Harald Cederlund

Comparative characterization of digestate versus pig slurry and cow manure – Chemical composition and effects on soil microbial activity

The growing number of biogas plants in Europe has resulted in increased production of nutrient-rich digestate with great potential as fertilizer for arable land. The nutrient composition of digestate varies with the substrate treated in the biogas plant and may contain compounds that stimulate or inhibit soil microbial activity. This study compared 20 digestates (D) with 10 pig slurries (PS) and 10 cow manures (CM) regarding their chemical content and their effect on soil microbial activities, i.e. potential ammonia oxidation rate (PAO) and soil respiration. The results showed no significant differences within the D group when divided based on substrate type. i.e. manure dominated vs. other organic waste materials in any of the tests. In general D contained significantly higher concentrations of ammonium while the concentrations of total carbon and volatile fatty acids were higher in PS and CM than in D. The D showed both stimulating and inhibiting effects on PAO, while all CM and all PS except one showed inhibiting effects on PAO. However, PAO activity was negatively correlated with the content of volatile fatty acids in the residues indicating that these compounds may be the cause of the inhibition. The maximum respiration activity (hpeakmax) was lower and the time point for the maximum respiration activity (tpeakmax) occurred earlier for D compared with CM and PS. This earlier peak time could be indicative of a high proportion of easily degradable carbon in D compared with PS and CM. However, the utilization rate of carbon, i.e. the proportion of added organic C converted to CO2-C during 12days, did not differ significantly between D, PS and CM, indicating that overall carbon quality in the different fertilizers was still roughly comparable. In short, our results suggest that digestates were different compared with PS and CM but without posing a higher risk with respect to their impact on soil microbial activity.

Environmental problems with the use of diuron on Swedish railways

Lennart Torstensson, Harald Cederlund, Elisabet Borjesson and John Stenstrom, of the Swedish University of Agricultural Sciences in Uppsala, outline the problems caused by using diuron along railway tracks in Sweden.

Functional microbial diversity of the railway track bed.

Railways constitute relatively unexplored microbial habitats. Little is known about the amounts, activities or distributions of microorganisms and their associated heterotrophic capabilities on railway embankments. The aim of this study was to investigate the microbiology of two Swedish railway tracks in order to fill some of the gaps in the available information. We estimated microbial biomass by means of substrate-induced respiration, microbial activity as basal respiration (BR) and as a kinetically derived parameter (r) hypothesised to correspond to the active fraction of the microbial biomass. It was confirmed that the microbial biomass and activity were low as compared with agricultural soils and that their distributions were distinctly positively skewed. Spatial Kriging revealed that covariance structures were sustained on a scale smaller than the employed sampling grid (<1 m). Substrate richness (SR), as measured with Biolog ECO plates, was used as a quantitative measure of functional diversity. SR correlated to microbial activity and SIR, indicating that functional traits were lacking where the microbial biomass was low or less active. The dependence of microbial activities on basic soil characteristics were inferred by separately designed general linear models. Water content was found to be the most important factor moderating basal respiration and functional diversity, whereas the organic matter content was identified as the most important covariate for SIR. Multivariate analysis of the carbon source utilisation patterns of the Biolog plates with equivalent average well-colour development revealed homogenous substrate utilisation among samples. This indicates that the microbial functional potential is randomly distributed in the railway track bed. In combination, our findings imply that the ecosystem functionality of railway embankments may be seriously hampered as compared with agricultural soils. This has consequences for the risk assessment of herbicides applied to railways.

Degradation and leaching of fluroxypyr after application to railway tracks.

Fluroxypyr is an auxin-type herbicide used for postemergent control of broad-leaved weeds in agriculture and in nonagricultural environments such as railways. The overall aim of this study was to assess the potential environmental impact from fluroxypyr application to railway tracks and to elucidate some of the factors that control its environmental fate. In laboratory studies, we examined the degradation of fluroxypyr and the formation of its metabolites fluroxypyr-methoxypyridine (F-MP) and fluroxypyr-pyridinol (F-P) in soil from two Swedish railways. We also investigated the degradation and leaching of fluroxypyr in three different railway plots treated with fluroxypyr (360 g ae ha). The half-life of fluroxypyr in soil samples ranged between 28 and 78 d. An estimated mean 48.6 ± 20% of the fluroxypyr was converted into F-P and 8.0 ± 2% into F-MP. The main metabolite, F-P, was rapidly degraded, with an average half-life of 10 ± 5 d. However, F-MP was not degraded to a significant degree in any sample, resulting in slowly increasing concentrations throughout the experiment. This pattern of relatively rapid degradation of F-P and slow accumulation of F-MP was also observed in the field. The persistent nature of F-MP may be of concern if fluroxypyr is used repeatedly at the same location. Fluroxypyr was detected in the groundwater beneath the track at all three locations studied in concentrations exceeding the EU limit of 0.1 μg L for pesticides in drinking water, and F-P was detected in the groundwater at two of three locations. The most important factor controlling fluroxypyr degradation rate in soil was the soil water content, which modulated microbial activity and presumably also fluroxypyr availability to microorganisms. Our findings imply that fluroxypyr may not be a suitable herbicide for weed control on railway tracks.