEC Soil Energy for Provision of Soil thermal in fertility and structural benefits, but structure biological nutrients properties processes other soil types showed no benefits Soluble Particulate Humus Resistant at all. The findings reported here are from a study of 29 different soils, sampled between 0-10cm, from nine Even if its effectiveness is proven, locations in south east Australia. whether or not biochar stacks up as an economically viable proposition will depend on the cost of the feedstock While animal residues and returns from renewable energy add to the carbon generated in the pyrolysis process. pool, grazing needs to It will also depend on whether it is be managed to avoid included in an emissions trading excessive removal of scheme where biochar producers or crop residues. users may receive credit for stabilising organic carbon.

Useful resources:

■ CSIRO biochar fact sheet www.csiro.au/resources/Biochar-Factsheet.html ■ Australia and New Zealand Biochar Researchers Network www.anzbiochar.org ■ Carbon Farmers Of Australia www.carbonfarmersofaustralia.com.au ■ CSIRO technical publications www.csiro.au/resources/Biochar-climate-change-and-soil.html

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Acknowledgements: Dr Jeff Baldock, CSIRO; Daniel Dempster, University of Western Australia; Dr Frances Hoyle, Department of Agriculture and Food Western Australia; Dr Evelyn Krull, CSIRO; Allan Mayfield; Dr Peter Wylie. produced by