Important biological indicators sensitive to soil quality assessment
Earthworm species and population
The population and diversity of earthworm is an important biological indicator of soil quality. They are instrumental in organic matter decomposition, particularly by processing the soil in their guts. They support aggregate stability, improve water-holding capacity, and moderate pore size and infiltration rate (Stockdill 1982). In the southern agricultural region of Saint-Lawrence Valley-Canada, significant relationship was observed between two endogen earthworm species and soil aggregate stability as soil quality indicators. The presence of Aporrectodea caliginosa was correlated with silt and clay contents, while Allolobophora chlorotica was related with organic amendments (Lapied et al. 2009). Incorporation of organic matter in soil as amendments regulates soil temperature and aeration. Earthworms have been regarded as anthropogenic land use indicators. In Netherlands, they are used as a biological indicator of soil quality, whereas in Germany as an indicator for soil biological site classification, a practical way to define the use of soil, based on the structure of the earthworm community, their abundance and biomass (Rombke et al. 2005). These organisms are widely recognized indicators of soil health due to their sensitivity to pesticides and heavy metals (Cikutovic et al. 1999), and organo-metallic compounds (pentachlorophenol) (Booth et al. 2000, Bunn et al. 1996). The science of earthworms in soil quality improvement is still in progress. Suitable research techniques are required for exploring the knowledge obtained in the micro-scale within upper soil layers to a field scale or landscape scale. Furthermore, suitable framework is required to understand the role of earthworms in the biogeochentical cycles.
Ants and termites
Ants are considered as important invertebrates as they are very good indicators of soil quality. The ants are preferred for the study due to their abundance, macroscopic size, and sensitivity to soil degradation. It has been observed that soils from ant nests (Messor andrei) contain more nutrients and harbor major groups of soil micro fauna and flora, e.g., bacteria, fungi, nematodes, miscellaneous eukaryotes and microartlrropods as compared to adjacent non-ant soil from a semi-arid, serpentine grassland in California. The relevance of ants as biological indicators exist particularly in restoration processes after adverse soil impacts (e.g., mining, dumping of hazardous wastes) as their prevalence depends on the composition and diversity of plant communities projecting them as a better predictor of soil health than plant species diversity (Andersen et al. 2002, Boulton et al. 2003). Moreover, association of ants with termites have been regarded as indicators of land recovery due to the enhanced carbon and nutrient levels post processing of soil organic matter through their enzymatic systems (De Bruyn and Conacher 1990). Cammeraat et al. (2002) analyzed the effect of Messor bouvieri (seed harvesting ants) on soil fertility, infiltration of water, soil structure, and hydrophobicity of semi-arid soils in Spain and they reported that ant nests had a lower soil reaction, higher organic carbon concentrations and inorganic nutrients, greater structural stability, and significantly higher infiltration rate than the soils of the adjacent areas.
Termites are also used as soil quality indicators. Roose-Amsaleg et al. (2005) smdied the nests of soil-feeding termite (Cubitermes) of different ages (fresh to mature to old) in tropical rain forest of Lope-Gabon Africa and found significantly higher concentration of potassium and phosphorous, clay and fine silt, organic matter, water retention capacities and cation exchange capacity in mature nests as compared to control soils. In savanna ecosystems in Colombia, Mora et al. (2005) studied the dynamics of 10 soil enzymes (xylanase, amylase, cellulase, a-glucosidase, p-glucosidase, P-xylosidase, JV-acteyl-glucosaminidase, alkaline and acidphosphatases, and laccase) to characterize the functional diversity of soil-feeding termite (Ruptitermes sp.) and soil pellets formed by two species of leaf-cutting ants (Acromyrmex landolti and Atta laevigata) in comparison to a control soil; it was observed that some of the soil pellets have unique enzymatic profile than the control soil and the diversity of these structures and species is related to different pathways for the decomposition of organic matter (Jimenez et al. 2008).