Introduction

Carbon is the building block and essential for all life on earth. Every living organism needs carbon to sustain life whether for physical structure, or as an energy source, or both. Carbon is able to cycle through the earth's atmosphere, oceans, biosphere (living things) and geosphere (soils). The total amount of carbon on earth is fixed, as we cannot create more and we cannot remove carbon from the earth's system. Carbon can be found as a gas, and in solid and liquid forms, but the total amount of carbon on earth always remains the same.

Carbon is the primary component of all fossil fuels (coal, oil and gas) that we burn to create power. Energy usage has increased the amount of carbon dioxide (CO2) in the atmosphere through burning fossil fuels.

In an agricultural system, carbon is cycled through the atmosphere (gases - CO2 and CH4), through plants (via photosynthesis) and animals (food), and through the soil (via microbes). The production of food affects the amount of carbon in the soil as harvesting plant and animal products removes carbon from the agricultural system. Best Management Practices (BMP) or good management systems will increase the amount of carbon stored in our soils and poor practices will reduce the amount of carbon being stored or sequestered. By increasing the amount of carbon stored in the soil as humus and recalcitrant carbon, we can significantly offset the amount of CO2 in the atmosphere and also improve the health of our soils. The carbon cycle and agriculture - http://vro.dpi.vic.gov.au/dpi/vro/vrosite.nsf/pages/soilhealth_organic_carbon-cycle

Monitoring carbon in farming and grazing soils is an essential part of any successful BMP. In future programs, farmers will potentially be paid for sequestering carbon in soils and for reducing methane emissions into the atmosphere. To learn more about Australian Government Programs Click here.

Key findings from Australian Govt Carbon Research

1. Past conversion of native land to agriculture has depleted soil organic carbon by 40-60 per cent but this provides significant future potential.
2. Soils used for crop production in Australia may only be able to slow or halt the rate of carbon losses, and not sequester additional atmospheric carbon.
3. Improved crop management practices have resulted in a relative gain on average of 0.2-0.3 tonnes carbon per hectare per year.
4. Pasture improvements have generally resulted in relative gains of 0.1–0.3 tonnes carbon per hectare per year.
5. Application of organic materials has potential to increase soil carbon levels.
6. The greatest soil carbon gains are likely to follow major shifts in management.
7. Soil carbon improvements are greatest in the first 5-10 years and then diminish over time.

The following table is a summary of Australian Government BMP practices for sequestering carbon.

Summary of major management options for sequestering carbon in agricultural soils
Management	Sequestration potential	Confidence in estimate	Justification
1. Shifts within an existing/mixed cropping system
a. Maximizing efficiencies	NIL-Low	Low	Yield and efficiency increases do not necessarily translate to increased C return to soil
1. water-use
2. nutrient-use
b. Increased productivity	NIL-Low	Low	Potential trade-off between increased C return to soil and increased decomposition rates
1. irrigation
2. fertilisation
c. Stubble management	Low	Medium	Greater C return to the soil should increase SOC stocks
1. Eliminate burning/grazing
d. Tillage			1. Reduced till has shown little SOC benefit
		Medium	2. Direct drill reduces erosion and destruction of soil structure thus slowing decomposition rates; however, surface residues decompose with only minor contribution to SOC pool.
1. Reduced tillage	Low	Medium
2. Direct drilling	NIL-Low
e. Rotation			1. Losses continue during fallow without any new C inputs – cover crops mitigate this
		Medium	2. Pastures generally return more C to soil than crops
1. Eliminate fallow with cover crop	Low		3. Pasture cropping increases C return with the benefits of perennial grasses (listed below) but studies lacking.
2. Inc. proportion of pasture to crops		High
3. Pasture cropping	Low-Moderate
		Medium
	Moderate
f. Organic matter and other offsite additions	Moderate-high	High	Direct input of C, often in a more stable form, into the soil additional stimulation of plant productivity (see above).
2. Shifts within an existing pastoral system
a. Increased productivity	NIL-Low	Low	Potential trade-off between increased C return to soil and increased decomposition rates
1. irrigation
2. fertilisation
b. Rotational grazing	Low	Low	Increased productivity, inc. root turnover and incorporation of residues by trampling but lacking field evidence
c. Shift to perennial species	Moderate	Medium	Plants can utilize water throughout year, increased belowground allocation but few studies to date
3. Shift to different system
a. Conventional to organic farming system	NIL-low-moderate	Low	Likely highly variable depending on the specifics of the organic system (i.e. manuring, cover crops, etc.)
b. Cropping to pasture system	Low-moderate	Medium	Generally greater C return to soil in pasture systems; will likely depend greatly upon the specifics of the switch
b. Retirement of land and restoration of degraded land	Moderate-high	High	Annual production, minus natural loss, is now returned to soil; active management to replant native species often results in large C gains

Soil Carbon - Measuring soil health

Monitoring the carbon levels in your farming and grazing soils is an essential tool in measuring the success of your management system. Soil carbon is an essential parameter in any soil nutrient analysis program. If you have tested your soils in the past decades you will probably have a record of either soil carbon (SOC), organic matter (OM) or humus. All of these numbers are related and can be used to gauge changes in soil health.

Carbon is a food source for microbes and plants, acts as a sponge to hold water (4 times its own weight), buffers impact of droughts and pesticides, provides a home for microbes and holds most nutrients so they do not leach.

Many scientists are developing methodologies to assess the various types of carbon found in soil. These include:

1. Organic matter - total material being broken down by microbes - unstable, can be lost in a few months.
2. Humus - stable carbon which may be in the soil for hundreds of years
3. Charcoal - Recalcitrant carbon - stable for thousands of years.

To date, there is no agreed, low cost method of testing all three types of carbon. However, to gauge agricultural success, an increase in any or all of these carbon types is success.

Carbon in Animals - Methane efficiency of grazing system

All grazing animals digest grass based pastures, varying from native pastures to improved pastures, grown on healthy balanced soils to poor low fertility soils. The quality of the soils impact on the quality of the pastures and the management systems also impact on the quality of the pastures. The quality of pastures will impact on the amount of methane emitted by livestock as methane is produced as a by-product of animals digesting these pastures. CSIRO is working on a series of trials to assess the methane losses on various pasture species, supplements and practices. Dr Ed Charmley is managing the research, based at Lansdowne Research Station, Qld.

A Grazing BMP has already demonstrated that a cow grazing Leucaena will emit 30% less methane than a cow on tropical pastures. For Lucerne, Stylo and Burgundy Bean, increasing the content of legume in the diet, reduced the emissions of methane per kg DM intake. The CSIRO project is researching the emissions from various pastures, various supplements and land management practices. (Check the research)

Bio-Char - From CSIRO Bio-char factsheet

Biochar is a stable form of carbon with significant potential for use in carbon sequestration and in improving soil condition. Biochar is a stable form of charcoal produced from heating natural organic materials (crop and other waste, woodchips, manure) in a high temperature, low oxygen process known as pyrolysis.
Due to its molecular structure, biochar is chemically and biologically in a more stable form than the original carbon form it comes from, making it more difficult to break down. This means that in some cases it can remain stable in soil for hundreds to thousands of years.

It is important to note that there are many different types and qualities of biochar. The key chemical and physical properties of a biochar are greatly affected by the type of material being used and the conditions of the pyrolysis process.

Some studies have shown that biochar can aid in:

• retaining nutrients and cation exchange capacity
• decreasing soil acidity
• decreased uptake of soil toxins
• improving soil structure
• nutrient use efficiency
• water-holding capacity

Some studies that have reported positive effects with regard to crop production often involved highly degraded and nutrient-poor soils, whereas application of biochar to fertile and healthy soils does not always yield a positive change. Research is ongoing.

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