The Microbiology of Railway Tracks Towards a Rational Use of Herbicides on Swedish Railways

Cederlund H. 2006. The microbiology of railway tracks – towards a rational use of herbicides on Swedish railways. Doctoral thesis. Swedish railways are regularly treated with herbicides in order to keep the track beds free from weeds. However, finding appropriate preparations and dosages that provide a good weed control but that are still environmentally acceptable, has proven to be difficult. This thesis investigates some fundamental aspects of the microbiology of railways, such as amounts, activities and spatial distributions of microorganisms in the track bed, in order to provide the knowledge base for a more informed use of herbicides on railway tracks. The railways investigated were characterized by overall low but highly variable rates of respiratory activity. Distributions were positively skewed and autocorrelation distances were short. Microbial biomass measured as substrate-induced respiration (SIR), microbial activity measured as basal respiration and a kinetically derived parameter r corresponding to the active fraction of the SIR covaried significantly with the organic matter content of the ballast. For basal respiration and r, the most important covariate was the water content of the ballast and microbial activity was higher when determined in the autumn during moist conditions. The functional diversity, assessed as substrate richness on Biolog ECO plates, was low but highly variable and covaried with SIR, indicating that functional groups of microorganisms were missing where the microbial biomass was low. The substrate utilization patterns were homogeneous among the railway samples, which suggest that heterotrophic microorganisms are randomly distributed on railway tracks. Degradation of diuron in fine material of railway ballast followed first-order kinetics and thus did not support growth of degrading microorganisms. The metabolites DCPMU and DCPU were formed in all samples and accumulated in most of them. The mineralization of MCPA followed growth-linked degradation kinetics and was enhanced where the railway track had been previously treated with MCPA. This enhancement was related to higher numbers of MCPA-degraders and higher specific growth rates (µ) of these in the previously treated track. The yield (Y) correlated to the nitrogen content of the railway, indicating that the formation of microbial cells from MCPA on railways is nutrient limited. These findings indicate that it would be sensible to use metabolically degradable herbicides and to apply them using weed-seeker techniques in order to decrease the likelihood of groundwater beneath the track being contaminated. Abbreviations/terms/common names AOB ammonia oxidizing bacteria AWCD average well-colour development CLPP community-level physiological profile CTC 5-cyano-2,3-ditolyltetrazolium …