Soil invertebrates as bioindicators of urban soil quality

Abstract

This study aimed at relating the abundance and diversity of invertebrate communities of urban soils to chemical and physical soil characteristics and to identify the taxa most sensitive or tolerant to soil stressors. The invertebrate community of five urban soils in Naples, Italy, was sampled. To assess soil quality invertebrate community indices (Shannon, Simpson, Menhinick and Pielou indices), Acarina/Collembola ratios, and the soil biological quality index (QBS) were calculated. The chemical and physical characteristics of the soils strongly differed. Abundance rather than taxa richness of invertebrates were more affected by soil characteristics. The community was more abundant and diverse in the soils with high organic matter and water content and low metal (Cu, Pb, Zn) concentrations. The taxa more resistant to the urban environment included Acarina, Enchytraeids, Collembola and Nematoda. Collembolans appeared particularly sensitive to changing soil properties. Among the investigated indices, QBS seems most appropriate for soil quality assessment.

Introduction

Soil is one of the most heterogeneous ecosystems on the planet. It plays an irreplaceable role in the biosphere: it governs plant productivity and allows organic matter degradation and nutrient cycles (Pietramellara et al., 2002). Human activities, using the soil for agriculture, building and transport, negatively affect soil functionality, leading to alterations of several processes that could weaken the ecosystem. Urbanization is associated with a variety of effects on the soil system, including pollution, conversion of indigenous habitats to various forms of land use, habitat fragmentation and loss, and soil community changes (McIntyre, 2000).

Unfortunately, little is known about the impact of human activity on organisms in urban soils. Soil invertebrates are excellent candidates for studying how human activity impacts the environment (McIntyre, 2000). Foremost of these is the variety of roles played by invertebrates in the soil system (i.e., organic matter degradation, nutrient cycling and bioturbation). Additionally, invertebrates are abundant, relatively easy to sample, and they can quickly respond to soil disturbance (McIntyre et al., 2001). Finally, since soil invertebrates respond to habitat structure, different species assemblages could highlight differences in soil properties and kind and degree of soil pollution (Nahmani and Lavelle, 2002). As invertebrates are sensitive to changes in soil conditions, they can be considered valuable indicators of soil disturbances (Nahmani and Lavelle, 2002).

Urbanization may have several effects on soil invertebrate communities. In particular, in the short term, species diversity and abundance decrease (Battigelli and Marshall, 1993, Eitminaviciute, 2006, Gongalsky et al., 2010), and tolerance to pollution may increase. In the long term, the increase in the number of tolerant individuals in the community and the replacement of pollution-sensitive species by less sensitive ones can lead to a different species assemblage inside the community (Salminen et al., 2001).

Nowadays, the main properties studied to characterize the soil invertebrate communities are abundance and species diversity (Nahmani and Lavelle, 2002, Sattler et al., 2010). Unfortunately, these two community properties by themselves are not exhaustive to explain the effects of pollution. In order to try to solve this lack, several indices have been proposed. Density and diversity can be integrated into synthetic indices, such as Shannon, Simpson, Menhinick and Pielou indices. Unfortunately, these indices do not take into account the ecological role of each taxon. In addition, some of them, affected by highly abundant or very rare species, can provide destroyed information: in many cases, in fact, high values of these indices could derive from the presence of invasive species rather than from the presence of species that are well structured in the community (Parisi et al., 2005). To overcome these hurdles, other synthetic parameters have been proposed in the scientific literature, such as the ratio between the number of species of mites and collembolans, and the soil biological quality index (QBS). High values of the ratio Acarina/Collembola suggest high soil quality, because it has been established that in degraded soils the number of Acarina species decreases (Jacomini et al., 2000). Nevertheless, this index is not reliable for all ecosystems, and the values sometimes are not comparable (Jacomini et al., 2000). The QBS index is applied to soil microarthropods separated according to their morphology, and it is based on the assumption that at higher soil quality, the number of microarthropod groups well adapted to soil habitats will be higher (Parisi et al., 2005).

The aim of this study was to relate the quantitative and qualitative species composition of the invertebrate communities (Enchytraeids and Arthropods) of five soils in the city of Naples, Italy, to the chemical and physical soil characteristics (i.e., pH, water holding capacity, organic matter content, total content and water-extractable metal concentrations) in order to identify the taxa most sensitive or tolerant to soil stressors. In order to assess soil quality and species composition data were integrated by calculating several indices. As a measure of traffic-induced pollution, soils were analyzed for Cu, Pb and Zn, because these metals were shown to be most indicative of urban pollution due to traffic (Davis et al., 2001). In addition, earlier studies on urban soils from Naples and other Italian cities showed that concentrations of these metals were most elevated compared to control soils while other metals seemed of little relevance (Manta et al., 2002, Maisto et al., 2004, Maisto et al., 2006).