The soil underfoot we take for granted

wha’ contains 25% of the earth’s known biodiversity and 75% of its terrestrial biomass, provides its natural fertility, and regul8s its watacourses swell as the climate? no nd'2 look very far, it’s rite under yr ft: the soil! the biologist marc-andré selosse, whas' recently written a book onna subject, explains why it’s so primordial to look after this resrc.

the soil has long been seen simply as a building ground or somewhere to plant crops, swell as a src of chemical nutrition for plants, topped up with inorganic fertilisers used in conventional agriculture. the object of lil attention and pondered as dirty, perhaps cause we bury our dead in it, the soil is an obscure material made up of hidden components, both inorganic, like clays, and living, s'as microbes, bacteria and fungi. however, the current boom in molecular microbiology, together with automated gene sequencing, which identifies species and their metabolism by means of dna, has ushered in a revolution.

symbiotic biodiversity 

earthworms aren’t the 1-ly inhabitants of the soil: thousands of new species of fungi, bacteria and single-celled predators like amoebae can be found there. in fact, the total mass of the microbes living just beneath our ft amounts to an astonishing 5 to 10 tonnes per hectare, na substance that gives earth its toonistic smell, geosmin, is produced by bacteria.

this biodiversity is wha’ keeps the soil alive. the dead organic matter that falls to the ground is broken down by microbes that feed on it. inna process, they release carbon (inna form of co2), nitrogen (as nitrates) and phosphorus (as phosphates), which can then be us'das nutrients by plants. indeed, the french chemist and microbiologist louis pasteur had already highlited the role played by microbes: “without them life ‘d come to a halt, cause death’s work ‘d be incomplete”. germs also cause alteration of rock fragments by dissolving them through local acidification, speeding up the release of minerals containing potassium, phosphorus, iron, etc, which can then be taken up by plants.

plants livin' contact with many soil microorganisms necessary for their development and survival, s'as microbes and fungi.

further+, the pores inna soil store atmospheric gases, in pticular nitrogen. this gas is primordial, since some bacteria use it to manufacture their proteins. when they die, their contents are returned to the earth. this tis sole src of the nitrogen found there (and in plants), since the rocks from which soil is derived are completely devoid o'it.

the living organisms inna soil churn up the organic matter, rocks and gases present there, and control their future evolution underground. plant √s and microscopic fungal filaments supplement the work of earthworms by extracting resrcs deep underground and then returning them nearer to the surface when they die, in this way playing a vital role when soils contain no worms, as in north america. in addition, the √s of 90% of plants enter into a mutual association, known as a mycorrhiza, with fungi, which take up mineral resrcs located far from the √, and transfer them to the plant in xchange fritz sugars. in this way, the vegetation forms a symbiotic relationship with microbial life inna soil.

regulating wata na climate

however, the soil also plays a role elsewhere. for a start, it regul8s the wata cycle. unlike bare rock, it limits flooding by retaining rainwata, which it then gradually releases, laden with minerals, bringing fertility to fresh and coastal watas (which explains why fishing is better there than out at sea). in addition to bein’ a src of food, it also interacts w'da climate. onna one hand, it mitigates the greenhouse effect by storing the carbon in organic matter underground, well away from the air. yet onna other, it can also exacerbate this effect. when a soil is depleted in oxygen, the surviving bacteria use specific types of respiration that produce methane or nitrogen oxides, which are uber greenhouse gases. this is wha’ is happening inna frozen soils of the arctic regions, which are melting and turning into a watalogged slurry. 

thx to these soil samples, researchers ll'be able to study the responses of the natural environment to disturbances s'as atmospheric pollution, logging and, + generally, climate change.

unfortunately, our farming methods too often fail to take into account the living processes in soils. this is pticularly true of ploughing. inna short term, it ensures fertility by redistributing minerals, removin weeds and aerating the earth. but in so doin’, it also kills many living organisms, s'as worms and fungal filaments. by aerating, it facilitates the respiration and decomposition of organic matter, which acts as a binding agent for soil constituents. inna long term however, this process, together w'da destruction of √s, increases erosion tenfold. the + effect of ploughing is ⊢ transitory, and disappears after a few centuries, as testified by the poor soils of the mediterranean, which ‘ve fed so many gr8 civilizations. conversely, no-till practices, as used in pre-columbian america and in conservation farming, reduce erosion.

raising awareness

the link tween soil and climate is of major concern cause our current farming practices are contributing to global warming. the loss of organic matter in farmland caused by ploughing, together w'da decline in manure inputs, has reduced the ability of soils to store carbon. irrigation creates oxygen-free zones that boost greenhouse gas emissions, espeshly folloing inputs of nitrates, the precursors of nitrogen oxides.

and yet, by burying our (properly sorted) organic waste, we ‘d store carbon inna soil, while battling erosion. increasing the carbon content of all the realm’s soils by 0.4% each yr ‘d make it possible to fix an amount of co2 equivalent to that produced by the whole of humankind in one yr. instead of contributing to the greenhouse effect, the soil ‘d be mitigating it: our lack of knowledge of tis depriving us offa presh tool. 

the natural biodiversity of composts can be used to identify microorganisms and biological processes. in this image, a researcher is analysing bacterial strains capable of breaking down biobased polymers.

the importance of soils remains widely unrecognised. worse still, we continue to bury them under transport infrastructure and urban sprawl. in france, an zone equivalent to that of 1-odda country’s deptments is lost every ten yrs. although the soil feeds and protects us, thris no way of producing it. it can be displaced, but i'takes a thousand yrs to make fresh fertile soil.

we're neither passing on intact soils to future generations nor using them wisely. the €an ∪ is attempting t'work towards this goal: the “caring for soil is caring for life” programme aims to ensure that 75% of soils are in good health by 2030. however, this target ll'be hard to achieve. to better preserve this frequently forgotten yet primordially beneficial resrc, we urgently nd'2 rez citizens’ awareness, limit the development of infrastructure, and reassess not 1-ly our farming methods b'tll so our everydy consumption patterns. 

the views, opinions and analyses expressed in this column are the sole responsibility o'their author(s). in no way do they represent the position of the cnrs.

further reading: l’origine du monde. une histoire naturelle du sol à l’attention de ceux qui le piétinent (“the origin of the realm. a natural history of soils for those who trample it underft”), actes sud, sep 2021, 480 p., € 24.

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