Effect of Cd-containing wood ash on the microflora of coniferous forest humus

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, exceed s the level allowed for fertilizers (3 mg kg(-1)) used in agriculture. To i nvestigate the combined and separated effects of Cd and ash on the forest h umus microflora, 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)), we re applied at a fertilization level of 5000 kg ha(-1) in a laboratory micro cosm study. The trial consisted of 60 microcosms (five replications per tre atment), which were incubated in darkness at +20 degrees C and a constant r elative air humidity of 60%. After two months the humus in the microcosms w as sampled. Analyses of CO2 evolution to measure the overall microbial acti vity and of phospholipid fatty acid (PLFA) pattern to measure microbial com munity structure were performed. The substrate-use patterns of Biolog EcoPl ates were analyzed as a measure of bacterial functionality. Finally the bac terial H-3-thymidine incorporation in the presence of different concentrati ons of Cd and the number of colony forming units (cfu) of bacteria on nutri ent agar in the presence of 0, 5 and 20 mg Cd l(-1) agar were applied to me asure Cd tolerance. The use of pumice (pH of humus under the pumice 4.0) di d not induce any changes in the above variables compared to two untreated m icrocosms (humus pH 3.9). Pumice was therefore used to distribute the Cd ev enly over the humus surface in order to estimate the possible effect of Cd without ash (pH of humus under the ash 7.0). The application of ash increas ed the microbial activity, changed the PLFA and substrate-use patterns and increased cfu compared to the humus under pumice. The form and level of Cd in the ash had no further effect on this result. In the humus under pumice the level, but not the form of Cd decreased the microbial activity and chan ged the PLFA pattern compared to the unspiked pumice. None of the treatment s induced bacterial tolerance to Cd. Ash thus protected the humus microflor a from the harmful effects of Cd. (C) 2000 Federation of European Microbiol ogical Societies. Published by Elsevier Science B.V. All rights reserved.