On-farm carbon dynamics

By: David Walker, Executive Officer
June 27, 2010

One reason we talk so much of the potential for managing soils and vegetation to store (sequester) carbon in the soil is the fact that the soil contains about twice as much carbon as the atmosphere. Relatively small changes in soil carbon could have a significant impact on greenhouse gases.

Prompted by concern about climate change issues, the Lower Apsley River Landcare Group near Walcha have taken the initiative to get a better understanding of greenhouse gas emissions from their farms and how these relate to the wider carbon cycle in their landscape. 

They were concerned about how the level of greenhouse gas emissions from agriculture, particularly the livestock sector, was being reported, bearing in mind that agriculture is said to contribute up to 20% of Australia's emissions. 

They were also concerned that current carbon accounting rules did not consider the full range of carbon sinks that occur on farms. They also wanted to put the level of agricultural emissions into perspective relative to the amount of carbon cycling through the soils and vegetation of a rural landscape and highlight the potential for bio-sequestration of carbon on farms.

The Lower Apsley River Landcare Group are landholders to the east of Walcha, mostly finewool grazing country, but also producing lamb and beef. Formed in 1994, the group has focussed its activities on revegetation for biodiversity and shelter and on works to improve the water quality of the river. They won the NSW Landcare Group award for 2007/2008.

They started by organising the Walcha Carbon Forum in June 2007, and thereafter decided to carry out a study to put some figures on the pools and fluxes of carbon (and CO2 and other greenhouse gases, all expressed as CO2 equivalents, shown as CO2-e).

Their findings show that large amounts of carbon are stored in various parts of the Lower Apsley landscape. The top 30 cm of soil alone was estimated to contain, on average, 224 tonnes of CO2- e per hectare and the native remnant trees a further 26 tonnes per hectare. Planted trees currently store about 5 tonnes per hectare and, at any one time, pastures contain about 6 tonnes per hectare of CO2-e. These total 261 tonnes CO2-e /ha. 

Over the 16,500 ha of the Lower Apsley properties, these carbon stores add up to a whopping 4.3 million tonnes of CO2-e contained in the soils and vegetation (remembering that this carbon has been stored over perhaps thousands of years).

These stored amounts dwarf the annual emissions attributed to ruminant livestock agriculture. The annual emissions from the farms average about 1.2 tonnes per hectare per year, being made up of 1.1 t/ha CO2-e as methane from livestock, 0.1 t/ha as nitrous oxide (N2O) from fertiliser and animal waste, and 0.03 t/ha from on-farm fuel and electricity use.

Perhaps a disappointing finding is that annual carbon sequestration by trees (the remnant trees occurring on the landscape and those that have been planted by the farmers) only totalled less than half a tonne per hectare, with older remnant vegetation 0.25 t/ha, and planted trees 0.24 t/ha. Thus the annual sequestration by the trees is only about 41% of the annual emissions mentioned above.

Although not measured or estimated in this study, potentially large amounts of carbon could be sequestered in the soil of the Lower Apsley by building soil organic matter.

A 0.1 percentage point increase in carbon as a percentage of the top 30cm of soil would sequester approximately 200,000 tonnes CO2-e (which is more than 10 times the total annual emissions from all sources for 2008/2009).

Farmers of the LARLG were particularly interested in the flows of carbon through their pastures and livestock and how these interact with the atmosphere and soils. To illustrate these flows, an attempt was made to generate a carbon cycle diagram based on a simplified pasture-livestock ecosystem of the Lower Apsley. 

Click here to download a pdf of the audit report - it is well worth a read.

To my mind, there are some important messages with this report. Firstly, farmers are interested in, and more than capable of, implementing and managing research aimed at gaining a better understanding of the science underpinning their productions systems. The Lower Apsley Landcare Group illustrates the professionalism and competence of our farming community.

The second message is that there are still major uncertainties surrounding our hopes that modifying our land management will enable us to solve the greenhouse gas emissions problem by using our pastures to remove the excess levels of carbon from the atmosphere and store it in the soil.

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