Carbon-mineral interactions and bioturbation: an earthworm invasion chronosequence in a sugar maple forest in Northern Minnesota

Abstract

European earthworm species have been introduced into previously glaciated hardwood forests in North America over the past centuries. The invasive earthworms reorganize soil structure, losses of carbon and nitrogen, and the reduction in abundance and diversity of understory communities. One of the direct impacts of earthworms on soils is increased bioturbation, which has domino effects on soil properties that include decrease in litter layer and thickening and increasing bulk density of the A horizons. Such enhanced interactions between organic matter and minerals due to invasive earthworms and earthworms in general have been studied in the context of soil carbon cycle. In this study we attempt to better understand exotic earthworms' impacts on the sorption of organic matter on mineral surfaces and how this fundamental process determining the soil carbon storage and turnover is affected by bioturbation and different earthworm functional groups. Despite the reduction of total C inventory with the arrival of endogeic species, such reduction is largely derived by the loss of light density fraction C. In contrast, the C inventory in heavy density fraction - which we associate with mineral-sorbed - shows little change, which is consistent with likewise stable inventories of mineral surface areas and C-covered mineral surface areas. Mineral-sorbed C pool appears to be in dynamic equilibrium across the diverse ecological stages of earthworm invasion. Our study suggests that the direction and size of changes in soil C inventory in response to bioturbators in general and invasive earthworms specifically will be strongly dependent upon the soil depth profiles of mineralogy and texture.