Soil contamination due to consistent application of chemical fertilizers and pesticides has been a global concern. Persistent contamination not only impacts soil quality, but has far-reaching environmental consequences including for human and animal health. Soil health management is crucial for ensuring sustainable agricultural production and maintenance of biodiversity. Fertilizers and pesticides are a necessary evil for soil health. Though they continue to be critically important tools for global food security, their undesirable effects cannot be overlooked, particularly when sustainable agriculture is the universal focus. Apart from a range of widely discussed and well-known adverse effects of chemical fertilizers and pesticides on the environment and human health they have also been held responsible for strongly influencing the ecological functions of beneficial soil flora and fauna, which are crucial for maintaining nutrient balance and their availability to plants.
The reactive oxygen species (free radicals) and their effects
The reactive oxygen species (ROS) mainly include oxygen compounds and nitrogen compounds (Table 1). ROS are extremely reactive and toxic to cells. They cause cellular damage through nonspecific reactions with their components, i.e. sugars, proteins, lipids, nucleicacids (DNA and RNA) and virtually all biomolecules found in the cell. ROS arise as a consequence of the diverse metabolic processes in cells, and their concentrations are controlled by both antioxidant enzymes and antioxidants. Results of scientific investigations indicate that free radical concentrations in cells might increase under the influence of various environmental factors, i.e. chemical pollution, including soil contamination, as well as physical factors, such as ionising and ultraviolet radiation, ultrasound, air pollution, tobacco smoking and fires.
Table 1. Selected examples of reactive oxygen and nitrogen species (Bartosz 2003, Karbarz 2010).
Soil biota, which mostly includes microorganisms and soil animal populations, are adversely affected by enhanced levels of free radicals. Free radicals play an important role in organic matter degradation, and their involvement in the breakdown of lignin. Lignin is a polyphenolic compound and depolymerisation is an essential part of the degradation process. This is achieved by the action of specialised lignin-degrading soil fungi and involves enzymatic hydrolysis and oxidation by hydroxyl, and possibly other, free radicals. The quinones that are produced in humification are derived from the fungal depolymerisation of polyphenols, such as lignin. Studies using electron spin resonance (ESR) spectroscopy have shown that humic substances contain stable organic free radicals. It has been proved that the degree of humification is positively correlated with the concentration of free radicals. Thus it appears that, at least in part, humification proceeds by the action of reactive free radicals, such as hydroxyl, on phenol-containing materials such as lignins causing depolymerisation. In order to form the stable semiquinone radicals present in soil organic matter, reactive free radicals must be scavenged, and the oxidative chain reaction thereby terminated. The scavenging compounds, which are likely to be phenolic, act as antioxidants in scavenging the reactive free radicals. Scientists believe that accumulating residues of chemical fertilizers and pesticides in soil could induce free radical generation and therefore an increased antioxidant level has to be ensured to counter their deleterious effects.
What are antioxidants?
Antioxidants are substances that occur most often in low concentrations when compared to oxidisable compounds and which delay or inhibit their oxidation. These compounds inhibit oxidation reactions by reacting with oxidising agents (preventive antioxidants) or with oxidation intermediates, such as chain-breaking antioxidants. The preservation of the pro-oxidative/anti-oxidative balance in the cell requires the interaction of the antioxidant enzyme system and antioxidant molecules. Scientific research shows that the first line of defence against the adverse effects of free radicals includes macromolecular antioxidants: enzymes – catalase (CAT), glutathione reductase (GRd), glutathione peroxidase (GPx), and superoxide dismutase (SOD) ; amino acids – cysteine and glutamic acid; hormones – DHEA and melatonin; coenzyme Q10; and the endogenous tripeptide glutathione (g-Glu-Cys-Gly), which occur in the cytoplasm, mitochondria and nucleus of the cell. The natural antioxidants (low-molecular-weight antioxidants of natural origin) most often supplied to the human body are vitamin C (tomatoes, red pepper, cruciferous vegetables, onion, garlic, red beets), cryptoxanthin (red pepper), lycopene (tomatoes), polyphenols (cruciferous vegetables and buckwheat – phenolic acids and catechins; potatoes and coffee – chlorogenic acid; tea and cocoa – catechins; soy – isoflavones; red beans and blueberries – anthocyanins; onion – quercetin), vitamin E, and vitamin A.
Antioxidants in the soil environment
Soil is an extremely dynamic system. Numerous chemical reactions are constantly taking place in it. Some of the most common ones are redox reactions. The simplest example of these is the biodegradation of organic matter, which involves free radicals. Redox processes in the soil take place with varying intensity. Literature reports confirm that the most antioxidant substances in the soil are found among the humic compounds contained in humus. Humic acids contained in humus have strong antioxidant properties in the soil. The high content of antioxidant humic substances in the soil may influence the chemical processes taking place in the rhizosphere, especially the transport of water and nutrients, redox reactions, secretion of organic acids, sugars, phenols and amino acids by plant roots, and chelate formation. The level of antioxidants in soil and plants is affected by agrochemical residues and heavy metals. Excessive amounts of heavy metals transferred to plants from the soil adversely affect the level of antioxidants in plant products, and through consumption of these plants have a detrimental effect on human health. During the decomposition of soil organic matter, phenolic compounds known for their antioxidant properties are generated. Phenolic compounds present in the soil environment include a broad spectrum of chemical compounds, such as phenol or chlorophenols. The chemical compounds in humus known for their antioxidant properties include phenolic acids (derivatives of benzoic and cinnamic acid), flavonoids (flavones, flavanols, flavonols, isoflavones and anthocyanins) and metabolicproducts of soil microorganisms.
Total antioxidant activity of soils and selected methods for its determination
The total antioxidant capacity (TAC) of soils is a measure of their ability to prevent unfavourable processes and oxidation reactions through compounds contained in the soil that can enter into redox reactions. Hence, the total antioxidant capacity of soils can be measured to determine the intensity of redox processes in the soil environment. This parameter makes it possible to compare TAC not only of different soils, but of their soil horizon levels as well. This allows for the estimation of the degree of degradation of the soil, including soil organic matter.
Currently, all of the methods described for the determination of the antioxidant capacity require the test substance to be in a dissolved form. Therefore, to study the antioxidant properties of soil, a soil extract must be prepared. Many authors emphasise that in the case of soils a procedure must be developed for the extraction of all antioxidant substances, not only those that are easily soluble in water. Insoluble antioxidants may also be present in the soil, especially in resistant organic matter. This is a very difficult analysis to perform accurately as it should be kept in mind that the choice of extraction method significantly influences the quality of the determination. The extraction may be carried out with distilled water, ethyl alcohol or acetone.
Two categories of methods are used to determine TAC, also referred to as antioxidant activity:
- Reduction of metal ions to ions with a lower oxidation state using a tested antioxidant.
- Scavenging of free, stable radicals.
Antioxidants in the soil and the incidence of diseases
Antioxidants present in the soil, which are transferred to foods derived from the soil, can have a positive effect on the course of diseases associated with oxidative stress. Antioxidants present in the soil, and later transferred to food grown in the soil, can have a positive effect on the course of diseases associated with oxidative stress. Oxidative stress plays a central role in the development of chronic and degenerative diseases such as cancer, autoimmune diseases, ageing, cataracts, rheumatoid arthritis, cardiovascular and neurodegenerative diseases. It also contributes to the formation of tumours, osteoporosis, Alzheimer’s and Parkinson’s disease. In humans, several mechanisms have been found to counteract oxidative stress by producing antioxidants that are either naturally produced in situ or also supplied externally by food and/or supplements.
Figure 1. a). Morphological anomaly in the earthworm Eudrilus eugeniae from soil deficient in antioxidants and treated with the herbicide Pretilachlor at 0.75 kg a.i./ha. b). The healthy earthworm from soil enriched with antioxidants after farm yard manure amendment at 10 g/kg.
Elevated soil antioxidants can minimise effects of contaminants on biota
The authors and their research group have done extensive studies on the deleterious impact of agrochemical residues on soil mesofauna (earthworms) and microbes. They observed that soil deficient in antioxidants, could significantly improve its TAC after suitable amendment with diverse types of organic manures, which are rich in antioxidants. A positive correlation of TAC with organic carbon was observed. The results of their studies have indicated that soils with high TAC significantly reduce the adverse impact of chemical contaminants on the soil organisms. Research findings on the earthworm Eudrillus eugeniae showed that a potential soil contaminant, pretilachlor had minimal impact on the animal in soil with high TAC. The morphology, skin and setal (locomotory organelle)structure of the earthworms were intact in antioxidant rich soil relative to deficient soil (Figures 1 and 2). Soil with high levels of antioxidants too facilitated microbial exoenzyme secretions which are crucial agents for decomposition of organics and mineralisation of nutrients.
Figure 2. Scanning electron microscopic photographs of the setal structure in the earthworm exposed to Pretilachlor at 0.75 kg a.i./ha. a). Abnormal setae in antioxidant deficient soil. b). Normal setae in soil enriched with antioxidants after amendment with vermimanure at 5 g/kg.
To ensure sustainable agricultural practices and a healthier environment, it is important to monitor soil parameters, including their physicochemical, biological and antioxidant properties on a regular basis. If the soil has favourable physicochemical and biological properties, it may be assumed that it should also have a high content of antioxidants. These substances are taken up from the soil together with agricultural and horticultural raw materials, and, as a consequence, they are consumed in plant-derived products, and will have a positive effect on our health. Regular and widespread measurement of soil antioxidant capacity would make it possible to prevent many diseases associated with oxidative stress.
Authors: C.S.K. Mishra, Suryasikha Samal, Pratik Acharya, Stuti Pragyan Pradhan and Tanushree Moharana, Department of Zoology Odisha University of Agriculture and Technology, College of Basic Science and Humanities, Bhubaneswar, India
Read the article online at: https://www.worldfertilizer.com/special-reports/26052021/the-importance-of-high-antioxidant-levels-in-farm-soil/