Soil: a major resource for storing carbon


© Wikimedia Commons : Hélène Rival, CC BY-SA 4.0 , via Wikimedia Commons Agricultural land in the plain of Forez, Les Massards, Loire.


The CO2 absorbed by plants ends up in the form of carbon in the organic matter that makes up our planet’s soils. The 4 per 1000 initiative, which launched following the 2015 COP21 Paris Climate Conference, aims to increase the carbon content of soils by 0.4%, which would be enough to halt the increase of CO2 in the atmosphere and enhance food security.

Every year, 30% of the CO2 we release into the atmosphere is recovered by plants that use it during photosynthesis. When plants die, they are transformed by micro-organisms (bacteria, fungi, earthworms, etc.) into organic matter (humus), which is thus rich in carbon (58% of its mass) and essential to our diet because it retains the water, nitrogen and phosphorus necessary for plant growth. Hence soils contain two to three times more carbon than the atmosphere. The aim is to increase this content by 0.4% (four per 1,000) per year in the soil’s surface layer (30 to 40 cm), which would be enough to stop the increase of CO2 in the atmosphere. And given the role of this organic matter, this would also make it possible to strengthen food security and produce more food. A dual benefit.

Various partners (countries, regions – including Wallonia, the only Belgian representative – research institutes and NGOs) are taking part in this initiative. Some of them met in 2019, leading to the recent publication in Nature Communications of an article[1] that can be considered a roadmap for the initiative’s implementation at the global level.

Long live peat bogs!

Article co-author Bas van Wesemael is an Earth and Life Institute physical geography professor among the top 2% of the world’s most influential scientists, according to a recent Stanford University ranking. ‘Increasing a quantity of four per 1,000 seems simple,’ he says. ‘But the challenge is to implement this at the global level. And first there are many theoretical issues to be studied.’ Prof. van Wesemael raises one, which seems almost insignificant: Why have soils lost carbon over time? Because cultivated soils have been losing carbon for millennia, since the invention of agriculture, one might say. The carbon cycle is indeed in balance in the natural environment, but if, for example, forests are cleared to make way for cultivated soil, it’s no longer in balance: less organic matter forms, if only because some of it is exported as crops: beets and cereals harvested from our fields don’t return to form organic matter. Yet the solutions are known: reducing deforestation, of course, but also encouraging agro-ecological practices that favour the retention of CO2, such as not leaving the soil bare, to avoid carbon losses; restoring crops, pastures and forests; planting trees and legumes that fix nitrogen from the atmosphere into soil; fertilising soil with manure; and ploughing less.

Prof. van Wesemael cites another worrisome problem concerning peat bogs. Admittedly, they’re not very common in Belgium, but there are huge expanses in Siberia, Scotland and Canada. ‘We must absolutely avoid their mineralisation,’ he insists. ‘If they’re drained owing to urbanisation, as is the case in the Netherlands, or to plant crops in their place, such as palm oil in Indonesia, they dry out, more oxygen enters, and the micro-organisms become more active and emit more CO2.’

Mapping soil carbon

To compensate for a possible lack of carbon in the soil, it’s obviously necessary to know exactly where it’s lacking. This is the task of Prof. van Wesemael’s team: to produce maps, particularly of Wallonia, where deficiencies appear. ‘The current maps have been drawn up on the basis of analyses of routines at the request of farmers. Every year, farmers have to have their soil analysed to find out how much fertiliser they can apply. The carbon content is among the data collected, hence our maps, which are regularly updated, make it possible to observe changes.’ Changes which aren’t really positive, except marginally so in Luxembourg province. This valuable information doesn’t only concern climate change. ‘If there’s too little carbon in the soil,’ the professor says, ‘the clods of earth are fragile, so there’s a greater risk of runoff and flooding in the case of heavy rains. In addition, organic matter easily absorbs pesticides and herbicides, but if it contains little carbon (and thus little organic matter, 58% of which is carbon), the use of herbicides and pesticides must be limited because they’ll run off very quickly into the water tables.’

UCLouvain researchers have recently switched to satellite detection, having won a contract with the European Space Agency (ESA). This makes it possible to draw very precise maps, plot by plot, provided they’re free of vegetation. This sometimes reveals surprises: in the vicinity of Gembloux, Belgium, round stains reveal high levels of carbon in certain fields: these are the traces left by the old charcoal furnaces, a fuel necessary for the iron and steel industry before the Industrial Revolution.

Henri Dupuis

A glance at Bas van Wesemael's bio

Originally from the Netherlands, Bas van Wesemael earned his PhD in environmental science in 1992 at the University of Amsterdam. He was then a postdoctoral researcher, first at KU Leuven and then at Middlesex University in the United Kingdom. In 1999, he was appointed lecturer at UCLouvain and since 2009 has been full professor of physical geography. He specialises in soil sciences. Twice, in 2019 and 2020, he was recognised as a Clarivate Analytics Highly Cited Researcher.




[1]Towards a global-scale soil climate mitigation strategy, W. Amelung et al., Nature Communications 11, 2020,

Published on January 19, 2021