Agronomy_ environment

The Maillard reaction in agronomy and environment

by Eric Lichtfouse

The Maillard reaction is playing a crucial role wherever organic matter is decomposing, e.g. in soils, waters, sediments, sewage sludges, etc. In soils the Maillard reaction lead to the formation of brown polymers that give a dark-brown colour to the top of the soil strata (see picture). This darkening is due to the polymerisation of small organic molecules issued from the decomposition of plants (leaves, shoots, and roots) and from soil biota such as worms, ants, algae, fungi and microbes. The Maillard reaction can thus explain at least partly the storage of stable, organic matter during thousands of years. This key role is important for the survival of humans, as explained below.

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First, in temperate areas humus can be viewed as a "sponge" that is able to store water for plant life. Without humus, rainwater is rapidly drained through soil surface towards deeper layers, far from roots. Consequently, without plants, there would be no food production. Here, it is noteworthy that intense cultivation practices lead inescapably to the slow degradation of humic matter of agricultural soils: one can easily observe that agricultural soils have usually a lighter colour than dark forest soils, as the result of intense tillage of the topsoil.

Humus can further be viewed as the food reservoir of plants, soil organisms, and, in turn, humans. Indeed, humus stores substantial amounts of organic nitrogen, which can evolve into mineral nitrogen (nitrates, ammonium) used for growth of plants and microbes. Here again, intense agricultural practices lead to the degradation of soil humus. In turn, "the humus food reservoir" becomes smaller. Consequently, farmers must add more fertilisers (nitrates, ammonium) to reach similar crop yields. A major part of those fertilisers do not even reach the roots but migrate down to contaminate groundwater. This addition of an excess of fertilisers then induces the degradation of humus, and, in turn, the humus food reservoir becomes smaller. And so on…

Humus as an organic, polymeric network is also able to efficiently trap pollutants such as pesticides, toxic metals, hydrocarbons and radioactive compounds. The storage of pollutants in humus is not environmentally benign because, one day or another, those pollutants may be freed from the humic reservoir then be transferred to the waters we drink, the plants we eat and the air we breathe.

Humus plays also a major role regarding greenhouse gases. Why? Simply because humus is able to store plant carbon, and plant carbon comes from atmospheric carbon dioxide (CO₂), a well-known greenhouse gas. On the other hand humus can also produce carbon dioxide by decomposition of organic debris. Therefore, humus is able to both produce and store carbon, depending on agricultural practices. For instance, forest soils tend to accumulate more carbon than producing carbon dioxide, whereas intense cropping and deforestation tend to release more carbon dioxide in the atmosphere than storing carbon in soils.

To conclude, soil humus is often regarded as a place to dump various wastes, a medium that can not be eaten, drank or breathed. This is wrong because, indirectly, we eat, drink, and breathe humus. Humus is a fragile resource of high importance for food production and water quality. Humus can store pollutants. It regulates greenhouse gases. All those issues become even more critical with the fact that the global amount of soil organic carbon (30 10¹⁴ Kg) is 6 times higher than that of organic carbon of all living organisms (5 10¹⁴ Kg).

Dr. Eric LICHTFOUSE
Earth Sciences Department
INRA-University of Burgundy
6, Bd Gabriel
21000 Dijon, France
Eric.Lichtfouse@u-Bourgogne.fr
Tel/fax 33 3 80 39 63 72
http://www.u-bourgogne.fr/ACE/Z00Licht.html
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