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Invasion of north temperate forest soils by exotic earthworms has the potential to alter microbial biomass and activity over large areas of North America. We measured the distribution and activity of microbial biomass in forest stands invaded by earthworms and in adjacent stands lacking earthworms in sugar maple-dominated forests in two locations in New York State, USA: one with a history of cultivation and thin organic surface soil horizons (forest floors) and the other with no history of cultivation and a thick (3–5 cm) forest floor. Earthworm invasion greatly reduced pools of microbial biomass in the forest floor and increased pools in the mineral soil. Enrichment of the mineral soil was much more marked at the site with thick forest floors. The increase in microbial biomass carbon (C) and nitrogen (N) in the mineral soil at this site was larger than the decrease in the forest floor, resulting in a net increase in total soil profile microbial biomass in the invaded plots. There was an increase in respiration in the mineral soil at both sites, which is consistent with a movement of organic matter and microbial biomass into the mineral soil. However, N-cycle processes (mineralization and nitrification) did not increase along with respiration. It is likely that the earthworm-induced input of C into the mineral soil created a microbial ‘sink’ for N, preventing an increase in net mineralization and nitrification and conserving N in the soil profile. [ABSTRACT FROM AUTHOR]

Earthworms have been shown to produce contrasting effects on soil carbon (C) and nitrogen (N) pools and dynamics. We measured soil C and N pools and processes and traced the flow of 13C and 15N from sugar maple (Acer saccharum Marsh.) litter into soil microbial biomass and respirable C and mineralizable and inorganic N pools in mature northern hardwood forest plots with variable earthworm communities. Previous studies have shown that plots dominated by either Lumbricus rubellus or Lumbricus terrestris have markedly lower total soil C than uncolonized plots. Here we show that total soil N pools in earthworm colonized plots were reduced much less than C, but significantly so in plots dominated by contain L. rubellus. Pools of microbial biomass C and N were higher in earthworm-colonized (especially those dominated by L. rubellus) plots and more 13C and 15N were recovered in microbial biomass and less was recovered in mineralizable and inorganic N pools in these plots. These plots also had lower rates of potential net N mineralization and nitrification than uncolonized reference plots. These results suggest that earthworm stimulation of microbial biomass and activity underlie depletion of soil C and retention and maintenance of soil N pools, at least in northern hardwood forests. Earthworms increase the carrying capacity of soil for microbial biomass and facilitate the flow of N from litter into stable soil organic matter. However, declines in soil C and C:N ratio may increase the potential for hydrologic and gaseous losses in earthworm-colonized sites under changing environmental conditions. [ABSTRACT FROM AUTHOR]