Effects of decomposition of leaves on bacterial biomass and on palatability to Lumbricus terrestris L

The decomposition of leaves of Taraxacum officinale, Hypericum perforatum, Dipsacus silvester and Miscanthus sinensis was monitored under laboratory conditions (20°C, 90 to 100% relative humidity) for 2 to 4 weeks. At weekly intervals, weight loss, C-to-N-ratio, secondary plant metabolites, bacterial biomass and palatability to the earthworm Lumbricus terrestris were determined. The first order constants for dry weight loss were 0.092, 0.012, 0.005 and 0.004 d −1 for T. officinale, H. perforatum, D. silvester and M. sinensis, respectively. Secondary metabolites in green leaves of all plants mainly comprised polyphenolic compounds (bitter drugs, coumarins, flavonoids). During initial decomposition, the overall pattern of secondary plant metabolites was characterized by a qualitative and quantitative reduction. After one week of decomposition, the patterns of secondary plant metabolites became similar in all species and remained visually unchanged during further decomposition. A potential inhibition of bacterial growth and palatability to L. terrestris could be demonstrated in bioassays with extracts of secondary plant metabolites from H. perforatum and D. silvester. On plants, however, an inhibition of bacterial growth and a reduction of palatability to L. terrestris was only obtained during the initial decomposition of senescent leaves of H. perforatum which is known to possess toxic secondary plant metabolites. The bacterial growth and the palatability to L. terrestris on leaves of T. officinale, D. silvester and M. sinensis were mainly influenced by the C-to-N-ratio of the leaves.