The Earth's epidermis under the microscope
To be sure that a soil can play a productive role (in terms of agriculture, sylviculture, etc.) and that it can fulfil its ecological functions, it is not enough that it be undamaged by any form of pollution. In addition, the soil needs to be of good biological quality and to show signs that it will remain so in the long term. Specific indicators are required to evaluate this ability.
These indicators are often overlooked. Life on Earth would be impossible without the soil. Without this highly complex mineral and organic layer which is perpetually reorganised, and which is often rich in water, gas and living organisms, the biosphere would not be what it is. Plant life binds itself to the soil, so that it can draw the required nutrients for growth. In this intermediary place between rocks and the atmosphere, carbon is stored, organic matter is decomposed and the bio-geochemical cycle of the elements takes place: nitrogen, potassium, carbon, etc. Finally, the soil is an important reservoir for biodiversity and particularly for micro-organisms: in addition to a significant number of protozoa and fungi, it can contain nearly 10 billion bacteria per gram!
Despite the significance of this for human activity, it wasn't until the 1980s that various European countries began to put legislation into place aiming to protect this natural resource. According to 2006-2007 Etat de l'environnement wallon (State of the Walloon Environment) figures, the European Union covers at least 50 million hectares of soils which are damaged in various ways, including through erosion, salinisation, reduction in organic matter, compression, impermeabilisation, and of course impoverished biodiversity. The financial cost of this damage is enormous: estimates for the European Union, which vary according to the method adopted, range from 8 to 40 billion euro per year! The fact is that a soil which has been compacted by cattle, impoverished in organic material or regularly flooded no longer offers the same potential protection. As for the most severe forms of degradation (industrial pollution, for example), this can render soil unfit for any kind of human use for several years.
In 2009, the Service Public de Wallonie (SPW, Department for Soil and Waste) asked the Microbial and Plant Ecology Laboratory at the University of Liège to conduct research to identify biological indicators of soil quality which were most relevant for the Walloon region. Like other parts of the European Union, the Walloon region is bound by a wide range of increasingly comprehensive legislation, largely emanating from Europe. The aim of this legislation is to sustainably support biological productivity, but also to maintain the quality of the environment in the long term and to promote healthy animal and plant life. However, although physics and chemistry are essential, they are insufficient alone to characterise the modifications which the soil undergoes. Soil which is perfectly de-polluted, for example, does not necessarily provide the ideal conditions for life to develop and to contribute to plant production. "Organisms in the soil play an essential role in how the soil functions" explains Monique Carnol, head of the Laboratory. "They break down organic matter which, once mineralised, provides nutritional elements. Soil organisms also play an important role in terms of soil structure."