If TCS and TCC accumulate in biosolids-amended soils to the point where soil fertility or groundwater quality is adversely impacted, the agricultural market for these materials could disappear very quickly. This would require other outlets for biosolids disposal, all of which would likely be more costly and less environmentally friendly than land application. This fact, combined with the potential threats to human health posed by TCS/TCC, justifies further study regarding the fate and impact of these compounds in soils where biosolids are land applied.
The proposed work is to investigate the impact of triclosan (TCS) and triclocarban (TCC) on soil microbial community structure, biomass, and diversity. It would also investigate the role that these communities play in degrading TCS/TCC and cycling of nitrogen in soil. Lastly, it would also elucidate environmental conditions that favor biotransformation of the compounds in soils, determine process rates, and identify types of microorganisms involved in the transformation. The hypotheses to be tested are well defined, and the experimental approaches to test the hypotheses are well-designed. Preliminary data collected by the research team suggests that this approach will be successful and yield valuable information.
The experimental plan includes the use of a variety of complimentary techniques for characterizing the community structure in combination with techniques to determine function and potential activity of the microbial populations. Three soil types will be tested in order to include soils representative of those that have had long-term exposure to TCS/TCC, those that have not had long term exposure, those that are primarily aerobic, and those that are anaerobic.
This project examined the consequences of land application of treated sewage sludge (biosolids) containing triclosan (TCS) and triclocarban (TCC), antimicrobial compounds widely used in consumer products such as antibacterial hand soaps. These chemicals have been widely detected in biosolids at reasonably high levels and they do not degrade quickly in soils. Because of their antimicrobial properties, the primary research question was whether their addition to soil would damage key ecosystem functions performed by soil microorganisms, such as nitrogen cycling. We found that TCC and TCS were detectable in all biosolids sampled at levels from 2-30 mg/kg and that they were not effectively removed during conventional sludge treatment. Although the compounds were retained to some extent by biosolid amended soils, they were released to water sufficiently to be of potential concern. Three types of methods were used to determine the effect of elevated TCC and TCS concentrations in biosolids on soil microorganisms. First, the ability of the soil microorganisms to process various types of nitrogen (e.g., ammonia, nitrite) were measured. Second, chemical indicators of microbial stress (e.g., starvation) were monitored. Finally, genetic markers specific for particular types of microbial populations (e.g., bacteria that oxidize ammonia) were measured. Tests were conducted by spiking three concentrations of TCS into soil alone or soil amended with biosolids. The varied methods provided a consistent picture of the effects of these compounds. Addition of TCS to soil or biosolid amended soil decreased nitrogen cycling potential, increased measures of microbial stress and decreased populations of particular organisms important in nitrogen cycling. However, amending soils with biosolids, which contain large amounts of available nitrogen and carbon, greatly stimulated microbial populations and activity levels so that adding TCS containing biosolids at the highest spiking level tested still provided a net benefit to soil nitrogen cycling.
