The soil-carbon transition curve as near-death experience
The carbon content of soils is improving in parts of East and Southeast Asia, while the public discourse is mostly about soil and land health degradation. This is similar to the pattern of tree cover or forest transition and the processes are indeed linked. The CGIAR Research Program on Forests, Trees and Agroforestry is built on the forest transition concept and we now have its belowground counterpart, says Meine van Noordwijk
The Scientific Committee on Problems of the Environment (SCOPE), which, since 1969, has been a forum where scientists can interact with the interests of policy makers and others, is now preparing to publish its 73rd volume, building on a topic identified in a chapter in the UNEP yearbook 2012: emerging issues in our global environment, that is, the benefits of soil carbon. SCOPE’s volume will consists of 27 background chapters, which are in the final stage of peer review and editing for release before the end of 2013. In a workshop held in Ispra, Italy, 18–22 March 2013, synthesis chapters were discussed. It was here that the concept of a soil-carbon transition curve emerged (see illustration).
The curve provides a conceptual link between I) a phase of loss of soil carbon that usually follows when land is cleared from its natural vegetation (forest or grassland) and converted to agricultural use; II) a stage of low carbon content where loss of productivity and buffering functions implies a risk of systemic collapse and land abandonment; and, finally, after a near-death experience; III) a stage of recovery where global and local benefits of increasing soil carbon align.
In analyzing the soil-carbon transition curve with a group of nine scientists (Tessa Goverse, Christiano Ballabio, Steve Banwart, Tapas Bhattacharyya, Marty Goldhaber, Nik Nikolaidis, Elke Noellemeyer and Yongchun Zao ) from around the world (working in Europe, Asia, Africa and North and South America), we found that different functions of ecosystem services are negatively affected in the declining phase of soil carbon but not always in the same order and to the same extent because a lot depends on the climate, direct exposure to wind, the sun and intensive rains.
Breakdown of organic matter means a net release of nutrients that are available for crops. Many long-established cropping practices—such as slash-and-burn land clearing, soil tillage (ploughing), liming of acid soils and drainage of wetlands—lead to breakdown of soil organic matter. The positive effect on crops of the breakdown of soil organic matter, however, cannot continue indefinitely. Before rock bottom is reached, so to speak, soil productivity has already declined dramatically and farmers often decide to abandon the land. Abandonment usually leads to stabilization and even a slow recovery when the roots of re-established vegetation do their work of building up soil carbon again.
A key concept is that of ‘buffering’: a soil with a good structure allows rainfall to infiltrate, stores water and allows its gradual release to plants’ roots; it also temporarily binds nutrients that might otherwise be available in excessive quantities and keeps them for future use; a healthy soil also contains many rhizosphere organisms that respond to the presence of plants’ roots, competing for the organic substrates that the roots provide and forming a ‘foodweb’ of consumers, consumers of consumers and so on. The foodweb regulates all its components and generally prevents single groups from emerging as pests and diseases to the plant. Soil’s biological buffering role is key to agricultural production but we cannot claim to fully understand it. Maintaining soil organic matter contents is, however, one of the safest ways to maximize the chances of effective buffering. Good soil structure is assisted by organic matter in its role of stabilizing aggregates and macropores.
Budiman Minasny and colleagues (2011) have compiled evidence for a ‘good news’ story: after decades of decline, soil carbon contents on Java are on the increase; a turning point has been reached. Similar data exist for South Korea and China.
The soil-carbon and tree-cover transition curves also relate to the economic concept of the Kuznets curve, in that environmental degradation initially increases when people emerge from poverty. This degradation creates new forms of poverty and deprivation of essentials for a desirable quality of life and these form a trigger to act and reverse the environmental degradation. The relationship between environmental quality and poverty is thus non-linear and may have thresholds that are critical because they can lead to ‘poverty traps’. For national governments and downstream populations, an early transition is desirable from the initial loss of carbon to sustainable use with stable or increasing soil carbon. To achieve this, policymakers need to understand the underlying reasons for the decisions made by farmers and other land managers.
As authors, we hope the concept of a soil-carbon transition curve will find its place among the demographic, agrarian, tree-cover and environmental transitions that jointly describe the Sustainable Development Goals, because we urgently need indicators of progress that are measurable and for which our governments can be made accountable. Europe has already made a significant step: its payments in support of farming, agriculture and maintenance of rural landscapes are now conditional on an increase in soil carbon. Land health, soil wealth and an escape from rural poverty are indeed closely linked.
Edited by Robert Finlayson
Read more about soil carbon
This work relates to the CGIAR Research Program on Forests, Trees and Agroforestry.