Degradation of Pesticides

What happens to pesticides once they have been added to the soil?

Chemicals, such as pesticides, added to the soil are broken down in a similar way to organic matter. They are oxidised by the microorganisms that produce the appropriate enzymes. Pesticides generally have very complex structures and most are converted to several intermediate organic compounds before finally being converted to carbon dioxide and water.

When these kinds of chemicals are degraded metabolically, the microorganisms involved gain energy and carbon from the oxidation of the chemical. This allows the microorganisms to grow and multiply, and the size of the microbial population may increase as more of the chemical is degraded. Sometimes, microorganisms capable of degrading a particular chemical grow and multiply until there is a higher proportion of these microorganisms in the soil. When a community of microorganisms in the soil adapts in this way, the chemical can be broken down more rapidly than it would have previously. This can be a problem when rapid degradation of a pesticide may reduce its effectiveness against pests.

Some chemicals added to the soil are degraded by the microorganisms without any particular benefit to the organism such as nutrients or energy supply. The population of microorganisms remains constant while the concentration of the chemical in soil decreases. The different modes of interaction between synthetic chemicals and microorganisms are a result of the enormous diversity of organisms in soil.

Why are some chemicals easier to degrade than others?

The ease with which a chemical is degraded depends on the complexity of its structure. More complex structures degrade more slowly either because fewer microorganisms in the soil produce enzymes capable of degrading them or because of the inaccessibility to microbes.