par Emma Nicolas (EnvIM 2018)
When thinking about planet Earth, the first adjective that comes to one’s mind is usually the “blue planet”. Indeed, water composes no less than 75% of the Earth’s total surface (Fig.1). But what about the ground? If water is absolutely necessary to life (and actually complex life came out from water), soils shelter all terrestrial species, including humans. Not only have they been giving us places to settle since we exist, but soils have also been providing us food, and more than 90% of our nutritional needs are covered by vegetal products or animals fed by soils, directly or indirectly. Nowadays, only one species, ours, appropriates itself 25% of what the continental biosphere could potentially produce entirely. What consequences has this anthropogenic monopolization on the soils? One of them is an increased erosion that can lead to dramatic issues, and this soil erosion is directly linked with climate change.
The power of soils
Supporting the development of vegetation, animals, micro-organisms, fungi, and other species, soils are also essential for organic matter decomposition, and water cycle regulation (by infiltration, filtration, runoff, storage, …). Furthermore, they play an important role in depollution, by immobilizing and degrading pollutants through the action of micro-organisms, and they also are crucial in fighting climate change. Indeed, carbon sequestrated by plants during photosynthesis is stored in soils, and thus they are fundamental for carbon cycle (Fig.2). Finally, who would have thought that one major antibiotic of the 20th century would have been elaborated from a soil fungus? Indeed, soils are reservoirs for potential future medicines and biocatalysts, and studies are dedicated to soil’s biodiversity with this aim in view. Soils hold a lot of treasures resulting from their composition… yet, what exactly is a soil?
According to Lozet and Mathieu, in the “Dictionnaire de Science du Sol” (2002), a soil is a “product of alteration and re-organization of the upper layers of crust, through the action of life, atmosphere, and through the exchanges of energy and of organic and inorganic matters”.
A soil is a stack of horizontal (more or less) layers that have precise biological, chemical and physical properties. Those layers are referred to as soil “horizons”. From the bedrock to the surface, all the horizons of a soil constitute its profile (Fig.3).
More specifically, a soil is constituted of particles of various size and form, that can either be spaced out by pores or aggregated together. Its components are organic and/or inorganic. The first ones come from products of animal origin and vegetal products accumulated at the soil’s surface. The second ones are derived from decomposing organic matter and correspond to the production of nitrates, sulfates, carbonic gases, phosphates and other mineral compounds. Decomposition occurs through biologic pathways: micro-organisms (i.e. bacteria and fungi) digest cellulose composing organic matter. Minerals are also present in soils. A distinction can be made between primary and secondary minerals. Primary minerals are very resistant (i.e. almost unmodified) and usually originate from the parent rock, whereas secondary minerals are produced in the soil through the modification of existing minerals or through synthesis after complete destruction of primary minerals.
One major element of soils’ composition is humus. It originates from the accumulation of a black residue that resists decomposition. Closely linked to climatic and drainage conditions, to parent rock and plants nature, but also to mineral compounds of a soil, humus has specific properties. Being the major site of microbial life in soil, humus reflects a soil fertility through its physical properties on soil structure or water retention, but also because it is a nitrogen, sulfur and phosphorus sink. Humus is a buffer zone that is essential for almost all plants nutritional metabolisms and is a considerable source of growth substances. Not only humus is the “food-pantry” of soils, but it also is essential for their structuration. A well-equilibrated humus confers a better restructuration capacity to soils, after agricultural works for instance, as well as a better natural resistance to compaction. It also stores water and gives it back to plants when needed, and lets the air enter the soils. Humus is indispensable to soils, hence it is crucial to preserve this life-layer to protect them.
How soils play a role in climatic change
If well managed, soils can help to mitigate climate change. Indeed, one step of the carbon cycle is the soil: atmospheric carbon is captured by plants during photosynthesis and is then stored in soil through micro-organisms during ten to a hundred years. Soils contain two to three times more carbon than the atmosphere, and small variations of this carbon sink can thus either worsen or counterbalance climate change. Two solutions for the soils to store more carbon would be either preventing them from losing its carbon by avoiding erosion for instance, or making carbon enter in soils. This is what the “4 pour 1000” initiative, launched by France in 2015 during COP21, suggests. According to this international initiative, increasing carbon stock of 0.4% each year in the 40 first centimeters of soil could stop the increase of CO2 in the atmosphere. With the aims of increasing soils’ adaptation to climatic changes, limiting global temperature increase, as well as contributing to the food safety goals, this volunteer initiative is intended to be a complement of the essential measures taken for a global and general decrease of greenhouse gas emissions in the overall economy. It is incumbent to each member of the “4 pour 1000” initiative to define the way of participating to it.
How? Two components structure the initiative. The first one is a multi-stakeholder (state and non-state bodies) actions program for a better management of soils’ carbon. On one hand, these actions consist in the local set up of agriculture practices that are favorable to soils restauration, to their organic carbon stock increase, and to the protection of carbon and biodiversity rich soils. On the other hand, this actions program consists in the set up of training and knowledge sharing programs, the financial support of soils’ carbon stock restauration/preservation projects, the draw up of public policies and adapted tools, or the development of soil respectful agriculture products supply chains.
The second component of the “4 pour 1000” initiative is an international research and scientific cooperation program concerning the link between carbon stored in soils and food safety. It is structured around four interrelated scientific issues. The first one is the evaluation of righteous agricultural practices and their consequences on CO2 sequestration. The second set of scientific studies is the assessment of carbon storage potential in soils according to the different regions and systems and the study of the related mechanisms. Finally, the two last issues addressed in this program are, on one hand, the support of innovations and their stimulation through adequate politics, and on the other hand, the follow-up of soils’ carbon stock variations and their estimates, especially for agricultural purposes.
Diminishing deforestation and encouraging agroecological practices would be the keys to make it happen. Indeed, as naked soils are more prone to carbon losses, ensuring a permanent vegetal cover would avoid these carbon debts. Restoring degraded forests and pastures, collecting water at the bottom of plants, and feed soils with manure and compost could also be solutions in this view.
Today soils of all the world are endangered by different factors linked to humans’ activities, such as pollution, or changes of soil use. A natural process, erosion, also threats soils, and one environmental issue is that this process is accelerated by human activities.
What is erosion?
Even if they both are natural phenomena, and both are necessary to sedimentation, two kinds of erosion are to be distinguished. The first one is water erosion and is essentially linked to rainfalls. In this case, soils are degraded by the movements of its materials.
According to the French ministry of Ecology, Sustainable development and Energy, in 2015, water erosion in France was estimated to 1.5 tons per hectare and per year (t/h/y). However, this number varies according to the different types of soil existing in France. For instance, arable lands in northern France are particularly vulnerable because of the lack of vegetal cover during some periods of the year. Hence, a strong spatial heterogeneity concerning water erosion vulnerability exists in France, and Bretagne, Picardie, Nord-Pas-de-Calais and Haute-Normandie present risks of losing soils by water erosion that reach more than 5 t/h/y on more than 10% of their territories. Those soil losses are taken seriously as they are irreversible and as water erosion can generate mudslides that have a lot of consequences on humans’ activities, such as public and private goods’ degradation or accidents. But mudslides also have terrible consequences on the environment. Indeed, mudslides not only enhance rivers’ pollution, as well as groundwater pollution in karstic areas, but they can also convey dissolved or suspended phytosanitary products and lead to drinkable water shortages. Moreover, hydric erosion leads to an enhanced sedimentation and in these conditions, sediments reaching the rivers can accelerate the banks erosion, fill in wetlands and rivers, and thus damage water quality, as well as biodiversity.
Soils can also be naturally degraded by wind erosion. This kind of erosion is triggered by specific wind intensities and soils characteristics, as well as an absence of protecting vegetal cover. In the same way as water erosion, some soils are more vulnerable to wind erosion than others. In France for instance, more than a quarter of Languedoc-Roussillon and Provence-Alpes-Côte d’Azur lands are the most prone to wind erosion.
How humans’ activities impact continental erosion?
During Holocene, humans have been impacting soils more and more. Indeed, our activities have generated huge changes of soil use, and two of them have had (and are still having) significant impacts on continental erosion (not to mention greenhouse gases emissions they also trigger): deforestation (Fig.4) and agriculture. Those two are closely related, as agriculture is the main cause of deforestation at a global scale. Increasing pressure on arable lands takes root in an increasing food demand, together with an exponentially increasing demography. Intensive farming also requires more lands for animals, as well as for producing their food.
When removing forest cover, soils are not protected against meteoric waters and wind anymore. They thus are more mobile, and surface horizons (the most fertile and mobile ones) are more susceptible to be blown off by wind or to be leached out by rain. Then, eroded particles can be transferred to other regions of the world or move to rivers by water run-off. One perfect illustration of deforestation-linked erosion is the picture of the Betsiboka Estuary astronauts took in 2004. After heavy rains following the Gafilo cyclone, the estuary was so red that it looked like it was “bleeding”: the red color was due to the drainage of iron oxide rich eroded sediments. Indeed, between 2010 and 2014, the average annual deforestation rate was 99000 h/year, and 44% of Madagascar forest area have been destroyed between 1953 and 2014 (Vieilledent et al., 2018) (Fig.5).
Solutions exist in order to prevent deforestation from reaching the point of no return. For instance, China has recovered forest thanks to dynamic reforestation campaigns (Clarini, 2018). According to a 2010 United Nations report, boreal and temperate forests are estimated to have recovered more than 25 million hectares of their surface between 1990 and 2010 (in France, forests gained 80000 km2 in 150 years, according to the “Office National des Forêts”), but primary forests are still cut down, and this leads to tremendous biodiversity and carbon stock losses.
Increasing needs for agriculture lead to increasing deforestation that enhances erosion by making soils naked and thus more mobile. Today, it is estimated that an average of 1 kg of soil is lost every kilogram of harvest. This is not the only role agriculture plays in erosion… indeed, agricultural practices may strongly exacerbate water and wind erosions due to intense tillage, numerous agricultural machinery crossings on the soils, or by letting soils naked during part of the year. One solution farmers can implement in order to limit erosion is maintaining a permanent vegetal cover, as well as increasing organic matter content of soils by letting crop residues on soils surface, spreading compost, diminishing tillage, making rotations including plants with high residue contents…
Advocating adapted agricultural methods might be a key to tackle erosion issues, as testified by soil protection policies launched in the United States that allowed erosion rate to decrease from 9 t/ha/y to 6-7 t/ha/y between 1982 and 2002 (Nearing et al., 2017).
Conducted by the Farm Service Agency (FSA) of the United States, the Conservation Reserve Program (CRP) is a land conservation program consisting in several initiatives, among which the “Highly Erodible Land Initiative” aims at preventing soil erosion. In the long-term, the objective of the CRP is re-establishing valuable land cover in order to support a decreased soil erosion, an increased water quality, and a recover of wildlife habitat. Therefore, farmers can enroll in the program in exchange of a yearly rental payment. They then concede to stop agricultural production on environmentally sensitive lands, and to plant species that will enhance environmental health and quality of those lands. For instance, six practices are affiliated to the “Highly Erodible Lands Initiative” giving recommendations on the establishment of permanent introduced grasses, the establishment of permanent native grasses, or on tree planting. Two of theses practices concern the permanent wildlife habitat, as well as rare and declining habitat. These six practices hence provide advices on the kind of seed to plant, the time of the year they should be planted, the type of tillage, or the kind of herbicide and the way to use it.
Fixing soils for a better future
If more initiatives like the “4 pour 1000” initiative aiming at adapting our soils in order to tackle climate change are not conducted by public authorities, erosion will continue to increase. Promoting soil preservation through communication campaigns, sensitizing farmers to alternative solutions such as reduction of tillage, rotation of cultivated plots, inter-seeding, giving them tools to operate a soil-friendly transition appear to be solutions to stem soil erosion linked to overcultivated soil. For those measures to be well accepted and better handled, financial assistance to farmers through the transition phase is an important point to mention. These initiatives can be taken locally, but also nationally.
By the way, how is soil erosion and degradation handled in France and Europe?
In France, there is no specific regulation that directly concerns soil degradation and erosion. Hence, in order to cope this lack of specific regulation, the fight against erosion rests on the application of different legislations dealing with subjects that are indirectly linked with soil degradation and erosion. At the collective scale, for instance, the SAGE (“Schéma d’Aménagement et de Gestion des Eaux”) sets general objectives at the watershed scale, regarding the use and preservation of water resources, aquatic ecosystems and wetlands. Handling of erosion risks can be part of a SAGE within the aim of reducing superficial water turbidity and eutrophication linked to agricultural pollutions. Furthermore, the state can intervene through two procedures. The first one is the inundation risks prevention plan (“Plan de Prévention des Risques “Inondation” (PPRI)). The PPRI imposes restrictive measures (integrated in the local urban plan) regarding soils’ occupation, hence reducing risks and vulnerability. The second procedure is the initiation of an action program (culture rotation, tillage reduction, …) on erosion zones that the prefect and the Environmental Code delimited, and the non-compliance of such measures leads to contraventions. However, voluntary approaches are favored.
In addition, collective networks of agricultural and non-agricultural actors are put into action in order to develop agroecology, reflect together on common issues, and share knowledges. That is what DeCo Agro-eco, a French national network tries to implement, hence fostering the creativity of the different stakeholders.
More broadly, at the European Union scale, erosion is tackled via the Common Agricultural Policy (CAP). This policy consists in giving financial support to farmers meeting specific environmental conditions. For instance, in order to be financially helped by the CAP, they must ensure an appropriate land management and ensure a minimum soil cover. Within the CAP, agri-environment measures are payments encouraging farmers to get involved in commitments such as managing low-intensity pasture systems, making environmentally favorable extension of farming, conserving high-value habitats and associated biodiversity… Moreover, cross-compliance can be provided: in exchange of direct payments, farmers comply with standards regarding to requirements of preserving lands in good environmental and agricultural conditions. The CAP can also financially support rural development programs in the view of reducing erosion issues.
Last, but not least, coastal erosion, not developed within this article, is a big issue for territory losses. Rise of sea level is one of the main reasons for coastal erosion, but coastal infrastructures, as well as industrial sand extraction are also factors influencing coastal erosion at a global scale: once again, human activities are to blame. According to the IPCC scenarios on the evolution of greenhouse gases concentration in the atmosphere, sea level should become between more than 20 and 140 cm higher than today (Nicholls and Cazenave, 2010).
Facing these threats, European citizens are increasingly aware that soils are a common good essential for life, that they need protection and a more sustainable management. In 2017, the European Commission validated the launching of a European citizens’ initiative, People4soil. Although only 212 000 votes have been obtained out of the million required, and even if there still is no directive for soil protection in Europe, this initiative represented a (late!) start and good news for the “old continent” to find effective solutions against soil erosion.
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Vieilledent, G., Grinand, C., Rakotomalala, F.A., Ranaivosoa, R., Rakotoarijaonna, J.R., Allnutt, T.F., Achardal, F., 2018. Combining global free cover loss data with historical national forest-cover maps to look at six decades of deforestation and forest fragmentation in Madagascar. bioRxiv – doi: 10.1101/147827.
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To go further
ADEME, 2010. La vie cachée des sols, plaquette du programme GESSOL. ADEME : 7021 – ISBN : 978-2-11-128035-9.
Lorin, T., Lallemand, F., El-Shafey, A., Darboux, F. (2018, May). Quelques conséquences locales et régionales des changements d’usages des sols liés aux activités humaines. Retrieved on January 2019, from http://planet-terre.ens-lyon.fr/article/erosion-sols.xml#r13
Chambre d’agriculture et de région du Nord-Pas de Calais, 2013. Guide de l’érosion : Lutter contre l’érosion.
Ministère de la Transition écologique et solidaire (2017, April). Quels sont les pouvoirs des sols ? Retrieved on January 2019, from https://www.ecologique-solidaire.gouv.fr/quels-sont-pouvoirs-des-sols