137Cs as an indicator of earthworm activity in soils

Abstract

We have measured the bomb-fallout radionuclide 137Cs in soils under various management practices. In bulk soil samples taken at various depths from forested woodlots, we found a deeper distribution of 137Cs in soils with greater numbers and biomass of the anecic earthworm species Lumbricus terrestris. In soils of similar texture under no-till management we found significant increases in earthworm populations relative to conventionally tilled (moldboard plow) soils. Activity of 137Cs was depleted in the upper 3 cm of the no-till soils which may be a result of (1) dilution due to the addition of fresh organic matter to the soil surface and/or (2) dilution in surface horizons by sub-soil material containing no 137Cs transported by earthworms casting it at the soil surface. We have also measured the 137Cs content in coatings of individual L. terrestris earthworm burrows with depth in a forested and a conventionally tilled soil. Some earthworm burrow coatings were devoid of 137Cs activity, suggesting that those burrows were likely to be active prior to any bomb fallout (i.e. before the 1940s). Other burrows showed significantly higher radioactivity suggesting more recent earthworm habitation. Our data suggest that the distribution of 137Cs is related to the activity of earthworms in the soils. The methodology developed can be used to qualitatively measure the age of burrows and thus better quantify the rates at which L. terrestris can store soil organic matter below the plow layer. Although the soils studied were free of Chernobyl-derived 137Cs, this technique holds potential for evaluating the influence of fauna on soil processes in the areas affected by the Chernobyl fallout.