Short and long-term effects of wood ash on the boreal forest humus microbial community

The short-term effects of loose and hardened wood ash on the coniferous forest humus layer microbes were studied 1–3 years after fertilization. The experiment was performed using two fertilization levels (3 and 9 t ash ha−1) and repeated in two coniferous forest stands of different site fertility. It was hypothesized that the effects of hardened wood ash on soil microbes are of less magnitude when compared to loose ash due to the slower dissolution rates. The long-term effects of loose ash (3 t ash ha−1) were studied in four forest stands of different site fertility 18 years after ash application. In order to study ash effects, the microbial activity (basal respiration- and thymidine incorporation rates) and microbial community structure (PLFA pattern) were determined. The results showed that irrespective of the forest site fertility, ash fertilization induced the same responses in the humus layer. It raised the microbial activity and changed the community structure. The changes were related to the dose and form of ash applied. Applying the same fertilization rate induced comparatively more changes to the loose wood ash sites than hardened wood ash sites, due to the detected slower dissolution of hardened ash into the humus. The effects of wood ash were long-term. Changes in the humus microbial activity and PLFA pattern were still detectable after 18 years.