The influence of earthworm community structure on the distribution and movement of solutes in a chisel-tilled soil

Abstract

Although it is known that deep-burrowing earthworms can increase the depth of penetration of solutes in soil, their comparative effect on solute transport when they are added to communities composed solely of epigeic and endogeic species is not clear. Ionic tracers were applied as artificial rainfall to the surface of plots with or without added deep-burrowing earthworms ( n=3). We determined the soil water content, total nitrogen, soil organic carbon (SOC), strontium, and bromide concentrations in soil samples taken from a 320 mm×380 mm, 56-position grid with 40 mm sample spacing. Earthworm inoculations increased populations of deep-burrowing Lumbricus terrestris L., and decreased those of the surface-dwelling L. rubellus Hoffmeister, and had no impact on geophagous species; compared to the earthworm communities in control plots. The average soil concentrations of SOC, bromide and strontium were higher where earthworms were inoculated compared to controls. Solute concentrations, SOC, and soil water content all decreased with depth. Synthetic data derived from multivariate analyses were used as an index of solute transport intensity. Solute transport intensity was greater at any given depth in response to earthworm inoculations, compared to that in controls. Although we observed a characteristic breakthrough of solutes to greater depths where deep-burrowing earthworms were inoculated, there were no differences in the spatial distribution of solutes between the two earthworm communities studied. We concluded that although the activity of deep-burrowing species allowed solute breakthrough to greater depth, the more diffuse distribution of solutes was the result of the dominant random burrowing activity of epigeic and endogeic earthworm species, which comprised the larger proportion of earthworms in these communities.