by Nahed Msayleb
The use of pesticides to control plant diseases that live in the soil affects beneficial microorganisms that are necessary for nutrients to circulate and hence limits soil fertility. Accordingly, the need for more eco-friendly ways to control plant diseases is growing. A potential alternative to the synthetic pesticides that deplete soil biodiversity and fertility is ozone gas: a powerful oxidation agent that could kill microorganisms without having detrimental effects on the environment.
Soil is a natural capital, which is by definition “a stock of natural ecosystems that yields a flow of valuable ecosystem goods or services into the future”. This means that soils are stocks that can provide a flow of plant production, and this flow can be indefinitely sustainable. Soil also provides many services as a system including plant anchorage and support, as well as decomposing and mineralizing plant debris (wastes) which renders nutrients available to plants and the cycle goes on. This flow of services requires that the soil functions as a system, and its sustainability hinges on its biodiversity.
The use of synthetic pesticides in the treatment of soil pests (disease-causing organisms), especially broad-spectrum and soil sterilizing fumigants (e.g. methyl bromide) affects the soil fauna and flora (i.e. beneficial microorganisms that are non-pests, or non-disease causing). This disrupts the cyclic processes that make the soil a functional system and natural capital. Among the most important beneficial microorganisms are those that perform the following processes: nutrient mineralization and/or transformation (e.g. nitrobacteria and nitrozomonas that change the chemical form of nitrogen and make it available to plants); nutrient fixation into the soil (e.g. nitrogen fixing bacteria); nutrient mobilization and soil aeration (e.g. soil worms); and those playing a symbiotic role with plant roots helping in nutrient absorption (e.g. rhizobacteria, mycorrhizal fungi). The harm of synthetic pesticides also extends beyond the location of their application. Pesticides residues and byproducts deposited in nature could be transported by water, air, soil and animals to undesired places where they could continue harming the environment.
Soil processes may not be disturbed when pesticides are first applied because the soil has a certain level of resilience. Resilience helps the soil to recover from the damage of pesticides and to continue providing ecological services. Resilience depends on the diversity of soil biota, along with other soil characteristics like texture, structure and organic matter content. The stock of soil fertility and services becomes threatened when soil biodiversity declines and resilience drops.
When plant diseases are treated with synthetic pesticides both the disease-causing organisms and beneficial microorganisms are killed. With the decline in beneficial microorganisms, disease organisms may grow unchecked and cause plant growth to suffer more. The persistent dependence on synthetic pesticides can disrupt natural biological control systems and may be associated with pest outbreaks, widespread resistance development, adverse effects on non-target organisms, and detrimental effects on the environment and human health. When pesticides are used, the need for pesticides grows over time in order to control pests, while soil fertility and productivity drop. In other words, the use of synthetic pesticides is considered a reinforcing feedback loop, whereby the bad effects are exacerbated and the stock is depleted (Fig. 1). To offset this negative impact, a restoring intervention with balancing feedback (negative feedback loop that offsets the effect of the pesticide use on the stock) should be used to reinforce the stock and to stabilize soil biodiversity and productivity. An ideal balancing intervention could be the control of pests below the damage threshold, without breaking their balance with beneficial microorganisms (that is depleting the soil biodiversity).
My research topic was about investigating the potential of ozone gas (O3) to treat soil pathogens and to boost the sustainability of soil productivity. Ozone was selected based on its characteristics as a potent oxidant with strong germicidal properties, and given that it has been implemented successfully against numerous pathogens including viruses, bacteria, fungi, protozoa and metazoa. It is often used to disinfect drinking water and wastewater, and disinfest ships ballast water due to its oxidizing properties. Ozone has also been applied to prevent mold on stored corn, and to degrade mycotoxins. Postharvest processing of fruits and vegetables with ozone gas or ozonated water inactivates pathogens and spoilage microorganisms. During the ozonation process, both beneficial microorganisms and disease-causing organisms are affected. However, the rate at which ozone is needed to put soil pests under control does not need to reach the level of complete soil sterilization. The goal of soil ozonation is merely to bring the populations of disease organisms into balance with beneficial microorganisms.
In contrast to other disinfection methods and synthetic pesticides used in pest control, the use of ozone as a disinfection agent has the advantage that it does not produce pollutants, because its rapid decomposition only produces oxygen. This means that no secondary harm is exerted on the soil biodiversity through residual toxins, in contrast with synthetic pesticides. In addition, ozone has a short half-life in the soil – in the order of minutes – before it decomposes into oxygen. The only drawback of ozone use in soil treatment is its health hazard on the appliers if inhaled. This could be easily avoided by using proper protective masks and appropriate training.
Three pathogens were selected for this research based on their economic importance, and the variety that they represent: plant-parasitic nematodes, which are microscopic round worms that cause severe crop yield losses; Phytophthora sojae, a predominant soybean disease that causes root and stem rot; and Fusarium oxysporum, which causes Fusarium wilt, an economically important disease in hydroponic systems.
Ozone gas was generated by corona discharge from oxygen or air and delivered into a reactor including the soil sample with the targeted disease organism (Fig. 2). Samples were treated with different amounts of ozone at two different temperatures to assess the effects of both the dose and temperature on how well ozone treats soil diseases.
The findings of this research clearly indicate that ozone may be an efficient and sustainable alternative to synthetic pesticides, including gas fumigants, nematicides, fungicides, and the treatment of Fusarium wilt. Ozone gas could be applied through fumigation under a tarp, like the application of methyl bromide. Soil ozonation may be adopted in organic agriculture, and may be suitable to integrate with cultural practices, integrated pest management and the treatment of resistant pests varieties where necessary and applicable. These are promising results for the system of soil biodiversity, where ozonation soil treatment proved to be a viable controller of soil pests, without eradicating beneficial soil organisms. Hence, ozonation could be an intervention with a balancing feedback effect on the stock of soil biodiversity, and helps to maintain the sustainability of soil productivity.
Nahed Msayleb is a 2010 fellow of the Fulbright Science & Technology Award, from Lebanon, and a PhD Candidate in Sustainable Agriculture at Iowa State University at Ames, in the department of Agricultural and Bio-systems Engineering.