Combined effects of earthworms and forest age on below- and aboveground processes in the Mid-Atlantic region

Background/Question/Methods

The deciduous forests in the Mid-Atlantic region are dominated by non-native lumbricid earthworms, although native species also occur. To investigate how earthworm community composition and abundance affect belowground processes and tree seedling growth we set up a field manipulation experiment at the Smithsonian Environmental Research Center in Edgewater, MD. A total of 66 experimental plots were set up in successional (70 yrs) and mature (150 yrs) Tulip-poplar-Oak associations. We manipulated earthworm abundance and leaf litter input, and planted seedlings of Tulip poplar, Red maple, Red oak, and American beech. The experiment lasted for two years during which we regularly monitored density, biomass and species composition of earthworm assemblages and measured soil respiration. Soil moisture, temperature and air temperature were also continuously monitored using a wireless sensor network. At harvest, soil bulk density, pH, N pools, C:N ratio, potential N-mineralization rates, and enzyme activity were determined. We used quantitative PCR to assess the community composition of soil fungi. We also determined the extent of mycorrhizal colonization and biomass of roots, shoots and leaves.

Results/Conclusions

In general, forest age had a significant effect in almost all of the measured factors and processes. Both mycorrhizal groups were more abundant in mature than in successional forests. Both ectomycorrhizal (ECM) and arbuscular (AM) fungi were less abundant in the earthworm removal plots. There was a significant positive earthworm effect on the rate and thermal sensitivity of soil respiration. Soil respiration was consistently higher on tulip poplar plots than on beech plots indication a strong influence of plant residue quality. At the end of the experiments soil pH increased under both plots, but more under tulip poplar. Local variation of N-mineralization and nitrification rates was high, and no treatment effects were detected. Oak and beech seedlings were smaller in high density earthworm plots, while the reverse was true for maple and tulip poplar seedlings. Non-native earthworms have significant ecosystem effects, even in forests where native earthworms still occur. However, these effects appear to be dynamic, and depend on forest type and land use history.