Agrochemical Contamination of Soil: Recent Technology Innovations for Bioremediation
Pesticides and chemical fertilizers are important tools in modem agriculture to improve the quantity and quality of food production. These agrochemicals minimize the economic losses caused by weeds, insects, and diseases. However, extensive and unscientific application of agrochemicals in the crop fields and its consequent ecotoxicity has called for special attention of scientific community all over the world to look for effective and sustainable mechanisms for then' minimal use. The environmental impact of hyper doses of pesticides and chemical fertilizers has been invariably detrimental. Adverse impact of these agricultural inputs could cause serious damage to soil biota which play vital role in maintaining soil health. The toxic intensity of such hazardous chemicals varies with time, dose, organism characteristics, environmental presence and nature of chemicals. Their presence in environment determines the risk of persistence (Ahmad and Ahmad 2014, Uqab et al. 2016).
The consequence of ecological maladies caused by recurrent use of the diverse types ofpesticides has necessitated the development and implementation of new technologies to reduce or eliminate their use or residues. Earlier, conventional technologies such as landfills, recycling, pyrolysis, etc. were used to neutralize pesticide toxicity, but these methods subsequently proved to be less efficient and cost prohibitive. Besides, these methods led to formation of toxic intermediates which are detrimental to the environment. Persistence of pesticides and chemical fertilizers in the environment might occur due to their physico-chemical properties or absence of organisms able to degrade them. Various physical factors such as light, temperature or humidity could lead to loss of toxic substances up to some extent by either volatilization or degradation process. Degeneration process could be enriched by presence of certain organisms in soil. Hence, these biota could be used to improve elimination of the undesirable pollutants from the environment through bioremediation. The ability of organisms to bioremediate pesticides is mainly based on their biodegradation activity of complex synthetic compounds (Paul et al. 2005, Jain et al. 2005). Bioremediation of agrochemicals is an eco-friendly as well as cost-effective option available to eradicate environmental contamination.
Impact of pesticides and fertilizers on soil
Based on different sources of contamination, soil pollutants are mainly categorised into various categories such as sewage sludge and industrial wastes, agrochemicals and metals, polycyclic aromatic hydrocarbons and radioactive substances. In addition, several anthropogenic activities have been observed to generate soil pollutants, the medium mostly gets affected due to direct application of agrochemicals.
Zhang et al. (2011) documented the outrageous consumption of agrochemicals in major countries. As per the reports available, pesticide consumption in the world has presently reached 2 million tonnes. European countries such as Italy, Spain, UK and the USA were reported as major consumers of pesticides. The utilization rate of Europe was 45% followed by USA (24%) and rest of the world (25%). Pesticide consumption in Asia has also reached an alarming level. Among the Asian countries, the pesticide application rate in China is the highest followed by Korea, Japan and India. In India, after the first green revolution post independence, the rate of agrochemical application has increased significantly in agricultural fields for high yield crop varieties. Presently, India is considered as the largest producer of pesticides in Asia and ranks 12th in world (Abhilash and Singh 2009).
The demand of pesticides increased in India during 1960s in the form of the chlorinated hydrocarbons and insecticides which was gradually replaced with organophosphates due to its faster natural degradation. Unregulated usage of these organophosphates in agricultural fields has threatened the soil ecosystem. Impacts of soil toxicity and loss of fertility due to pesticides have been reported in different areas of India (Kumar et al. 2016, Kliajuria 2016). Insecticides such as organochlorines, organophosphates, carbamates, and pyrethroids are conventionally applied in different parts of the country. Besides the use of organic herbicides, the demand for synthetic herbicides such as 2,4-D. glyphosate, butachlor, nitrogen based chemical fertilizers such as urea, ammonium nitrate, ammonium sulphate, phosphates in the form of nitro-phosphate and potassium fertilizers has increased manyfold. Some by products of industrial wastes such as phosphogypsum and paper mill sludge, which are used to fortify the soil, could also prove detrimental to the soil biota at elevated concentrations (Nayak et al. 2011, Nicolopoulou-Stamati et al. 2016, Mishra and Samal 2018, Samal et al. 2019a, b).
Agrochemicals can remain in soils and sediments for a long tune and enter the food chain directly or percolate down to the water table. These chemicals could directly accumulate within the adipose tissue of animals or through biomaguification in higher trophic level organisms, such as mammals and cause health hazards over time because of the enhanced levels of toxic compounds within the body (Ortiz-Hemandez et al. 2014). Accumulation of pesticides in soil may affect the soil fauna like nematodes, microarthropods, earthworms, and diverse types of microbes.
Agrochemicals at high concentrations are likely to harm natural predators of pests and pollinators. Pesticide contamination in soil poses a serious threat to both below ground and above ground organisms and reduces the population of millions of tiny organisms including protozoa, fungi, bacteria, and soil arthropods such as Collembola, Hymenoptera and Araclmida (Aktar et al. 2009, Geiger et al. 2010). Some authors observed histopathological alterations in tissue of the earthworm E. fetida, E. kinneari exposed to various concentrations of Dimethoate, fluorides and herbicides (Sharma and Satyanarayan2011, Lakliani et al. 2012). Alterations hi histo-morphological structures, metabolic and antioxidant enzymes like lactate dehydrogenase, acetylcholinesterase and catalase of Eudrilus eugeniae exposed to elevated concentrations of pesticides monocrotopohos and glyphosate have been reported (Samal et al. 2019a). Severe skin lesions and setal conformational alterations have been observed due to pesticide exposures (Figure 1). Serious damage to musculature of the body wall in different species of earthworms has been found due to agrochemical hypertoxicity (Figure 2).
Figure 1. Scanning electron microscopy photograph of E. eugeniae exposed to elevated concentration of monocrotophos and urea, (a) skin lesion exposed to monocrotophos, (b) setal damage with monocrotophos, (c) setal damage with glyphosate
(Samal et al. 2019a).
Figure 2. Muscular anomalies in the body wall of the earthworm E. eugeniae exposed to high concentrations of agrochemicals, (a) monocrotophos, (b) glyphosate (Samal et al. 2019a). The arrow mark indicates degeneration of muscle
fibres at high concentrations of the pesticides.