Forrest soils contain the largest terrestrial reserves of carbon on the planet and this carbon is stored there in a more or less sustainable manner 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, these carbon stocks naturally decompose, emitting large amounts of carbon dioxide and methane, two greenhouse gases. At Cemagref, the French Centre for Agricultural and Environmental Engineering Research – www.cemagref.fr in France the aim of an ongoing doctoral thesis conducted in a partnership with ADEME (Agence de l’Environnement et de la Maîtrise de l’Energie) 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.

Under the auspices of the European project IRISE (Impact of Fire Repetition on the
Environment) that researcher Lauric Cécilion has developed the Near-Infrared Spectrometry Soil Carbon Measurement method and this is an important step in dealing with the possible problem of greenhouse gas emissions from the soil. Near-infrared spectroscopy has proved itself to be an essential tool in quantifying the build-up of carbon in the soils at a large scale. But whilst this new instrumentation can measure the amount of carbon in the soil, what can prevent it escaping in the form of greenhouse gasses?

There is a solution to this problem according to Kristine Nichols, a microbiologist at the Agricultural Research Service (ARS) in the USA. A soil constituent known as glomalin provides a secure vault for the world’s soil carbon. Glomalin is a sticky substance secreted by threadlike fungal structures called hyphae that funnel nutrients and water to plant roots. It acts like little globs of chewing gum on strings or strands of plant roots and the fungal hyphae. Into this sticky “string bag” fall the sand, silt and clay particles that make up soil, along with plant debris and other carbon-containing organic matter. The sand, silt and clay stick to the glomalin, starting aggregate formation, a major
step in soil creation.

On the surface of soil aggregates, glomalin forms a lattice-like waxy coating to
keep water from flowing rapidly into the aggregate and washing away everything,
including the carbon. As the builder of the formation “bag” for soil, glomalin
is vital globally to soil building, productivity and sustainability, as well as
to carbon storage.

Nichols uses glomalin measurements to gauge which farming or rangeland practices
work best for storing carbon. Since glomalin levels can reflect how much carbon
each practice is storing, they could be used in conjunction with carbon credit
trading programs.

In studies on cropland, Nichols has found that both tilling and leaving land
idle—as is common in arid regions—lower glomalin levels by destroying living
hyphal fungal networks. The networks need live roots and do better in
undisturbed soil.

When glomalin binds with iron or other heavy metals, it can keep carbon from
decomposing for up to 100 years. Even without heavy metals, glomalin stores
carbon in the inner recesses of soil particles where only slow-acting microbes
live. This carbon in organic matter is also saved, like a slow-release
fertilizer, for later use by plants and hyphae.

Adapted from materials provided by USDA/Agricultural Research Service.

References:
a. Cemagref. www.cemagref.fr
b. USDA/Agricultural Research Service (2008, July 2). Glomalin Is Key To
Locking Up Soil Carbon.