Soil is no quick fix for climate change

As the worst bush fires in a generation subside in New South Wales, the debate rages on about the role that climate change played. Meanwhile, Australian Prime Minister Tony Abbott has introduced bills to repeal the country’s carbon tax – which was designed to control emissions of greenhouse gases – and replace it with a “direct action” policy.

One of the central assumptions of the policy is “carbon farming” – the management of soil (and plants) to hold more carbon and offset a significant fraction of the country’s CO₂ emissions. This reflects a consensus that soil can be managed to mitigate climate change. However, the evidence on which this consensus rests is thin at best.

Ferreting carbon away into soil is a seductive prospect. Soil contains more carbon than the whole atmosphere and all of the world’s plants combined. This carbon finds its way into the soil in organic matter, which is mostly bits of dead plants and animals, and carbon compounds that “leak” out of living roots.

In most ecosystems, there is almost as much life in the form of microbes below ground as there is in the form of plants and animals above ground. This teeming microbial community breaks down organic matter into inorganic nutrients – including nitrogen, sulphur and phosphorus – that can then be taken up by plants. Compared with their natural state, most of the world’s agricultural soils have been depleted of organic matter. Therefore, it is good for soil to put that carbon back.

The reason that carbon is so important to soil is that it is by far the dominant fuel for microbes. Carbon in organic matter provides most of the energy needed by the microbial community to do the job that the rest of Earth’s ecosystem depends on: turning waste into fertilizer. Microbes have evolved to scavenge this energy source whenever it becomes available. It is therefore paradoxical that there is any carbon left in soil at all.

In fact, we really don’t know how soil holds on to its carbon. Yet, everywhere we look, carbon is present in soil with nitrogen at a ratio of approximately 10:1.

The prevailing assumption used in climate models is that some of this carbon is locked up in organic matter in forms that microbes find hard to break down. If that was true, the universal 10:1 ratio is hard to explain, because one would expect more variation depending on the kinds of matter available. In fact, the most recent research shows it is wrong. Microbes have adapted to decompose carbon in all its forms with similar ease.

It turns out that most of the carbon that is retained in soil may already have been processed many times by microbes. The reason it is retained is that it finds itself in a microenvironment in soil where microbes are prevented from using it because, for example, their activity is constrained.

The ratio of air to water in soil is a major constraint on microbial activity. Too much water, or too little, limits activity. The features of soil that hold on to water against the pull of gravity – and so control the ratio of air to water – are pores smaller than about 50 micrometres, or 50 thousandths of a millimetre. We need to optimize these structures to keep more carbon in soil and maintain the other important microbial and soil functions.

We know very little about soil at this level of detail because we have not, until now, thought about soil as a habitat for microbes. We don’t know how or why these tiny structures change with time, but we do know that microbes are involved and so carbon is important. But we also know that soil carbon increases slowly and can be lost quickly, especially if soil structure is disturbed or the fresh supply from plants is cut off by drought.

A recent synthesis of observations of Australian soils suggests that the capacity of soil to store enough carbon to mitigate climate change may be technically and economically limited, and decreases with time. So, soils have a limit to the amount of carbon they can store, and it may not be enough to hold back climate change.

Moreover, its natural potential to lose carbon exceeds its potential to hold it, leaving stored carbon vulnerable to being released into the atmosphere by shocks over which we have no control, such as drought.

None of this means we shouldn’t try to increase soil carbon. We depend on soil for more than 99% of the food we eat, but all of the evidence suggests that, if we continue to farm it as we have done, we will lose most of the world’s topsoil to degradation and erosion within the next few hundred years. Farm yields are already lower than they could be in 40% of the world’s agricultural soils because of this.

Understanding how to “fuel-up” the soil with carbon is essential to restore the microbial habitats in soil that maintain fertility, hold water and build resilience. Understanding the opportunities and limits of this is probably the most important challenge we face in securing the future food supply.

Farming carbon to mitigate climate change is a distraction from the real priority for getting carbon back in soil – that is, securing soil itself.

Read more blogs on sustainability and environment.

Author: John Crawford is the Judith and David Coffey Chair in Sustainability and Complex Systems at the University of Sydney. He writes about environmental issues for the Forum:Blog.