HERBICIDES AND ENVIRONMENTAL EFFECTS
Herbicides and Soil Health
The effects of herbicides on soil microbiota are not well studied by researchers but are a matter of serious concern. Repeated and uncontrolled application of herbicides can upset the balance between beneficial soil microbes and pathogenic species, affecting related critical soil functions. Many herbicides have been shown to cause a transient reduction in microbial population, including cyanobacteria, Nitrobacter agilis (nitrite oxidizer) and other fungal species. This is interlinked with the soil’s biological equilibrium as any disturbances to the soil microbiota may result in reduced fertility. It may result in disturbed soil heterotrophic respiration, nutrient cycling and Organic matter decomposition (Singh et al, 2020). Many herbicides threaten the reproduction, growth and health of earthworms, which are critical components of the soil system. They are highly necessary for improving soil fertility, enhancing soil infiltration, nutrient cycling, decreasing runoff and also in the biocontrol of some parasites. A detrimental effect on their existence can also bring about negative impacts on soil health (Zarea and Karimi, 2012).
The risks posed by a herbicide can be estimated from its capability to persist in soil. Herbicides with higher persistence remain in the treated soil for a long period in their active stage. This can lead to another interlinked issues such as leaching (moved away from the soil via rainwater etc.), contamination of nearby water resources, upsetting the soil equilibrium and associated life forms and can also increase exposure risks to humans.
Herbicides and Aquatic Systems
The existence and maintenance of aquatic systems are no doubt of paramount importance as they are one of the major organs of the biosphere. Chemicals like Herbicides have been widely used in aquatic environments for the management of several aquatic and ditch bank weeds for a long time. Such use has been shown to induce negative impacts towards aquatic ecosystems. Many herbicides are toxic to fishes, aquatic invertebrates, crustaceans and also aquatic plants and algae. There are many studies which demonstrate the ability of herbicides to induce mortality in aquatic forms, especially fishes. This can ultimately disrupt the balance, stability and sustainability of aquatic ecosystems.
Contamination of aquatic environments can occur when herbicide residues in the soil are carried to nearby water resources by runoffs and drift from treated sites. Several studies have detected the presence of herbicide residues in common water resources following application even after months. The contamination of such resources can cause secondary toxic effects to human beings, domestic animals and also wildlife, primary targets being the aquatic life forms themselves. In Padaviya farming colony, Sri Lanka, farmers who drank water from glyphosate-contaminated resources followed by leaching from nearby agricultural lands were found to develop CKD or chronic kidney disease (Jayasumana et al, 2015).
Studies have also shown that herbicides can also contaminate marine environments and compromise the functioning of corals by disrupting the photochemical efficiencies of symbiotic algae. This can lead to coral bleaching (whitening of corals).
Herbicides, Plant Diversity and Food resources
Being the primary producers of the biosphere, Plants have an inevitable role in the functioning of different ecosystems. No plants are useless. They nurture, nourish and harbours the diverse life forms on earth. The precipitous development in modern agriculture came with multifaceted consequences as it was engrossed in economic benefits. One among them is the growth of monoculture practices. As a result of the same, farming communities gave prior important to crop and all the others that are not were considered secondary and unwanted. The practice of weed mitigation led to less arduous strategies like chemical control and thus herbicides began to be used for the mitigation of such “unwanted” plants or so-called weeds irrespective of their biological significance. Many such “weeds” have significant medical and consumable value. Also, frequent herbicide uses and weed management in many areas can pose a risk of habitat loss as many weeds are significant habitats for many organisms. Forestry applications for the management of silvicultural weeds also reduce the diversity of forest plant species and can lead to habitat destruction.
Like we discussed before, many plants considered unwanted in modern agriculture are of significant medical and consumable value. They were once considered important sources of nutritious food by indigenous communities and were part of traditional medicine. This knowledge is now being ignored due to the dominance of monocultural systems in agriculture. Many important food sources are destroyed in large scale for reaping economic benefits. In a way this practice contributes to nutritional insecurity in our country.
Presence of herbicide residues in crop produces is another major concern. Detectable amounts of residues have been found in diverse food products including harvested vegetables, cereals and grains. The persistence of herbicide residues in consumables is a serious issue as this can pose serious risks via prolonged ingestion. Various herbicides, especially HHPs like glyphosate, paraquat, oxyfluorfen, pendimethalin etc. have been extracted from vegetables collected from herbicide treated fields in detectable amounts. This is frightening as most of the crop produces available in the markets are collected from herbicide treated fields. We all are at risk of such exposure.
Herbicides and Animal Health
Herbicides, being hazardous chemicals can induce moderate to high toxic effects on various organisms. Studies have revealed that these chemicals can induce severe reproductive, teratogenic, carcinogenic and neurodevelopmental issues in exposed organisms. The leaching and drift of herbicides from agricultural sites to aquatic environments can threaten the aquatic life forms as they are sensitive to chemicals. Studies reveal that herbicides are able to cause alterations in arthropod diversity (Egan et al, 2014). Prolonged exposure to herbicides can affect the growth of significant soil fauna.
Herbicide poisoning in animals, especially herbivores is another frightening issue. Since they directly feed the plant species, the chances of acute toxicity in them are considerably higher. In the Wayanad district of Kerala, nearly 7 goats died of herbicide poisoning followed by ingestion of treated grass.
Egan, J.F., Bohnenblust, E., Goslee, S., Mortensen, D. and Tooker, J., 2014. Herbicide drift can affect plant and arthropod communities. Agriculture, Ecosystems & Environment, 185, pp.77-87.
Jayasumana, C., Paranagama, P., Agampodi, S., Wijewardane, C., Gunatilake, S. and Siribaddana, S., 2015. Drinking well water and occupational exposure to Herbicides is associated with chronic kidney disease, in Padavi-Sripura, Sri Lanka. Environmental Health, 14(1), pp.1-10.
Singh, M. K., Singh, N. K., & Singh, S. P. (2020). Impact of herbicide use on soil microorganisms. Plant responses to soil pollution, 179-194.
Zarea, M. J., & Karimi, N. (2012). Effect of herbicides on earthworms. Dynamic soil, Dynamic plant, 6(1), 5-13