In addition, all treatments but the control received 6 g Cr/ha/year as a result of the supplementary mineral
N fertilization.
The results presented in this paper show that there are only small differences between the treatments in
terms heavy metals in soil. On the other hand, in terms of fertilizing effect, the biogas residue performed
almost equally well as mineral fertilizer during.
Regarding the metal content of the year 2014 harvest of barley, there was however a trend of increased
metal uptake of nickel, cobalt and to some extent also chromium from the soil fertilized with sewage
sludge (Fig. 1). Even if this trend was not statistically significant it seems like the bioavailability of these
components has increased as a result of the addition of the sewage sludge. The increased uptake by the
crop can be the result of increased mobilization of the metals under the different conditions created by the
treatment regimes. However, for cadmium, both the plots fertilized with mineral fertilizer and sewage
sludge showed a trend of elevated uptake in the crop compared to the unfertilized control plots and plots
fertilized with biogas residues. The barley from both the unfertilized control plot and the biogas residue
plots shoed similar cadmium levels even though no external sources of cadmium has been introduced to
the control plots during the last 16 years.