Impact of quarry exploitation and disuse on pedogenesis

Abstract

Quarrying represents one of the oldest and most important human activities; the impacts of which have largely influenced vegetation and landscape. Despite its character as a vital and non-renewable resource, the soil has been poorly studied with regard to either the impact resulting from the exploitation of quarries or its restoration after exploitation. In the plain of La Crau (south-eastern France), numerous dry alluvial quarries were exploited in the 1970s. Their exploitation lasted about ten years and they were then rapidly abandoned. At that time, there was no legal obligation to restore sites after their abandonment. In this context, following various modes of exploitation (e.g. exploitation depth), various rehabilitation options were implemented: no rehabilitation, topsoil transfer, deposits of anthropogenic materials. The absence of legislation consequently resulted in heterogeneous areas after their abandonment. Today, we have the benefit of 35years of hindsight on the dynamics of these ecosystems resulting from these exploitation modes and rehabilitation options and thus various questions arise: (1) Did the diversity of the modes of exploitation result in heterogeneous soils 30years after abandonment? (2) At what stage of pedogenesis are the soils created in the abandoned quarries? (3) Did the various rehabilitation options have an impact on the traces of biological activity in the soils of the quarries? (4) Is it possible to identify methods of rehabilitation of quarries that would favour the rehabilitation of the soils and the recolonisation of the plant cover in order to improve the techniques in use or to propose alternative methods? The study of soil profiles (with a special focus on biological activity of animals and plants) and soil physico-chemical analyses (pH, CaCO3, C, N, C:N ratio, CEC, exchangeable K+, plant available P and fine soil particle contents) made it possible to study these questions. Our results confirm that all the soils resulting from the former exploitation modes and rehabilitation options of the quarries present their own specific characteristics (upper soil layer fertility, soil profile vertical organization) which remain very different, even after 30years, from the characteristics of the destroyed reference soil ecosystem. Pedogenetic processes are very slow or even blocked by the recurrent action of certain environmental parameters. In terms of ecological rehabilitation, only topsoil transfers undertaken under good conditions (same soil thickness as the reference steppe and no contact with the phreatic zone) have allowed to restore most of the physical and chemical properties recorded close to the reference ecosystem soil.