Soils: The last frontier

From time to time Science publishes a collected series of articles on one topic. Often these ‘Special Issues’ are around a theme of particular interest to environmentalists. Science, 304, 11 June 2004 carried a special issue on soil – ‘Soils – The Final Frontier’. The Science Website is at http://www.sciencemag.org However, only subscribers can gain access to the full text of articles. Science is available in all major public and university libraries.

It contains a world soil map and articles: Introduction: Ecology of the Underworld Soil and Trouble Wounding Earth’s Fragile Skin Defrosting the Carbon Freezer of the North The Secret Life of Fungi Soil Carbon Sequestration Impacts on Global Climate Change and Food Security Breaking the Sod, Humankind, History and Soil Ecological Linkages Between Above Ground and Below Ground Biota Interactions and Self-Organisation in the Soil-Microbe Complex While all the articles are worth reading it is the one on Soil Carbon Sequestration that I found the most interesting and with considerable relevance to some existing and some emerging practices in Australia. The thrust of the article is that soil contains both organic and inorganic carbon, the former in the form of humus. Soils rich in humus are both more productive and can sequester significant quantities of soil organic carbon (SOC). Thus retaining organic material in the soil can both improve crop yields and help reduce the addition of carbon to the atmosphere where it drives climate change. Due to a range of poor management practices around the world there has been both a loss of CO2 from soils to the atmosphere, and degradation of the productive capacity of soil.

These practices could be changed. ‘The global soil carbon pool of 2,500 gigatonnes (Gt) includes about 1,550 Gt of SOC and 950 Gt of soil inorganic carbon. The soil carbon pool is 3.3 times the size of the atmospheric pool and 4.5 times the size of the biotic pool. The SOC pool to one metre depth ranges from 30 tonnes per ha in arid climates to 800 tonnes per ha in organic soils in cold regions and a predominant range from 50 – 150 tonnes/ha.’ ‘An increase of one tonne of soil carbon pool of degraded cropland soils may increase crop yield by 20 – 40 kg/ha for wheat, 10 – 20 kg for maize and 0.5 – 1 kg for cowpeas’.

As well as enhanced food production returning damaged soils to their SOC capacity has the potential to sequester carbon emissions from fossil fuel burning of 0.4 – 1.2 Gt per year and to achieve this for the next 20 – 50 years until a new equilibrium is reached. This amounts to between 5 and 15% of present emissions. In many parts of the world crop wastes, for example wheat straw, are collected and used as fuel. This deprives the soil of humus, reduces the carbon sequestering capacity of the soil and reduces subsequent crop yields. The article argues that this practice needs to stop as it traps its practitioners into a cycle of declining yields linked to declining soil quality.

However, in Canada there is an active program emerging to use wheat straw as a feedstock for ethanol for transport. This program is being pursued as a source of renewable, greenhouse friendly fuel. It is not at all clear that the carbon balance of such a program would be beneficial. Similar programs have been suggested for Australia. It is clear that before any such program is actively pursued very careful carbon input/output analysis needs to be carried out including the matter of soil carbon retention. Bushfire and burning bushland to reduce the risk of wildfires also need to be considered in the context of soil carbon. Retaining fallen vegetation on the ground would eventually increase humus in the soil, improve soil texture, water holding capacity and productivity as well as sequestering carbon. It may be that eventually the tree canopy would become more dense, the understorey more open and wetter and less subject to wildfires. Some older residents who have lived in the Adelaide Hills all their lives have described the earlier stringy bark forests as similar to this. After reading this article it seemed to me that the role of fire management in the carbon cycle of our forests needs careful (re)consideration.

Notes by John Coulter.