Biomarkers in soil organisms. Their potential use in the assessment of soil pollution and remediation

Soil is “the top layer of the earth's crust, formed by mineral particles, organic matter, water, air and living organisms” (ISO 1996). Soil can also be defined as the interface between geosphere, atmosphere and hydrosphere. Soil has got a crucial role in environmental, economic, social and cultural development. It is the basis for food production, it facilitates accumulation and processing of minerals, organic matter, water and energy. Soil is also the habitat for a large amount of organisms and thus it plays an essential role in ecological terms. Soil also represents the basis of the human activities. However, the anthropic impact has largely exploited soil resources, inducing a slow but constant degradation process (Lal 2005). In particular, soil pollution has been strongly increased during (Aelion 2004, Ashraf 2014) the last decades due to: industrialization, which has increased pollutant atmospheric depositions onto the soil; the increasing use of chemical products directly or indirectly conferred into soil; intensive agricultural practices employing high amounts of biocide and fertilizers onto soil. Dry deposition is characterized by pollutants transferred simply by gravity or interception of the soil surface inside the trajectory of motion of the contaminant, while wet deposition is characterized by pollutants transferred into the aqueous phase and transported to the ground by direct impact (rainflow), or when, remaining suspended in the air, dragged from the meteoric events (washout). When toxic compounds reach the soil surface, they can undergo different processes such as infiltration, immobilization, transformation and accumulation. The growing use of toxic substances has caused alteration of soil structure reducing its fertility and increasing contamination of crops and groundwater and an overall disturbance of the ecosystemic relationships (Abbasi et al. 2013). Soil pollution can be distinguished in diffuse or local according to the source and effects of pollution processes. The first is caused by the introduction of significant amounts of organic and inorganic chemical products (from industrial, civil and agricultural activities) and can be originated, for example, from the atmospheric transport of pollutant containing substances and subsequent ground deposition. Soil diffuse pollution is critical because it usually affects large areas, contamination sources can be unknown and pollutants may evolve along with the soil matrix. Conversely, in local pollution contamination is limited to defined industrial areas or arise from disposal of industrial and civil wastes (e.g. landfills) (APAT 2004). Independently on the type of pollution, contaminated soils undergo changes to the microbiome structure that modifies the quantity and quality of the organic matter, leading to damages to the entire vegetation-soil ecosystem (Ashraf et al. 2014). The presence of pollutants in soils above certain levels entails a series of negative consequences for the food chain and thus for human health (Nesheim 2015). In the last two decades, soil pollution has caused an increasing interest for pollution monitoring and toxicity assessment by the scientific community and national or international environmental agencies. However, traditional approaches such as those based on chemical analysis and threshold values are far from being applicable in soil pollution assessment. As reported by some authors, in fact, pollutant bioavailability is the most critical parameter since it can be shaped by some soil characteristics such as pH or organic matter content, eventually affecting organism tolerance to certain pollutant types (Bradham et al. 2006, Spurgeon et al. 2006, Criel et al. 2008). For these reasons, a more comprehensive practice including chemical, biological and ecological evaluations is required for soil. In analogy, Chapman (1990) proposed for sediment assessment the Sediment Quality Triad, consisting of three evaluation stages, i.e. sediment chemistry measuring contamination levels; bioassay for toxicity assessment and macro-invertebrate community structure. Nowadays, the new strategies for evaluating ecosystem alterations are mainly based on effect-based approaches integrating chemical analysis with a series of prognostic biological markers. The use of biological parameters, i.e. biomarkers, can identify the adverse effect of pollutants thus representing a early warning of ecosystem changes as well as environmental risk. Since soil invertebrates are in direct contact with both ground and groundwater (Kammenga et al. 2000), they can be considered excellent sentinel organisms to be used for biomarkers measurement and for soil toxicity evaluation (Hyne and Maher 2003, Weeks et al. 2004). The aim of this chapter is to review the use of biomarkers utilized for soil monitoring, assessment and remediation analysing their advantages and limitations