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The invasion of north temperate forests by exotic species of earthworms is an important issue that has been overlooked in the study and management of these forests. We initiated research to address the hypothesis that earthworm invasion will have large consequences for nutrient retention and uptake in these ecosystems. In this special feature of Ecosystems, we present five papers describing results from our experiment. In this paper, we (a) introduce our experimental approach and conceptual model of how earthworms influence forest ecosystem processes, (b) describe the characteristics of the study areas and earthworm communities at our two study locations, and (c) provide a brief overview and synthesis of the main findings. The most dramatic effect of earthworm invasion was the loss of the forest floor at an undisturbed forest site, which altered the location and nature of nutrient cycling activity in the soil profile. Invasion changed soil total carbon (C) and phosphorus (P) pools, carbon—nitrogen (C:N) ratios, the loss and distribution of different soil P fractions, and the distribution and function of roots and microbes. Response to invasion varied with site characteristics and earthworm species. Our results suggest that exotic earthworm invasion is a significant factor that will influence the structure and function of northern temperate forest ecosystems over the next few decades. Regional evaluations of these forests will need to consider the presence or absence of earthworms along with other important ecosystem drivers, such as pollution, climate, and underlying soil characteristics. [ABSTRACT FROM AUTHOR]

We analyzed soil organic matter distribution and soil solution chemistry in plots with and without earthworms at two sugar maple (Acer saccharum)—dominated forests in New York State, USA, with differing land-use histories to assess the influence of earthworm invasion on the retention or loss of soil carbon (C) and nitrogen (N) in northern temperate forests. Our objectives were to assess the influence of exotic earthworm invasion on (a) the amount and depth distribution of soil C and N, (b) soil δ13C and δ15N, and (c) soil solution chemistry and leaching of C and N in forests with different land-use histories. At a relatively undisturbed forest site (Arnot Forest), earthworms eliminated the thick forest floor, decreased soil C storage in the upper 12 cm by 28%, and reduced soil C:N ratios from 19.2 to 15.3. At a previously cultivated forest site with little forest floor (Tompkins Farm), earthworms did not influence the storage of soil C or N or soil C:N ratios. Earthworms altered the stable isotopic signature of soil at Arnot Forest but not at Tompkins Farm; the alteration of stable isotopes indicated that earthworms significantly increased the loss of forest floor C but not N from the soil profile at Arnot Forest. Nitrate (NO3 ¯) concentrations in tension and zerotension lysimeters were much greater at Tompkins Farm than Arnot Forest, and earthworms increased NO3 ¯ leaching at Tompkins Farm. The results suggest that the effect of earthworm invasion on the distribution, retention, and solution chemistry of soil C and N in northern temperate forests may depend on the initial quantity and quality of soil organic matter at invaded sites. [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]