The Benefits of Soil Carbon

By Bryan Walker 15/02/2012

The UN Environment Programme’s just released Year Book 2012 includes a report The Benefits of Soil Carbon which looks at the vital role played by soil carbon in regulating climate, water supplies and biodiversity, and maintaining the ecosystem services that we depend on. The report is 15 pages long and well presented for general reading. I thought it well worth drawing attention to. I’ll point to a few of the report’s highlights here, in particular those that relate to climate change.

First, it offers a reminder of how important a carbon storehouse the soil is.  The top metre of the world’s soils stores more than three times the amount of carbon held in the atmosphere (approximately 2200 billion tonnes of carbon, two-thirds of it in the form of organic matter). This sequestration gives soils an essential role in climate regulation. However, soils are vulnerable to carbon losses through degradation. They also release greenhouse gases to the atmosphere as a result of accelerated decomposition due to land use change or unsustainable land management practices.

This is not a theoretical problem. With frequent reference to scientific papers the report says that since the 19th century, around 60 per cent of the carbon in the world’s soils and vegetation has been lost owing to land use. In the past 25 years, one-quarter of the global land area has suffered a decline in productivity and in the ability to provide ecosystem services because of soil carbon losses. Soil erosion associated with conventional agricultural practices can occur at rates up to 100 times greater than the rate at which natural soil formation takes place. Peatland drainage worldwide is causing carbon-rich peat to disappear at a rate 20 times greater than the rate at which the peat accumulated.

Climate change itself is expected to have significant impacts on soil dynamics. Among them the report points to the concern of experts that if permafrost thaws, enormous amounts of carbon might be released into the air, greatly intensifying global warming. Although the magnitude of this effect remains highly uncertain, recent estimates of frozen soil carbon are huge. According to some scientists, some 18.8 million km2 of northern soils hold about 1700 billion tonnes of organic carbon.

In discussing the intensification of land use the report notes among other factors that modern industrialized crop production relying on monocultures of highly efficient cash crops generally results in a negative carbon budget, especially when crop residues are not returned to the soil.

The report touches on a number of practices which have a deleterious effect on the carbon content of soils, but its main thrust is towards strategies to maintain and increase carbon stocks in soils. It covers three major land use systems, grasslands, croplands and forests. The press release outlines some of the key findings:

The Year Book encourages the development of universally agreed and reproducible field and laboratory methods for measuring, reporting and verifying changes of soil carbon over time.

Carbon stocks can be enhanced by ensuring that carbon inputs to the soil are greater than carbon losses. For example:

  • Forests have considerable potential for reducing greenhouse gas emission to the atmosphere by storing large stocks of carbon both above and below ground.
  • Improvements of grasslands offer a global greenhouse gas mitigation potential of 810 Mt of CO2 up to 2030, almost all of which would be sequestered in the soil.
  • In croplands, integration of several crops in a field at the same time can increase organic material, soil biodiversity and soil health, as well as increase food production, particularly for subsistence farmers.
  • Paludiculture is an innovative alternative to conventional peatland agriculture. It involves biomass cultivation on wet and rewetted peatlands, which can contribute to climate change mitigation by reducing emissions through rewetting of drained peatland soils, and by replacing fossil resources with renewable biomass alternatives.

Across the globe, there are examples of how multiple benefits can be delivered through effective management of soil carbon:

In Kenya, the World Bank’s BioCarbon Fund is providing the Kenya Agricultural Carbon Project with US $350,000 to pay smallholder farmers to improve their agricultural practices, to increase both food security and soil carbon sequestration.

From Dakar to Djibouti, the Great Green Wall initiative is a massive afforestation project to create a 15 km wide strip of trees and other vegetation along a 7000 km transect to improve carbon sequestration, stabilize soils and conserve soil moisture amongst others.

In China, similar approaches are being monitored to assess whether land degradation in arid areas can be reversed.

In Brazil, changes in crop production practices have been found to have significant effects on soil carbon stocks. Conversion to no-till techniques in soybean, maize and related crop rotation systems resulted in a sequestration rate of 0.41 tonnes soil organic carbon per hectare per year.

In Argentina, significant increases in soil carbon stocks have also been achieved, where farmers changed to no-till systems, along with enhanced benefits in water retention, infiltration and erosion prevention.

There’s no startlingly new information in the report, but it’s a useful document which hopefully will play a part in informing those who work on government and global policy matters. While there’s more than the mitigation of climate change at stake in the maintenance of the world’s soils, it is obviously an issue of enormous importance for its potential to either moderate the level of atmospheric carbon or increase it. The most alarming possibility of thawing permafrost is perhaps bracketed in a special category of its own as a consequence of global warming. But there’s more than sufficient at stake in the ways we regularly manage our soils to warrant the close attention the report calls for.