Influence of Cognettia sphagnetorum (Enchytraeidae) on birch growth and microbial activity, composition and biomass in soil with or without wood ash

In this laboratory study using microcosms with seedlings of silver birch (B etula pendula), we explored whether Cognettia sphagnetorum (Enchytraeidae) can retain its important role of accelerating decomposition processes in so ils and stimulating primary production under disturbance. We established sy stems with or without wood ash amendment (first-order disturbance) in the s oil, either in the presence or absence of C. sphagnetorum. To test whether the systems treated with wood ash are more sensitive to an additional distu rbance than the ash-free systems, the microcosms were later on disturbed by drought. To determine the influence of two disturbances on the enchytraeid s and populations of other fauna, and the possible changes in the system fu nctioning, measurements were made of the growth of birch seedlings, foliar N concentration, composition and biomass of soil microbial communities and leaching of N and dissolved organic carbon from the microcosms. Both wood a sh application and drought exerted a clear negative influence on the popula tions of C. sphagnetorum. However, populations of this species were very re silient and recovered rapidly after drought in the ash-free soils. In the a sh-free soils C. sphagnetorum tended to improve birch growth, increased the N content of the birch leaves, and decreased the root to shoot ratio. Howe ver, in the ash-treated soils enchytraeids had negative effects on these pa rameters. C. sphagnetorum impacted on neither N and C leaching nor soil mic robes, whereas wood ash decreased microbial biomasses and changed their com munity structure (as determined by phospholipid fatty acids method and dena turing gel electrophoresis) and substrate utilisation potential (Biolog met hod). It was concluded that C. sphagnetorum can retain its influential role under varying environmental conditions, but that the stimulating or retard ing effects of this species on system functioning can be context dependent.