Soil protection has become a priority of the European Union. The loss of organic matter is one of the various threats to the soils that have been identified by the European Commission. Agricultural soils as well as mountain soils are implicated; they are veritable carbon sinks, which, under the impact of climate warming, could become sources greenhouse gas emissions. Near-infrared spectroscopy is a very promising avenue able to quantify the build-up of carbon in the soils at a large scale.
The capacity of plants to store carbon dioxide emitted in the atmosphere via photosynthesis is well known. But did you know that soils are veritable natural carbon sinks? Consequently, forest soils contain the largest terrestrial reserves on the planet.
Carbon is stored there in a more or less sustainable fashion in the form of organic matter: microflora, soil wildlife, roots and plant debris, organic labile residue (sugars, cellulose) and more stable molecules (lignin, tannin, humines). However, in a context of climate warming, these carbon stocks decompose, emitting large amounts of carbon dioxide and methane, two greenhouse gases. At Cemagref, the aim of an ongoing doctoral thesis conducted in a partnership with ADEME is to develop a simple and cost-effective tool to quantify organic carbon storage in soils. Upstream, this work come within the future European Framework Directive on soil protection with one of the priorities being to make a list of the soils at risk in Europe.
A global understanding of the process
In the mountains, the storage of organic carbon is favoured by the temperature and moisture conditions of the environment and by the characteristics of the litter. To study the impacts of these different parameters, Lauric Cécillon’s research was conducted on a cold scree in a mid-altitude mountain area, located in the Southern Alps. A true field laboratory, this ecosystem shows highly contrasted micro-climate soil conditions and plants (pine trees, beech groves and fir forest, and an ecotone zone) simulating the long-term impacts of climate warming over a distance of a few hundred metres.
This researcher first focused on the process controlling the build-up of organic matter in the soil. The decomposition process was studied by experiments with litter bags. The mass losses of these litter bags were measured over a lapse of time. The process of aggregation of the organic matter was monitored by the analysis of thin slices of soil and by the physico-chemical description of the particulate organic matter.
Towards mountain soil mapping
In addition, this researcher has developed a new method for predicting the organic and microbial carbon stocks which favourably replaces the chemical methods of analysis. This method, based on near-infrared spectrometry, was developed on burnt soils, within the European IRISE project, then applied on approximately 1000 samples of mountain soil. In just a few minutes, the amount of organic carbon, total nitrogen, and microbial carbon, the bacterial activities of de-nitrification and potential nitrification, as well as two enzymes of soil degradation can be determined. This quick and reliable tool makes it possible to analyze a large number of samples cheaply, a significant advantage in the highly heterogeneous mountain environments. The aim of the next thesis will be to map carbon storage and soil quality at the scale of a natural preserve area located in the High Plateaux of the Vercors range.
From burnt soils to mountain soils?
It is within the European project IRISE (Impact of Fire Repetition on the Environment) that Lauric Cécilion has developed the near-infrared spectrometry soil carbon measurement method. Like agricultural soils and mountain soils, burnt soils are threatened by a drop in their stock of organic matter. This tool can measure the impact of the repetition of the fires on the soil quality. The researcher has also shown that the spectrometric analysis of earthworm castings makes it possible to select the plots based on how long ago fires occurred. Finally, the technique made it possible to validate the positive action of earthworms on the build-up of organic carbon and nitrogen and the richness of the microbial flora in the soils after burning.
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