Pictures of the fractions of carbon from rapidly decomposable raw pieces of plants and micro-organisms scanned using electronic microscopy.

Fractions of carbon from rapidly decomposable raw pieces of plants and micro-organisms scanned using electronic microscopy.

A new simple, fast and inexpensive technique for measuring carbon in soils

CSIRO's technique will help predict the carbon status of any region in Australia.

  • 7 March 2008 | Updated 14 October 2011

Background

Soil carbon is stored in a number of fractions with widely varying chemistry and stability. 

These soil carbon fractions can be converted to the greenhouse gas carbon dioxide, and comprise:

  • rapidly decomposable raw pieces of plants and micro-organisms that lasts for days or weeks
  • fine partly decomposed soil organic matter that takes years or even decades to disappear
  • charcoal-like carbon from countless grass-land fire events that lasts for hundreds of years.

Now a quick and cheap way of determining the soil carbon content and the amount of carbon found in each form has been developed by CSIRO’s soil organic matter research team to help predict the carbon status of any region in Australia.

Until recently, measuring the carbon contents within carbon pools could not be determined accurately enough or economically.

Measuring carbon in soils is increasingly important world-wide due to its potential conversion to the greenhouse gas, carbon dioxide.

Taking account of the net greenhouse gas emissions from land use will help us better understand and manage global climate change.

In Australia, a National Carbon Accounting System (NCAS) tracks greenhouse gas sources and sinks from the land.

Land-based sources and sinks are of key interest to Australia, forming around 30 per cent of the national emissions profile from activities such as land clearing, cropping, grazing and forestry.

The NCAS is used to:

  • determine Australia's land-based sources and sinks
  • track progress towards national emissions targets
  • inform policies and programs in vegetation and land management.

The new technique

Until recently, measuring the amount of carbon within these different fractions, as required by the NCAS model, could not be determined accurately and cheaply enough. 

A new rapid and inexpensive analytical method to determine the allocation of soil carbon to these fractions was therefore needed for practical use to provide information to the NCAS model.

Using mid-infrared (MIR) spectroscopy, the CSIRO team has been able to generate a spectrum of any soil similar to a 'fingerprint'. Such spectra contain a picture of all the various minerals and organic carbon fractions in the soil.

When this 'fingerprint' is combined with previous measurements of carbon fractions across a range of soil types and analysed using a complex mathematical process, the amount of carbon and its allocation to carbon fractions can be predicted easily for additional soils.

CSIRO’s soil organic matter research team – including Mr Jan Skjemstad, Ms Janine Taylor, Dr Les Janik and colleagues – have developed a suite of analytical techniques to allocate soil carbon to each fraction. However, this process requires the use of a number of complex and expensive laboratory techniques. 

The development of an MIR based method now makes these measurements both rapid and cheap.   

How the technique will be used

The team describes the MIR technique in a paper recently published in the Australian Journal of Soil Research.

To use MIR spectra for carbon modelling, spectra from a large test set of calibration soil samples are collected and then combined with previously determined allocations of carbon to the soil fractions for each of the calibration soils. 

Getting the laboratory data for calibration is very expensive but once it has been obtained only needs to be used once in the MIR prediction system.

The combined data is then analysed using a complex mathematical process called 'partial least-squares (PLS) analysis'.

The model from this process can then be used to easily predict the amount of carbon in its various forms for unknown soils.

This approach allows the NCAS system to rapidly predict the carbon status of any region in Australia and assess the role of soil carbon in budgets of global carbon so important to our strategies to manage climate change and has application throughout the world.

Find out more about our work in Understanding Climate Change.

  • Janik LJ, Skjemstad JO, Shepherd KD, Spouncer LR. 2007. The Prediction of Soil Carbon Fractions Using Mid-Infrared-Partial Least Square Analysis. In: Australian Journal of Soil Research. 45(2): 73-81.