Influence of earthworm invasion on soil microbial biomass and activity in a northern hardwood forest

Recent invasion and activity of exotic earthworms has profoundly altered the chemical and physical environment of surface soils in northern hardwood forests that previously had mor humus horizons. We investigated the influence of earthworm invasion on soil microbial biomass and activity in surface soils of Allegheny northern hardwood forests in central New York state. Earthworm activity in these sites had transformed surface soils with clear Oi, Oe, and Oa horizons (forest floor) overlying mineral soil, to more uniformly mixed organic-enriched A horizons. The highest concentrations of microbial biomass and activity occurred in the forest floor. Microbial biomass (assayed by chloroform fumigation–extraction) nearly doubled in surface (0–5 cm) mineral soils in response to earthworm activity, an effect that corresponded directly to redistribution of organic matter from forest floor into the mineral soil. Microbial activity in surface mineral soils was even more sensitive to the presence of earthworms than microbial biomass. For example, substrate-induced respiration (or maximum initial respiratory rate, MIRR) was 6.7-fold greater, basal respiration was 5-fold greater, and microbial respiration per unit microbial biomass (metabolic quotient, qCO2) was almost 3-fold greater in surface mineral soils where earthworms were present than in earthworm-free sites. Of the activity indices, only MIRR was higher when expressed on an organic matter basis. Surface mineral soils where earthworms were present thus appear to retain a high proportion of the microbial biomass and activity found in mor organic horizons. Our findings suggest that earthworm activity stimulates the activity of soil microorganisms, probably by enhancing organic C availability via processing and mixing of litter. The relative pattern in microbial properties did not change over the growing season; however, there were some seasonal changes in the proportional differences between worm and no-worm soils. Our results indicate interactions among earthworms, organic matter, and soil microbial activity that should alter the carbon and nutrient balance of northern hardwood forest surface soils, relative to non-invaded soils.