Chemical soil degradation as a result of contamination: A review

Chemical soil degradation after erosion is the second most abundant form of soil degradation and as such poses a threat to our finite soil resource, as it tends to render it less usable. It is therefore necessary to understand the means by which soils are degraded chemically. This review paper seeks to highlight some of the causes of soil chemical degradation. One way by which soils degrade chemically is through soil contamination; either by diffuse contamination or from localised sources. Drivers such as salinization, acidification of soils, chemical fertilizer application and use of pesticides all tend to aid the process of soil chemical degradation. The review paper sheds light on these drivers of degradation and also discusses some assessment methods developed to determine soil chemical degradation. In assessing chemical degradation, a combination of assessment tools and soil quality indicator parameters or single assessment tools may be employed. Some of these tools include ecotoxicological approach, soil quality test. A combination of two or more assessment tools aids in the process of restoration of the soil. Chemically degraded soils may be irreversible in most cases and as such its prevention will aid in agricultural sustainability.


INTRODUCTION
The practice of agriculture in the 21 st century is faced with many challenges. For researchers in the field of agriculture, it has become necessary to understand and be innovative in countering the effects of the problems being faced. These challenges are as a result of the many impeding factors threatening agriculture sustainability either through anthropogenic causes or natural factors. One of these factors is soil degradation. Most often the term soil degradation is conflicted with land degradation, which in academic sense differs greatly.
Soil degradation and land degradation have been clearly differentiated by several scientists over the course of time. FAO (2014) defines soil degradation as a change in the soil health status resulting in a diminished capacity of the ecosystem to provide goods and services for its beneficiaries while land degradation encompasses all the negative changes in the capacity of the ecosystem to provide goods and services.
Of the various forms of soil degradation, chemical intrusion in soils as well as chemical soil degradation is of essence and has been noted by most soil researchers lately. According to Logan (1990), chemical soil degradation is of no less importance compared to other *Corresponding author. E-mail: richmondnarh501@gmail.com. Tel: +4915750266010. Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License Table 1. Continental and global extent of chemical soil degradation in million hectares. Dry land zone is defined as the climatic region with an annual precipitation\evapotranspiration ration of 0.65 or less. The humid zone has a ratio of more than 0.65.
forms of degradation but it is often overlooked. An assessment report by EC (2013) iterates that, over 200 years of industrialisation has left a widespread of soil contamination, which has led to soils being chemically degraded in Europe. Chemical degradation refers to the accumulation of toxic chemicals and chemical processes which impact on chemical properties that regulates life processes in the soil (Logan, 1990). Suraj et al. (2001) stipulates that, a change in one or more of these soil chemical properties have a direct and indirect adverse effects on the chemical fertility of soils. Chemically degraded soils have the presence of large amounts of toxic chemicals interfering with activities of soil life processes. These toxic chemicals may also interfere with nutrient availability, nutrient uptake and nutrient element mobility. A report by Oldeman (1992), states that a total of almost 240 M ha of the world soil is chemically degraded and this amounts to about 12% of the total area affected by human induced soil degradation (Table 1).
Overlooking of chemical soil degradation has been due to the fact that its impact is not clearly visible from the onset. In recent times there has been an increase in soil contamination (a major cause of chemical soil degradation in Europe (EC, 2013), arising from persistent human influence. A report by Van-Camp et al. (2004) states that, consumer behaviour and the industrial sector are contributing to the increase in the number of potential sources of contamination. These sources may include municipal waste disposal, energy production and transport, mainly in urban areas. Soil contamination as a form of chemical degradation differs greatly in other part of the world as the chemical sources differ according to location and activity. The identified cause of soil chemical degradation can be related to the human activities of the inhabitants of that particular region.
Many publications pertaining to chemical soil degradation have placed focus on identifying the chemical elements responsible for the degradation. A thorough investigation into assessment methodologies is lacking in most publications. An understanding of the role played by factors such as location and end use of land in assessing degradation is lacking. For this reason standardization of assessment methodologies can not be achieved, therefore most assessment tools used are individually defined to suit their present need. This review paper therefore tends to highlight some of these assessment tools and the general shortcomings pertaining to using search tools. Kavvadias (2014) states that, chemical soil degradation is as a result of soil contamination from chemicals. This contamination process can be divided into diffuse contamination and source contamination.

Diffuse contamination
Diffuse pollution is an important threat to soil conservation and this is much evident in urban communities with multiple sources of emissions (Biasoli and Ajmone-Marsan, 2007). This is contamination that is associated with atmospheric deposition, certain agricultural practices and inadequate waste and wastewater recycling and treatment (Kavvadias, 2014). According to EEA (2014), most soils contaminated through diffusion of chemicals are often used as sites for the disposal of industrial and urban waste. The most important soil contamination substances from diffuse sources are atmospheric deposition of acidifying and eutrophying compounds or potentially harmful chemicals, deposition of contaminants from flowing water or eroded soils, and direct application of substances such as pesticides, sewage sludge, fertilizers and manure which may contain heavy metals onto the soil (Kavvadias, 2014).
In order for diffused chemicals to cause damage in the soil, some characteristic properties of the soil must come into play (EEA, 2014). One of such soil characteristic property is the soils unique composition. The unique composition of a soil determines how much water it can hold, the living organisms it supports, and which chemical reactions are likely to occur (EC, 2013). Based on the soil properties, the diffused chemicals may either react with other soil factors, get adsorbed to soil substances or may be leached directly into groundwater table thereby causing other forms of pollution. Due to the behaviour of these chemical substances in the soil, it is very difficult to determine their fate in the soil. Researchers often use the adsorption and solubilisation properties of the chemical to determine their fate, but this becomes difficult with diffuse chemicals as they exhibit both partial adsorption as well as partial solubilisation (EEA, 2014).
With the onset of diffuse chemical degradation, certain soil functions are hindered. The most important of these functions are the soil buffering, filtering and transforming capacity (EEA, 2014). With these functions lost or hindered, the soil loses its capacity to eliminate harmful chemical substances or reduce the effect on crop growth and yield. Also, if the accumulation of pollutants exceeds the buffering capacity, then soils of sediments can become the source of diffuse pollution to adjacent compartments such as for groundwater and surface water (Halm and Grathwohl, 2005).

Localised sources
According to EEA-UNEP (2000), local contamination is an emerging issue, which usually affects areas with a high density of urban agglomeration and with a long tradition of heavy industry, or vicinities of military installation. The major contaminants observed in these areas are heavy metals, organic contaminants such as chlorinated hydrocarbons and mineral oils (EEA, 2014). The result of all these contaminants present in the soil are, the loss of soil function, uptake of contaminants by plants, or the contaminants posing other forms of environmental threats such as water pollution and affecting human health through direct contact (EEA, 2014).

Salinization and sodification
Salinization is a process of chemical soil degradation, which greatly reduces soil productivity. Kavvadias (2014) defines salinization as the accumulation of water-soluble salts (including sodium, potassium, magnesium and calcium, sulphate, carbonate and bicarbonate) on or near the surface of the soil. Salinization involves the Tetteh 303 accumulation of different salts, but the increased content of exchangeable sodium (Na⁺) in a soil resulting to a completely unproductive soil is referred to as sodification (Kavvadias, 2014). There are several means by which salt accumulates in the soil and this is compounded by the activities of humans. According to Hedge et al. (2011) the source of soluble salts in the soil besides irrigation water are mineral weathering, fertilizers, salts used on frozen roads, atmospheric transfer of sea spray and lateral movement of ground water from salt containing areas.
Research done by West et al. (1994) identified that, there are three principal mechanisms of salinization. The first is salt accumulation, second is seepage of salt and the third is wind deposition. Salt accumulation occurs when leaching is induced due to reduced soil water, the salt content of the soil then accumulates at the surface or at some depth in the soil structure, and then following erosion, it becomes exposed. According to Ballantyne (1963), salinization can also occur when salt is leached into a perched water table and then seeps to a lower point in the landscape. The wind deposition relies on a suitable source of salt deposits where the wind carries them onto the surface of the soil at another location. Salinization is a common problem in arid and semi-arid regions where the rate of water loss from the soil through evapotranspiration is higher than the amount of rainfall received (Hedge et al., 2011).
It is influenced by a number of factors and the main influencing factors according to Kavvadias (2014) are climate, the salt content of the parent material and groundwater, land cover and topography. The EEA (1995), estimated salinization to affect around 3.8 million ha of land in Europe. This has become significant due to the rapid development of irrigation and increased demand for domestic water supplies causing a decline in conventional water resources. As a result waste water reclamation and reuse is increasing (Kavvadias, 2014). Ravilovich (1992) stated that, in clayey soils (Alluvial vertisol) increasing salinity problems are caused by irrigation with domestic effluent water. Salinization caused through irrigation can be prevented with appropriate irrigation patterns and measures that ensure that the right quantity of water required by the soil and plant is what is being supplied and not the excess.

Acidification
Acidification is the change in the chemical composition of the soil, which may trigger the circulation of toxic metals (Nagle, 2006). Acidification impacts negatively on the soil ecosystem thereby causing damage to plants. It also results in the alteration of soil water chemistry. Soil acidification results from pH decline or from acid deposition. The phenomenon of acid deposition arises from the deposition of emissions from vehicles such as SO 2 , power stations, other industrial processes and natural bio-geochemical cycles onto the soil surface mainly via rainfall and dry deposition (EC, 2013).
One of the soil types most affected by acidification is acid sulphate soil. According to Dent (1986), the extreme acidity of these soils is as a result of the oxidation of pyrite when pyrite rich parent materials are drained. Pyrite accumulates in waterlogged soils that are rich in organic matter and also dissolved sulphates from seawater. Upon drainage, oxygen enters the soil system and oxidises pyrite to sulphuric acid causing the pH to drop to less than 4. Acidified soils hinder the availability of some mineral elements in the soil either by reacting to produce forms that become bound to the soil particles or form complexes or the elements are leached further down the soil structure.

Chemical fertilizer
According to Savei (2012), non-organic fertilisers mainly contain phosphates, nitrate, ammonium and potassium salts. The fertiliser also contains large majority of heavy metals like Hg, Cd, Hs, Pb, Cu, Ni, and Cu. All these elements are known to cause soil degradation. Findings from research studies show that, the effects of chemical fertilisers on the soil are not immediately obvious, because the soil has strong buffering power due to their composition.
One major component of soil that is degraded through fertiliser use is soil structure. Savei (2012) states that, fertilising soils especially with industrial fertilisers such as NaNO 3, NH 4 NO 3, KCl, K 2 SO 4 , NH 4 Cl is known to cause deterioration in the soil structure. The continuous use of acid forming nitrogen fertilizers causes a decrease in soil pH (Moebius-Clune et al., 2011). Savei (2012) also reported a research carried out in the province of Rize in Turkey with one-way ammonium sulphate fertilizer application to tea resulting in an increase in acidity of soils and that 85% of the territory had a dropped pH below 4. The research also identified that, application of large amounts of potassium fertilizer was found to disrupt the balance of nutrients preventing plants from receiving the necessary nutrients for growth (2012).

Pesticides
Pesticides play a major role towards contributing to the modern day agriculture, which in a sense is enduring food security. However, the possibility of applied pesticides reaching the soil and causing degradation of some aspect of soil properties cannot be underestimated. Such possibilities are high when pesticides are applied at high rates over many years (Hance et al., 2001) and this leads to toxicity. Pimentel and Levitan (1986), in their paper stated that, there has been an estimate indicating that less than 0.1% of the pesticides applied to crops actually reach the target pest with the rest finding its way in the environment, contaminating soils, air and water. A concentration of pollutants tends to accumulate in the topsoil where most soil organisms live (Ulrich, 1987). Pesticides enter soils from spray drift during foliage treatment, wash off from treated foliage, release from granulates or from treated seeds in soils (PAN, 2010) as is shown in Figure 1. Moreover, most of these pesticides become persistent in the soil at varying degrees. The persistence of pesticides also limits the degree to which the chemical composition can be degraded as well as transported in the soil. Pesticides in soils have been identified to have major effect on soil microbes. Pesticides can cause significant irreversible changes in soil microbial populations (PAN, 2010). These soil microbes are important in maintaining soil fertility, thus pesticides, which seriously affect soil micro flora and micro fauna, may harm soil fertility (Nawab et al., 2002). Liebich et al. (2003) state that fungicides have been found to be toxic to soil fungi and actinomycetes, by causing changes in microbial community structure. Other bacteria species such as nitrification bacteria have also been found to be very sensitive to pesticides influence. According to Gigliotti and Allievi (2001), inhibition of nitrification was proved by sulphonylurea herbicides. PAN (2010) observed in a new study that, some organochlorine pesticides suppress symbiotic nitrogen fixation resulting in lower crop yields. They further report that authors found out that pesticide Pentachlorphenol, DDT and Methyl parathion at levels found in farm soils interfered with signalling from leguminous plant such as alfalfa, peas, and soybeans to symbiotic soil bacteria.
Some pesticides (Benomyl, Dimethoate) can also negatively affect symbiotic mycorrhizal fungi, which facilitate plant nutrient uptake (Chiocchio et al., 2000). A laboratory experiment that reproduced vineyard conditions in France showed that mixture of insecticides and/or fungicides at different environmental concentrations caused a neurotoxic effect in earthworms. After a long period of exposure or high concentrations, earthworms were physiologically damaged and could not cope with the high toxicity (Schreck et al., 2008) An integrated study on a roundup resistant soya field in Argentina showed deleterious effect of these pesticides on earthworm population. Earthworms avoided soil with glyphosate; their feeding activity and viability were reduced. Glyphosate and chlorpyrifos also caused several adverse effects at cellular level (DNA damage) that indicated physiological stress. Authors concluded that the effects observed on the reproduction and avoidance caused by glyphosate could contribute to earthworm decrease, with the subsequent loss of their beneficial functions (Casabé et al., 2007)

ASSESSMENT OF CHEMICALLY DEGRADED SOILS
In general view, the assessment of degraded soils is  seen as a complex issue and other scientific knowledge has kicked against the mono-disciplinarity approach in tackling the issue. According to Moebius et al. (2011), no standardized soil quality tests exist. WOCAT, ISRIC and FAO propose such tools as expert opinion, remote sensing, field monitoring, productivity measurements and participatory surveys as efficient for the assessment of degraded land, but there is no standardized tool for chemical degradation assessment. Table 2 shows one tool used by Senjobi et al. (2012) to assess degradation. They used soil indicator parameters to estimate the degree of degradation. In other terms, they determined the soil health by assessing certain soil parameters. Another approach defined as an assessment tool for soil contamination is the ecotoxicological approach.

Eco toxicological approach
This is a science that deals with the ecological effects of potentially toxic substances (Van Straalen, 2002). This approach seeks to shed light on the ecological role of soils during contamination. In ecological risk assessment, the analyst has two possible argument options; either argues the effect of pollutants and the risk to concentration or from concentration to risk. In the first approach, the analyst must define a certain maximally accepted risk of substances in the environment. This is done using toxicity tests in which organisms are exposed to graded series of concentrations and effects are measured at each concentration. The concentrations corresponding to the maximally accepted effect is then Figure 2. General design of a terrestrial model ecosystem, Sheppard (1997). estimated from the results by regression technique and expressed as EC10 (10% Effect concentration) In the second approach that is from concentration to risk the initiation of this approach begins with a site where certain concentrations of pollutants are present (Van Straalen, 2002). The objective is to determine the risk associated with the contamination. The scientific bases on which effective soil assessment tests can be run is not fully developed therefore posing a number of challenges on conducting and accepting test results. Rutgers et al. (2000) proposed another ecotoxicological soil assessment approach. In this approach they argued that, soil evaluations should be contingent on the intended land use of the site depending on whether the site is going to be used as a residential area, industrial estate or an agricultural field. This argument further raises the problem of non-standardisation of assessment methodologies for soil pollution. Their approach consisted of a triad dimension. In this method, an important role is played by bioassays in which soil organisms are exposed to samples taken from the site and their response is observed under standardized conditions.

Terrestrial mode ecosystem
There is a significant effort by the international community to standardise the soil assessment evaluations. According to Van Straalen (2002), in International co-operation programs sponsored by the European Union, a significant progress has so far been made in the standardisation and field validation of a type of soil microcosm called the Terrestrial Model Ecosystem (TME). This is an ecotoxicological risk assessment approach, which is gaining grounds in the scientific community. This approach is characterised by a number of factors, (i) It is necessary to use undisturbed soil columns taken from the field rather than artificially reconstructing a soil column from separate materials. (ii) There is the inclusion of living vegetation growing on the soil rather than using only the soil itself or only the litter layer. This will allow for interactions between soil living organisms and plant roots. (iii) It is necessary to take a column with a content of several litters rather than the small systems of a few centimetres used as microcosms. This will allow for larger organisms such as earthworms to develop more or less normally and it also takes away part of the micro scale variability.
Meeting these conditions then requires carrying out the actual test. The TME samples are then incubated for a period of time in the laboratory under artificial daylight and constant ambient temperature. While carrying out these processes, measurements are made. The soil columns taken from the field are equipped with funnels, which will allow the soil leachates to be contained in a flask as shown in Figure 2. A number of variables are then measured such as, nutrient concentrations in the leachate (e.g. ammonium, nitrate, sulphate), evolution of gases (e.g. CO 2, Nitrous oxide), decomposition of organic matter, microbial biomass, microbial community structure, enzyme activities, and invertebrate populations. Differences in the performance of systems taken at different locations may be evidence of altered ecological functioning of the soil (Van Straalen, 2002).
With many ways to determine the state of a soil health without any standardised system, there have been some conflicting interests with many proposals to consider. Van Straalen (2002) argues that, in soil assessment, the exceedance or non exceedance of standards used are often insufficient argument to conclude on the presence or the absence of risks because site specific factors such as microbial activity, age of pollution, pH, clay content may modify the risk associated to and extent that, it is not simply related to the chemically determined total concentration in the soil.

CONCLUSION
Due to the diversified forms of the causes of chemical soil degradation, generalisation of the assessment procedure may tend to underestimate the real condition of degradation; therefore in order to fully ascertain the extent of chemical degradation, it is best to first identify the type as well as the casual factors for the degradation. Having this information at hand will give a credible assessment result.
In instances where we combine soil quality indicators as well as assessment methods, priority must be given to the identified causal factor of the degradation. This is because there are always variations in the indicators we select which may be as a result of different impacts from different causal factors in the same locality. In recent times the extent to which humans influence chemical soil degradation poses more challenges in determining the standardised assessment tool. It is therefore necessary to develop assessment tools based on the location and activities in the vicinity under review.