**3. Contamination of soils**

Chemical elements play many roles, but it is possible roughly framing them in nutrients or

Have in mind that the same chemical element, in a certain concentration, can be a nutrient but

Biological indicators, such as microbial biomass, mineralizable nitrogen, microbial respiration, enzyme activity, and metabolic quotient, are fundamental for nutrient cycling and for estimates on soil ability to influence plant growth. In addition, microorganisms provide rapid responses to changes in the environment due to the abundance of metabolic and biochemical activities; therefore, they have great potential to be used as a tool to assess the quality of the soil [16].

The microbial biomass is represented by living components in soil organic matter [17]. This variable controls functions such as the decomposition and accumulation of organic matter or transformations involving mineral nutrients. Biomass provides information about changes in the organic properties of the soil, about changes caused by crops or by vegetation removal, about regeneration after topsoil removal, and about the effects from pollutants such as heavy

The nitrogen (N) in the soil is the organic compound indirectly available to plants. The N content conversion from its organic form to its mineral one (mineralization) is mainly carried out by

to different forms of organic nitrogen available for plants. From a soil viewpoint, this potential organic N conversion into mineral N (potentially mineralizable nitrogen) has been considered an important factor; therefore, it is an indicator recommended to measure the quality of the soil [8]. Soil respiration reflects the microbial activity and is defined as the biological oxidation of

This indicator occupies a key position in carbon cycles observed in terrestrial ecosystems due

Enzymes rule the biological catabolism of organic and mineral components in the soil. This process is closely related to organic matter, physical properties, and microbial activity and biomass; therefore, it is widely used to assess the quality of the soil, since it indicates changes

The respiratory coefficient represents the relation between carbon in the microbial biomass and total organic carbon (TOC). This coefficient indicates microbial biomass efficiency in

) through an enzymatic complex. From this point on, ammonia can be incorporated

), which is conducted by aerobic microorganisms [18].

) into ammo-

bacteria belonging to genus *Rhizobium*, which convert atmospheric nitrogen (N<sup>2</sup>

in microbial activity and the presence of pollutants in the soil [14, 20, 21].

• Macronutrients (Ca, K, Mg, N, P, S), which are mostly needed by plants.

toxic elements.

toxic in another one.

**2.3. Biological indicators**

metals and pesticides [16, 17].

organic matter into carbon dioxide (CO<sup>2</sup>

using the available carbon for biosynthesis [21, 22].

nia (NH<sup>3</sup>

to oxidation [19].

Soil nutrients can be divided into two groups:

16 Soil Contamination and Alternatives for Sustainable Development

• Toxic elements, which are considered heavy metals.

Soil contamination is one of the main environmental issues worldwide [1]. The unconscious use of the soil for agricultural activities, for the disposal of waste, chemicals, and industrial waste has been the cause of concern for centuries. Besides huge damages to the environment, a large amount of contaminated soils unviable for agriculture or for construction sites is the consequence of such inappropriate discharge [24]. Agricultural production has led to the increased use of pesticides to control pests and weeds. In addition to active toxic ingredients, many agricultural products contain potentially polluting elements or compounds, such as trace elements and emulsifying surfactants, among others [25, 26]. There has been simultaneous increase in the application of sewage sludge, industrial waste—composed of urban waste—and of agricultural waste for disposal or recycling purposes. These residues have high contents of organic matter and mineral elements capable of improving the chemical, physical, and biological properties of the soil. However, these residues may contain trace elements, pathogens, and many other substances that cause environmental damages [26, 27].

The term "soil contamination" refers to the presence of toxic substances belonging to chemical classes such as inorganic ions (metals), organic solvents, radioactive substances, pharmaceuticals, polycyclic aromatic hydrocarbons (PAHs), and pesticides (herbicide, insecticide, and fungicide) [28].

According to the International Union of Pure and Applied Chemistry (IUPAC), inorganic pollutants are described as "toxic elements." They comprise metal and metalloid elements formerly called "heavy metals." This nomenclature includes elements that, at low concentration, are biologically essential to living organisms. However, when these essential elements are observed at concentrations higher than the required ones or above the recommended limits, they also cause imbalance and exhibit some toxicity.

The displacement of water, soil, and air contaminants, as well as the interface between different compartments, is determined by processes related to the chemical properties of substances in the soil and to environmental compartments. Soil is one of the most complex matrices in the environment due to its heterogeneity. Different soil types present a wide variety of particle sizes and ecophysiological characteristics [14].

Contaminants that reach the lithosphere move by diffusion, that is, water moves through gaps between soil particles. Contaminant displacement speed depends on molecular weight and on the concentration of the contaminant gradient, as well as on soil characteristics such as humidity, clay type, specific area, cation exchange capacity, pH, redox potential, temperature, porosity, and permeability. Soil contamination presents intrinsic characteristics such as cumulative characters and low mobility of pollutants. However, this mobility becomes greater in soil recording relatively low pH or large amounts of sand to the detriment of clay, since these factors make the solubility of toxic elements easier in the environment. Consequently, the soil becomes susceptible to water leach to underground sheets and to other water bodies. Therefore, interactions among the chemical structure of contaminants, soil properties, and entry mode in the environment determine whether a specific substance is persistent and whether it is potentially hazardous to soil compartment [12–15].

substances available in a sample. They have the inherent ability to detect deleterious effects produced by a toxic agent, or mixture, on living organisms. Thus, these tests allow evaluating

Ecotoxicological Tests as a Tool to Assess the Quality of the Soil

http://dx.doi.org/10.5772/intechopen.82192

19

A large number of ecotoxicological trials have been developed, or improved, due to a wide variety of studied species and ecosystems. Aquatic, sedimentary, and terrestrial systems can be used in tests to verify the degree of contaminations caused by toxic agents and their pos-

Among the ecotoxicological tests available, the behavioral test (avoidance test) is a rapid method applied to determine the bioavailability of chemicals or the contaminants in the soil. Although this test is a simple and rapid assay, it has high ecological significance, since avoidance indicates site rejection and population decrease caused by stressor agents such as contamination [34].

The behavioral response results from the level of the organism, which can be defined as the action, reaction, or activation of a given system. This system is subjected to a set of specific conditions that represent the integration of biochemical and physiological processes [35].

Avoidance tests also substantiate the application of other ecotoxicological tests (acute and chronic), since they indicate whether a substance is influencing the physiological and meta-

Acute toxicity tests (mortality test) assess contamination after short-term exposure to high contaminant doses (from 24 h to 14 days). Overall, acute toxicity test results showed that lethality is the main effect of contamination; however, other manifestations such as decreased

Acute toxicity tests are relatively simple, are inexpensive, and can be applied to a wide variety of organisms. However, they have the disadvantages of not indicating the contaminants responsible for the observed toxicity and the effects of contamination on the dynamics of

Results of acute toxicity tests include LC50/LD50 values (concentration/dose causing mortality to 50% of organisms tested) or EC50 (effective concentration or concentration causing an

Chronic toxicity (reproduction test) tests are closely linked to results of acute toxicity tests, since sublethal concentrations are calculated based on LC50. These tests evaluate the effects of lower contaminant concentrations for long exposure time. The observed effects are sublethal and emerge when the toxic agent concentration the organisms are exposed to allows the survival of these organisms. However, these concentrations affect one, or more, biological function of these organisms, and it influences reproduction and egg development and growth [40].

effect, other than mortality, that is, immobility to exposed organisms) [38].

how hazardous these substances are.

sible ecological implications [32, 33].

bolic functions of test organisms.

mobility can be observed [36].

assessed populations [37].

*4.1.3. Chronic toxicity test*

*4.1.2. Acute toxicity test*

*4.1.1. Avoidance test*

The soil must be understood as a living being due to its characteristics, since its biota plays a fundamental role in physical-chemical equilibrium. The soil gets polluted by negative changes in the existing equilibria, which lead to conditions that impair, or makes unfeasible, the life in the assessed area. They also cause environmental damages that can take millennia to be fixed.
