Metabolic and cometabolic degradation of herbicides in the fine material of railway ballast

Abstract

Microbial degradation of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) and mineralization of 4-chloro-2-methylphenoxyacetic acid (MCPA) were studied in soil samples taken from the ballast layers of three Swedish railway embankments. The degradation of diuron followed first-order kinetics and half-lives ranged between 122-365 days. The halflives correlated strongly with microbial biomass estimated by substrate-induced respiration (SIR; R = -0.85; p<0.05) and with the amount of organic matter measured as loss on ignition (R = -0.87; p<0.05). Accumulation of the metabolites DCPMU and DCPU was observed in all samples and these were only detectably degraded in the sample with the highest SIR. Addition of ground lucerne straw to the ballast samples stimulated microbial activity and led to increased formation of metabolites, but further transformation of DCPMU and DCPU was not enhanced. Mineralization of MCPA followed growth-linked kinetics and the time for 50% mineralization was 44.5 ± 7.1 days in samples of previously untreated ballast. In samples of ballast that had been previously treated with the herbicide formulation MCPA 750, the time for 50% mineralization was reduced to 13.7 ±11.3 days. The number of MCPA degraders, quantified using an MPN technique, was clearly increased but highly variable. An average yield of 0.18 cells pg -1 of MCPA was estimated from the kinetic data. The yield estimates correlated with the amount of nitrogen in the ballast, indicating that mineralization of MCPA was nitrogen-limited in the railway embankments studied. This has practical implications for weed control using herbicides on railways.

Evidence for the importance of litter as a co-substrate for MCPA dissipation in an agricultural soil

“…A similar effect of rice straw addition on MCPA degradation in a perfused soil system was reported by Duah-Yentumi and Kuwatsuka (1982). Berger (1999) and Cederlund et al (2007) also found that the addition of straw enhanced the degradation of diuron by stimulating microbial activity. Poll et al (2010a) found significantly more MCPA-derived C bound to microbial biomass in litter-treated than in litter-untreated soils and suggested that litter addition favoured activity and C assimilation of MCPA-degrading microbes.…”

A new concept for reduction of diffuse contamination by simultaneous application of pesticide and pesticide-degrading microorganisms

“…Herbicides are applied regularly to railway embankments for weed control and environmentally friendly use of herbicides in areas with such soils is a difficult challenge. Studies of the microbiology of railway embankments have shown that MCPA-degraders are often present there, but that both the MCPA-degradation capacity and the general microbial biomass and activity are often very low (Cederlund and Stenström 2004;Cederlund et al 2007). Fast drainage through coarse soil material and microbial biomass and activity being located mostly in hot-spots, probably due to uneven distribution of organic material (Cederlund et al 2008), both contribute to increasing the risk of pesticide leaching.…”

“…Furthermore, the coarse material of e.g. railway embankments favours dry soil conditions and this, together with often low temperatures during large parts of the year in Sweden, limits degradation (Cederlund et al 2007). In our study, low degradation rates of MCPA were found in the uninoculated sand, with 76-102% of the initial amount still remaining after 3 days.…”

Stormwater quality management in rail transportation — Past, present and future

“…This could be explained by the coarse texture, low organic and nutrient contents of ballast and embankment materials. Therefore, PAHs and herbicides used in railway embankments usually had better mobility and prolonged persistence (Cederlund et al, 2007). Many of these organic compounds are attached to organic matters in soil.…”