Soil: the secret weapon in fighting climate change - a consequential review
Soil and carbon
The relationship between soil and carbon is complex. At a basic level, it begins with carbon dioxide (CO2) and water (H2O) being converted by plants into sugar molecules (e.g. CHO) through photosynthesis. This is then stored in vegetation and the carbon is transferred to the soil via dead plant matter. Soil micro-organisms break down the dead organic matter, and carbon is respired back to the atmosphere as carbon dioxide or methane (depending on the availability of oxygen in the soil). Some carbon compounds are easily digested and respired by the microbes, so have a relatively short ‘residence time’. Others compounds are stored in living vegetation and soil organic matter for a longer period. This results in soil providing a critical carbon sequestration service. It is estimated that 2,200 gigatonnes are stored in soils around the world.
Soil and carbon... and climate change
Therefore, increasing levels of carbon dioxide (CO2) and the associated rising temperatures will stimulate photosynthetic activity*... So surely the biosphere is able to help mitigate climate change?
Actually no, it is more complex. There is a dynamic system between the above-ground processes (e.g. plant photosynthesis) and below-ground processes (e.g. decomposition by fungi). This can be illustrated by the following three 'consequential feedback loops'**…
Consequential feedback loop, example one. There are 400 gigatonnes of carbon stored in permafrost regions*** around the world, where the carbon is in a frozen state and hence protected from microbial decomposition. In this positive feedback loop:
Climate change causes an increase in summer air temperatures which increases soil temperatures
The permafrost regions begin to melt and the stored carbon becomes metabolised (organic processes become active)
These microbial activities:
Generate heat which results in more melting
Convert the stored carbon into carbon dioxide and methane, accentuating climate change and increasing summer temperatures.
Consequential feedback loop, example two. In grassland soils, we can consider the relationship between ‘labile soil carbon’ i.e. soil that has a relatively rapidly turn-over (less than 5 years) and ‘inert or stable soil carbon’, largely unavailable to micro-organisms. In this positive feedback loop:
Increasing CO2 concentrations enhance the volume of labile carbon and altered rainfall patterns result in plant roots extending into deep soil layers
Together, these result in enhanced microbial activity (including at lower soil levels) causing the decomposition of previously stable soil carbon where microbes were previously not active
The release of this ‘stable’ soil carbon, in turn, intensifies climate change.
Consequential feedback loop, example three. Now, consider the release of carbon, climate change and wind erosion. In this positive feedback loop:
With more droughts and more variable climate, soils receive less rain. This results in less vegetation growth, which decreases nature’s ‘buffer' to the wind
In parallel, lower soil moisture decreases the ability of soil particles to bind together into larger, heavier aggregates, making the soil more vulnerable to wind erosion
Changing climate conditions result in increased wind speeds, which exert more force on the ground, and more wind erosion. Doubling the wind speed increases the erosion rate of soil by eight times
Increased wind speeds lead to more vegetation destruction (e.g. trees knocked down) making previously 'locked-in carbon' subject to decay and release of CO2.
Left unaddressed, climate change will further degrade land, releasing greater volumes of soil carbon and further exacerbating climate change. For instance, the global loss of soil organic carbon since 1850 is estimated at about 66 gigatonnes.
However, with responsible land-management, soil can provide a critical sequestration service**** and help mitigate climate change. The FAO has estimated that carbon stored in soils could be increased by 30–50 tonnes per hectare. One of the most effective ways to do this is through responsible soil stewardship and agriculture practices, therefore further investment is needed to protect our soils for our food and for the climate.
* For further details on the impact of elevated CO2 concentrations and increased temperatures See Reich, P. B., Sendall, K. M., Rice, K., Rich, R. L., Stefanski, A., Hobbie, S. E., & Montgomery, R. A. (2015). Geographic range predicts photosynthetic and growth response to warming in co-occurring tree species. Nature Climate Change. Chicago
** A consequential feedback loop refers to a situation where part of the output of a situation is used for new input. In climate change, a feedback loop is the equivalent of a vicious or virtuous circle; something that accelerates or decelerates a warming trend. A positive feedback accelerates a temperature rise, whereas a negative feedback decelerates it.
*** Permafrost. In geology, permafrost or cryotic soil is soil at or below the freezing point of water 0 °C (32 °F) for two or more years. Most permafrost is located in high latitudes (in and around the Arctic and Antarctic regions).
**** Carbon sequestration describes long-term storage of carbon dioxide or other forms of carbon to either mitigate or defer global climate change.
Heimann and Reichstein, 2008
FAO (2015) State of our soils