The cultivation of energy crops can upgrade polluted agricultural land that cannot be used to grow food crops. INRA research scientists in Versailles-Grignon have shown that these energy crops may have a beneficial effect on the macrofauna in soils contaminated by metallic elements. These results underline the value of biological processes to the in situ redeployment of historically polluted sites.
Soil macrofauna as a study object
In order to compensate for the use of fossil energy resources, biofuels that can exploit inedible lignocellulosic plant resources are seeing sustained growth. In this context, polluted soils could potentially represent cultivable areas for energy purposes.
During this study, INRA research scientists in Versailles-Grignon focused on soil fauna, and more specifically macroinvertebrates, which are small-sized animals without a skeleton that can be seen with the naked eye and live in soil or on its surface.
The objective was to study the effects of different crops on this macrofauna in polluted and non-polluted soils. The crops grown were perennial energy crops of miscanthus or poplar very short rotation coppice1, annual crops of wheat for livestock feed, and forest plots.
Two plains severely polluted by heavy metals
In the Paris region, the Pierrelaye plain (95) covers an area or more than 20 km2, and was historically used for market gardening. During the last century, it received untreated waste water from Paris for many decades. In the Lens region (62) in the heart of the Artois plain, metal manufacturing in an enormous area of 12,400 km2 produced large quantities of dust for almost a century.
This waste water spreading and atmospheric fall-out both contributed to polluting the soil with heavy metals. Zinc, lead, copper and cadmium are thus present in very large quantities at numerous sites. This heritage now complicates agricultural activities on these two plains.
Energy crops that boost the macrofauna in polluted soils
During this study, the scientists identified 10,500 invertebrates belonging to 130 species, 76 families and 23 orders, ranging from slugs to woodlice and centipedes, without forgetting earthworms, insects and spiders.
The researchers compared the macrofauna in polluted soils under annual wheat crops with that found in polluted soils under energy crops. They observed that soils under energy crops harboured more abundant macrofauna, with a number of individuals that was 3 to 9 times higher than that found under annual crops (6 to 18 individuals per m2 versus 2 individuals). As for species diversity, this varied little or not at all, while the proportion of resident invertebrates2 was higher under energy crops.
However, the beneficial effects on fauna should be qualified. Under these two types of crop, the transfer of heavy metals to certain macroinvertebrates may be facilitated because of the greater availability of pollutants in the polluted soils of these agricultural plains.
In addition, the macrofauna in polluted soils under annual and energy crops was still much less abundant than that observed in forest plots (contaminated or not), where more than 200 individuals per m2 were counted.
The researchers considered that the impact of energy crops on the macrofauna of polluted soils in the farming plains studied was neutral. In other words, the positive effects (increase in the number of individuals) and negative effects (bioavailability of heavy metals) cancelled each other out.
Nevertheless, a positive trend was observed. As a general rule, macrofauna appeared to benefit from an absence or reduction in soil tillage and the less widespread use of plant health products, two factors which limited disturbances and allowed a gradual stabilisation of their communities. Overall, these findings throw new light on the possibility of upgrading contaminated soils through the cultivation of energy crops.
1 very short rotation coppices are intensive crops of trees that throw out new roots.
2 Resident as opposed to "mobile". For example, earthworms are considered as residents, while winged insects are considered as mobile.
A SPECIAL TECHNIQUE TO COLLECT SOIL MACROVERTEBRATES
Traps and elbow grease are the principal tools used by the perfect (scientific) hunter of macroinvertebrates:
- a plastic bottle buried in the soil with its neck level with the surface, or in other words a trap, can collect invertebrates as they move across the ground;
- a spade, fuelled by a lot of energy and perspiration, to dig and loosen the soil which is then divided into heaps from which the invertebrates are collected.
On site, the researchers do not forget to record all the data needed to identify the samples thus collected.
Back in the laboratory, armed with a magnifying glass and microscope, they then just need to identify everything they have found. Several more weeks of work may be necessary to achieve this....