Mineralization of the herbicide MCPA in urban soils is linked to presence and growth of class III tfdA genes

Mineralization and sorption of 14C-ring labeled herbicide 2-methyl-4-chlorophenoxyacetic acid (MCPA) were quantified along with the tfdA gene abundance in 7 different soils. The soils tested were five gravel soils from urban locations, one soil from the embankment of a railway track, and finally an agricultural soil as a control. The mineralization experiments were performed with a concentration of MCPA of 5 mg/kg and incubated at 10 °C for a period of 60 days.With Kd values ranging from 0.04 to 0.41 l kg−1 the sorption experiments revealed that binding of MCPA to the six gravel soils was lower compared to the control soil which had a Kd value of 0.91 l kg−1. The potential for MCPA mineralization varied from less than 5 to over 55% mineralized in 60 days. The most rapid MCPA mineralization was observed in the soil from the Danish railway tracks with 55% mineralized after only 18 days. The mineralization data was fitted to degradation kinetic models, which indicated that growth occurred as a response to MCPA degradation in most of the soils.Soil DNA was extracted and tfdA genes responsible for the first step in MCPA degradation were quantified by real-time PCR (qPCR) at appropriate time points throughout the mineralization experiments. Indicating growth of specific MCPA degraders, the abundance of class III tfdA genes showed an increase during MCPA mineralization in those soils able to mineralize MCPA.These findings emphasize the importance of the presence of microorganisms that are able to readily degrade MCPA, to avoid groundwater leaching following use on urban gravel areas that possess low binding ability of the compound.

Research highlights
► Mineralization of the herbicide MCPA in urban soils is linked to presence and growth of tfdA genes.
► MCPA is degrade at very variable rates in urban gravel.
► MCPA degradation is dependent on the presence of microorganisms that carries the class III degradative gene tfdA.
► MCPA is degraded fastest in the gravel from railway that previously has been treated with MCPA.