Validated approaches to restoring the health of ecosystems affected by soil pollution

Abstract

The diagnosis of soil pollution in relation to the most commonly used remediation methods is an innovative way of tackling this problem. To this end, the theoretical framework of Ecosystems Health has been instrumental, since this area is presently emerging as a new language for general discourse on pollution. For example, the use of the term stressor for a pollutant implies the study of behavior responses in living organisms (at different levels of organization, from the cell to ecosystem) both in terms of impacts (toxicity) and tolerance towards the pollutant (their adaptation). Two very common scenarios in the central Iberian Peninsula have allowed us to approach the understanding of this area of study: (i) old abandoned mines and, (ii) old solid waste (urban and industrial) landfills. The soils of these systems are mainly polluted with heavy metals (Zn, Cu, Pb, Cd), they overlie different lithological substrates (granites, limestones and especially arkoses), and are highly representative of soils of the Mediterranean region (mainly regosols, luvisols and cambisols). The landfills of the mines or waste tips have slopes exceeding 40%, such that any leachates produced transfer pollutants to adjacent ecosystems at lower altitudes (stream bed pastures, marshes and surface water courses). Moreover, since the topsoil layer is usually sandy with low proportions of clay and organic matter, deep leachates also transport pollutants to ground water systems. We focused on the autotrophic component of the affected ecosystems for several reasons: toxicity tests could be used to examine the physiological and behavioral responses of organisms (mortality, injury, metabolic changes) as well as population (population density, risk of extinction) or community (structure, diversity, biomass, nutrient flow changes) variables. The build-up of a heavy metal in the above-ground part of a plant (phytoaccumulation) consumed by herbivores is also detrimental for health, due to transfer to the trophic network. Root systems may play a role in phytostabilizing heavy metals and in preventing them from passing to deeper soil layers. Finally, given the erosion problems of fine materials in landfills and waste tip slopes, vegetation helps avoid the movement of topsoil layer pollutants to other ecosystems. The methodological approaches validated by results obtained over the last twenty years can be summarized as: studies of polluted sites based on phytoecological sampling, analysis of soil chemical and physical properties, georeferencing of the heavy metals they contain for further sampling in areas showing the highest levels, collecting and chemically analyzing plants at these sites, and the use of soils with their seed banks to perform experiments in microcosms in controlled conditions. The idea is to use a combination of field and laboratory methods that simulate real scenarios in which soil pollution occurs. INTRODUCTION: THE COMPLEXITY OF DEGRADED ECOSYSTEMS WITHIN THE FRAMEWORK OF SYSTEMS SCIENCE With the title “From Biodiversity to Biocomplexity”, the monograph of the June 2001 issue of BioScience was devoted to the multidisciplinary step we need to take if we are to understand our environment. The main questions addressed as challenges for biologists at this start of a century are perfectly apt to introduce the subject of the work we describe in this chapter. There is no question that the field of biocomplexity will be in the forefront of research endeavors in the years to come, given it is a property of all ecosystems; or in other words, it is the thread that links many complex systems that are structured or influenced by living organisms, their components or biological processes. Interactions between live beings and all the facets of their external environment are an evident fact and research into these interactions, which involve multiple levels of biological organization and/or several spatial or temporal scales, is of great importance for everything related to environmental impacts. To undertake such research, long term data collection and monitoring are required to examine interactions at different spatial scales of the landscape and assess interplay between air, soil, water and the ecosystems they sustain. These issues will guide decision-making and help understand the functions of ecosystems and the changes they suffer in an ever-evolving world. Notwithstanding, the study of complex systems, among which we can include terrestrial ecosystems at polluted sites, is in large measure at a very early stage. Analyzing the individual components of a system offers no information about the properties of its combined components. The emergence of new properties when components change is a common phenomenon, but deciphering what makes new features emerge is a topic that has only just started to attract the attention of scientists. With these brief considerations only, we can begin to understand the current impact that the topic of restoring degraded ecosystems is having for biologists because of its complex nature. Biocomplexity is nevertheless difficult to describe and experimentally assess owing to its non-linear nature. Indeed, understanding biological complexity requires the development of new paradigms that cross temporal, spatial and conceptual barriers. Certainly, in the journal mentioned it is recognized that the development of the ecosystemic focus and its use to try to understand and resolve environmental problems, has been one of the major advances of Biology in the past 50 years. For many years now this has been our approach (Hernandez, 1989 and 1991) and in the subsequent sections of this chapter we describe some of the concepts we have established and tested to address the subject of restoring degraded ecosystems. Remediating and recovering degraded ecosystems is really a topic of systemic reality. This dictates a need to juggle the basic principles of systems analysis (complexity, interaction, uncertainty, multicausality) in an episystemic focus on ecological restoration. The applicability of this focus stems from the fact that ecosystems are dynamic systems that evolve and coevolve with human activity, though always evoking their stability, a term frequently used in relation to the response of an ecosystem to perturbation. This in reality is the picture we are faced with: both EU community policies and global warming that affect the dynamics of abandoned cereal fields or grassland systems, along with the environmental impacts of new emerging systems in structured landscapes such as landfills. The remediation of landscapes by reforesting with tree species is met with changes in soil structure, a feature not usually considered in practice that complicates the system to be restored. In the case of solid waste landfills capped with soils taken from surrounding areas, we have to face the complexity that arises from the secondary ecological succession, arising from the seed bank of the covering soil, intermixing with the primary succession of these new ecosystems. Not only does the impact of waste disposal need restoring but the “waste system” itself also needs to be remediated.