Mechanisms of pesticide
degradation in compost

By Craig Coker, Composting Specialist
NC Div. Of Pollution Prevention & Environmental Assistance

Rynk, R., et. al., "Occurrence, Degradation and Fate of Pesticides During Composting", Compost Science and Utilization, Vol. 7, No. 4, Autumn, 1999 and Vol. 8, No. 1, Winter 2000

The authors reviewed the findings of research reported in the currently available literature regarding the occurrence and transformations of pesticides through the composting process and the use of compost.

Pesticides degrade or move within the environment of a composting pile via numerous physical, chemical and biological mechanisms, including adsorption (binding to solid particles), leaching, volatilization (vaporization of a solid or liquid into a gas), chemical transformations, and biological degradation processes.

In general, the mechanisms responsible for pesticide degradation during composting are nearly the same as those that take place in soils. While the behavior of a pesticide in soils should be a reasonable estimate of how that pesticide would behave in composting, there are important differences. 

Composting involves higher temperatures, more organic matter, and generally more vigorous biological activity. Thus, processes such as adsorption, humification, biological transformations, and perhaps volatilization are more significant in composting.

One of the most important factors controlling the adsorption of pesticides is the organic matter (O.M.) content of soils (the higher the O.M., the higher the adsorption). Leaching is less likely to occur with composts due to the higher organic matter (and thus, higher adsorption); however, sediments of compost particles carried by runoff and wind are likely to hold more pesticides than soil particles.  It is not yet clear if composting induces more or less volatilization of pesticides. On one hand, the increased adsorption suggests less volatilization. On the other hand, processes such as turning windrows, forced aeration and larger, more porous composting media would favor increased volatilization.

Biological and chemical transformations also influence the fate of pesticides in composting. Microorganisms can only metabolize pesticides if they are bioavailable (e.g. water soluble and not adsorbed) and if they have a chemical structure compatible with the organisms’ enzymes that catalyze the biodegradation. Chemical transformations occur less frequently, but occur to initiate the degradation process, producing intermediate compounds that are more susceptible to biological degradation and eventual mineralization.

Research.  Investigations of pesticide residues in composting feedstocks and finished compost detected few of the target pesticides. The compounds that were detected occurred at low concentrations. The majority of the compounds detected were insecticides in the organochlorine category, including chemicals that have been banned in the U.S. for years. Generally, organophosphate and carbamate insecticides and most herbicides were rarely detected. 

Based on pesticide concentrations before and after composting, organochlorine pesticides were more resistant to biodegradation during composting. Mechanisms of degradation include: mineralization, partial degradation to secondary compounds, adsorption, humification, and volatilization. In general, research results suggest that the pattern of pesticide degradation during composting is similar to the degradation observed in soils, and that degradation did not always occur at accelerated rates during composting.

The majority of studies of the occurrence and degradation of pesticides in composting have been conducted on yard trimmings composts (herbicides are less well-studied given their lower rate of use). Rynk, et. al. report on studies conducted in Portland, Oregon; Westchester County, New York; Seattle, Washington; Illinois; Massachusetts, and New Jersey. 

In Portland, low concentrations of pentachlorophenol and chlordane were consistently detected. In addition, dieldrin, DDT, DDE, aldrin, chlorpyrifos, and dinoseb were detected in some samples. None were detected at concentrations above regulatory levels. 

In the Westchester County study, chlordane, lindane, captan and 2,4 – D were detected. Only chlordane was detected above the minimum USDA tolerance level for food. In New Jersey, only chlordane was found at detectable levels. 

The Seattle study found nine pesticides in the raw feedstocks, and eight in the finished compost, none of which were herbicides. All pesticides degraded after 90 days of composting, except dieldrin, which concentrated by 27%. 

The Illinois study found increased concentrations of methoxychlor, DDE, 2,4,5-T, and trifluralin in finished compost, but all compounds were well below applicable regulatory criteria.

Based on these studies, pesticide residues in compost do not appear to be a concern. Many of the detected compounds (chlordane, dieldrin and DDT, for example) have been banned in the U.S. for many years. Their occurrence in these studies suggest that it is difficult to "cleanse" the environment of these old pesticides. The absence of organophosphates, carbamates and most herbicides in composting feedstocks implies that these classes of pesticides are well degraded, diluted or lost to the environment before the feedstocks were collected for composting.

Composting tends to substantially decrease the concentrations of most pesticides, but the extent of degradation depends on the characteristics of the pesticide. Some compounds were observed to increase in concentration in finished compost, suggesting that these compounds degrade more slowly than the underlying organic matter. 

In general, the patterns of pesticide degradation in composting parallel the patterns found in soils. Those compounds that persist in soils tend to be resistant during composting as well. Those that disappear quickly in soils also disappear during composting.

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