Seeing the soil carbon under the trees

By Dr Mila Bristow and Dr Tim Smith

New research funded through the Carbon Farming Initiative (CFI) will determine the feasibility of carbon farming through reforestation. 

The four year project will compare the amount of soil carbon found in  planted forests versus adjoining agricultural or native forest systems. The planted forest types include dry sclerophyll species (eucalypt and Corymbia species), rainforest species, exotic pine, and exotic and native savanna species. 

Senior Scientist Dr Tim Smith with the Queensland Department of Agriculture, Fisheries and Forestry is leading the project in partnership with scientists from Queensland Government, University of Queensland, University of the Sunshine Coast and Charles Darwin University. The team will also be working with the plantation forest industry and pastoralists in Queensland and the Northern Territory.

“For farmers to participate in the carbon economy we need more accurate predictions of the costs and benefits associated with carbon,” Dr Smith said.

“This project will measure how soil carbon accumulates under trees. What drives this accumulation? How long it takes, and how it can be managed?”

Returning to a pine plantation he measured almost twelve years ago, Dr Smith and his colleagues will re-sample the soil carbon stored under these plantations. 

“We have a unique opportunity to study changes in soil organic matter and other soil properties under plantations over time,” Dr Smith said.

Combining the original methodology with new techniques, more soil types and more forest types, the team will study changes in the soil carbon pools over time and between land uses.

 Why do we care about soil carbon?

Soil carbon and organic matter sustain many plant and microorganism systems through the retention of nutrients, maintenance of soil structure and supporting root health.  Increasing soil carbon storage is a priority area under the CFI. Its viability as a greenhouse gas abatement technology for Australia’s agricultural sector has been recommended.

How much carbon is and can be stored, how to increase soil carbon and how stable soil storage will be, are all currently poorly understood.

 According to Lawson et al., Analysing the economic potential of forestry for carbon sequestration under alternative carbon price paths, at a carbon price of $29.10 CO2-e, it was estimated that approximately 25 million hectares of land would become economically suitable for reforestation, 40 % of which would be in Queensland. Estimates indicated this area of land would store approximately 623 million tonnes of CO2-e over the period 2007–2050 (Lawson et al., 2008). However, there is debate over the biophysical potential for reforestation and the degree of reforestation for carbon storage on agricultural land is likely to be strongly influenced by economic and social considerations. Balancing the biophysical potential and economic potential for reforestation is an important step to ensure participation in the CFI.

How does carbon get into the soil?

Plants take CO2 from atmosphere and store it in their biomass.  As plants grow and die they shed bark, twigs, leaves and roots.  This litter, deadwood and root material breaks down and some of the carbon is stored in the soil.  This stored carbon is referred to as soil carbon pools.

Soils store about 75% of the carbon on land. Changes in soil carbon occur slowly and the stability of the soil carbon pools depends on environmental conditions and land management practices. Deforestation, forest degradation, land-use change (e.g. conversion of forest to agriculture, or conversion of dryland farming to irrigated farming), fire, soil cultivation, fertilisation, turnover and manipulation, temperature and moisture, soil type and plant species all influence soil carbon pools. 

For more information, contact Dr Tim Smith, DAFF, Tim.Smith@daff.qld.gov.au, or Dr Mila Bristow, CDU, Mila.Bristow@cdu.edu.au.