Relationships between phenolics and soil microorganisms in spruce forests:Significance for natural regeneration

Plant-produced phenolic compounds inhibit soil microbial activity and are s uspected to be involved in the failure of natural regeneration of French su balpine Norway spruce (Picea abies) forests. This work evaluated relationsh ips between phenolic compounds and soil microorganisms in humus collected i n spruce forest during winter and summer seasons. After having identified a nd quantified different phenolics from humus samples, we incubated another set of humus samples with 10 times the concentration of naturally occurring phenolics, and effects on the density of microorganisms were compared to h umus with no addition of phenolic compounds. Furthermore, in order to follo w the degradation of phenolic compounds by microbes, the concentration of p henolics in the incubated humus samples was determined by HPLC after three and six days of incubation. The results indicate that humus microorganisms use phenolics as a carbon source, since almost all phenolic compounds had d isappeared after six days of incubation. Addition of phenolic compounds to the humus samples also affected the soil microbial populations so that bact eria were inhibited in the humus collected both in the winter and in the su mmer. However, there were differences between the experiments. Fungi and ce llulose hydrolyzers were stimulated in the winter humus experiment, while f ungi were unaffected and cellulose hydrolyzers were inhibited in the summer experiment. The addition of phenolic compounds stimulated ammonifiers in t he summer experiment. We anticipate that the role of soil microorganisms in the problem of natural regeneration failure is probably more important tha n previously thought.