Des fonctions au concept global de sécurité des sols

Soil performs many functions: supporting biomass production, supporting the landscape, providing raw materials, recording the past, filtering and purifying water, regulating water and carbon and nitrogen cycles, and serving as a reservoir of biodiversity.

• Soil provides the physical and nutritional support for most plants. It thus ensures food security for populations through agricultural biomass production. It also allows forest biomass production through forestry. Soils supply energy and mineral raw materials. According to INSEE, extraction of materials from French soil represented 637 million tonnes (Mt) in 2014. It mainly groups biomass and construction minerals, with the extraction of metallic ores and fossil fuels remaining marginal. • With the 2008 recession, reduced construction sector activity lowered non-metallic mineral extraction: 349 Mt in 2014 cover 90% of needs. If in 2014 biomass produced in France (287 Mt) meets 83% of needs, national fossil fuels (1.1 Mt) cover barely 1%.

Soils naturally participate in water filtration and purification. They also receive sludge from wastewater treatment plants. In 2012, 42% of sludge (415 thousand tonnes of dry matter) produced in France was spread over 2 to 3% of agricultural land. Soils also participate in major biogeochemical cycles of water, carbon, and nitrogen.

On a global scale, they store 2 to 3 times more carbon than the atmosphere and 3 to 7 times more than vegetation in the first meter. They constitute an immense biodiversity reservoir: their microorganisms are considered the most diverse and abundant living beings on Earth. Finally, soils preserve geological heritage, archaeological records, and information linked to paleoenvironments and climate evolution. In 2015, 72 nature reserves, including a quarter specifically geological, preserve remarkable heritage: outcrops, rocks, minerals, fossils, formations, or structures.

Human pressures on Earth have reached a tipping point beyond which abrupt global environmental changes cannot be excluded. Nine planetary boundaries have been identified that humanity must not exceed to develop in a safe ecosystem: climate change, biodiversity erosion, disruption of biogeochemical cycles (nitrogen, phosphorus), ocean acidification, land use change, stratospheric ozone depletion, global freshwater use, atmospheric aerosol emissions, and introduction of novel entities into the biosphere.

The control variables defined to estimate critical thresholds for these boundaries do not integrate the soil dimension. However, soils are at the heart of major environmental challenges (availability of quality water, biodiversity preservation, food security, climate change mitigation, etc.). Like aquatic and atmospheric environments and nature, they undergo degradation clearly identified in the nine planetary boundaries. Key figures on soils in France illustrate how they are involved in eight of the nine identified planetary boundaries.

Note: in orange appear the four limits already exceeded in 2015: climate change, biodiversity erosion, disruption of the nitrogen biogeochemical cycle (Rockström et al., 2009), land use change (Steffen et al., 2015). Transgression of one or more limits creates the risk of crossing thresholds beyond which abrupt changes may catastrophically affect the environment.

Ultimately, all knowledge acquired on French soils over thirty years allows us to address the five dimensions of the global soil security concept: capacity, condition, capital, connectivity, codification. These five dimensions further distinguish their optimal state, current state, and their use.

The "capacity" of soil answers the question "What can this soil do?". GIS Sol monitoring programs inform the "condition" of a soil, in other words, "Can this soil continue to do this?". A few projects on carbon stocks have addressed the economic assessment of soils (Gessol, EFESE) or their "capital". Many educational materials for varied audiences (informed, decision-makers, non-specialists) illustrate "connectivity" between soils and their users. Finally, some tools supporting territorial planning policies, soil decontamination, or greenhouse gas emission projection estimates illustrate "codification".