Quantifying earthworm bioturbation from changes in vertical bulk density profiles

The rate at which soil becomes physically mixed due to earthworm actions (bioturbation) has relevance for the fate of nutrients and pollutants and for the soil’s ability to sequester carbon. Nevertheless, methods to quantify bioturbation under field-like conditions are largely lacking. The soils of the Fennoscandian tundra offer a special possibility to quantify bioturbation, because they have developed in the absence of soil burrowing macrofauna. They commonly exhibit a thick organic layer on top of the mineral soil with a sharp layer boundary. The bulk density of the two soil layers differs markedly. Since bioturbation mixes both soil layers, the temporal changes in the bulk density profile of such soils may be exploited to estimate bioturbation rates in the field. In this study, we applied a model for earthworm bioturbation to observed changes in soil densities occurring in a mesocosm experiment with intact soil carried out in the arctic during four summers. We show that changes in soil density profiles can indeed be used to infer realistic earthworm bioturbation rates. Although uncertainties in parameter values were sometimes large, the results from this study suggest that soil turnover rates and endogeic earthworm soil ingestion rates in tundra soils may be as high as those reported for temperate conditions. Such large bioturbation rates can explain observed large morphological changes in nearby soils where dispersing earthworms have resulted in complete inmixing of the organic layer into the mineral soil. Our model is applicable to soil profiles with marked vertical differences in bulk density such as the soils of the Fennoscandian tundra where earthworms are currently dispersing into new areas and to layered repacked soil samples that are incubated in the field.