Effects of leaf litter consumption by millipedes (Harpaphe haydeniana) on subsequent decomposition depends on litter type

Most estimates of litter decomposition rates do not account well for the effects of soil macrofauna, and so may not accurately represent decomposition in ecosystems in which litter-transforming soil fauna are abundant. In coastal rainforests of British Columbia, Canada, millipedes are the dominant macrofauna and consume a substantial proportion of annual leaf litterfall, most of which is egested as faecal pellets. Little is known about the fate of millipede faecal pellets, which hinders accurate estimation of rates of litter decomposition in these ecosystems. We assessed the influence of feeding by the millipede (Harpaphe haydeniana) on decomposition of broadleaf (maple) and needle (Douglas-fir) litter by comparing rates of CO2 release during 56-day laboratory incubation from litter which had been ingested by millipedes and transformed into faecal pellets with that from unconsumed leaf litter. Changes in litter microbial communities as a consequence of millipede ingestion were assessed by comparing the PLFA profiles of uningested litter and faeces during incubation.Rates of CO2 release from litter and faeces were similar. CO2 release was greater in maple litter than Douglas-fir litter, and this difference persisted in faeces arising from the ingested litter. Maple litter had higher bacterial abundance and was dominated by Gram-negative bacteria; Douglas-fir litter was dominated by Gram-positive bacteria. Differences in bacterial abundances on the two litters were retained through millipede gut passage, but abundance of Gram-negative bacteria declined with gut passage of maple leaves, such that microbial communities were more similar in the two types of faeces. CO2 release was stimulated by grinding of litter and of faeces, indicating that enclosure of millipede faeces within pellets is partially responsible for the slow decomposition of this material. Similar decomposition rates of faeces and undigested litter indicate that effects of litter-transforming fauna such as millipedes on decomposition rates can be adequately accounted for in models by including estimates of the mass of litter assimilated. © 2012.