08/13/10

Researching Plant Uptake of Microconstituents

Two recent media reports cited studies that have found uptake by plants of traces of a few microconstituents found in biosolids.

Two recent media reports cited studies that have found uptake by plants of traces of a few microconstituents found in biosolids. 

Chenxi Wu and colleagues at the University of Toledo (OH) reported in Environmental Science and Technology on a greenhouse study testing soybean plant uptake of three pharmaceuticals and the antimicrobial chemicals triclosan and triclocarban.  Uptake was measured for two of the pharmaceuticals, as well as both antimicrobials.  However, maximum plant concentrations were in the tens of parts per billion (equivalent to a couple of minutes in ~32 years).  The study also found significant degradation of the trace chemicals in the biosolids and soil mixture, a fate of many trace chemicals according to several other studies.

Meanwhile, Scientific American reported online that a presentation at a meeting of the Ecological Society of America (http://www.scientificamerican.com/article.cfm?id=silver-beware-antimicrobial-nanoparticles-in-soil-may-harm-plant-life) found reduced growth in one plant species and reduced soil microbial biomass when silver nanoparticles were added with biosolids to the soil. 

In both studies, there is a well-known problem with methodology:  the contaminants of concern were spiked into the biosolids/soil mix.   This methodology is suspect and yields results that are probably not representative of real-world field conditions, according to several experienced biosolids researchers who have examined the fate of trace metals and organic chemical contaminants in biosolids for many years.  For example, research reported by O’Connor et al. (1990) compared the fate of trace chemicals in chemical spiked soils with the fate of the same concentrations of trace chemicals delivered to soils in biosolids.  The latter are considerably less bioavailable, because of equilibration and sorption with the biosolids matrix.

Details matter.  Clearly, if farmers using biosolids experienced the kind of dramatic decline (20%) in plant growth that the second study reported, they would not use biosolids.