A microcosmos study on the effects of cd-containing wood ash on the coniferous humus fungal community and the cd bioavailability

Background and Aims

The use of wood ash in forestry has been questioned because the cadmium (Cd) concentration of ash, which varies between 1 and 20 mg kg^-1 ash, exceeds the level allowed for fertilizers (3 mg kg^-1) used in agriculture. To investigate the effects of Cd and ash on the fungal community composition and Cd bioavailability of the humus layer of boreal, coniferous forests, pumice or wood ash, spiked with a water soluble (CdCl₂) or insoluble (CdO) form of Cd at three levels (0, 400 and 1000 mg kg^-1), were applied at a fertilization level of 5000 kg ha^-1 in a laboratory microcosm study.

Methods

After 2 months, the humus in the microcosms was sampled and extracted for total DNA to detect changes in the fungal community by using polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE) techniques. PCR was performed using the fungal 18S rDNA primers FR1 + FF390 and FR1 + NS1. The bioavailability of Cd was measured with a bacterial biosensor(Bacillus subtilis BR 151/pT0024) emitting light in the presence of Cd.

Results

Using the primer pairs FR1 + FF390 and FR1 + NS1, resulted in over 35 and 15 DGGE bands, respectively. Both primer pairs detected an ash, but no Cd effect. When using FR1 + FF390, a higher fluorescence was observed in one DGGE band of all ashed samples compared to the pumiced samples. With the primer pair FR1 + NS1, the ashed samples had a DGGE band which was not visible or only faintly visible in the pumiced samples. In addition, one DGGE band disappeared from the ashed samples. Humus layer water extracts showed that the Cd added with the pumice was in a bioavailable form. The luminescence intensity of the biosensor was dependent on the form and level of the Cd added. No luminescence was detected when the Cd was added with the ash.

Conclusions

Ash fertilization altered the humus layer fungal community, whereas the level and form of additional Cd in the ash had no influence because it was not bioavailable.