Distribution of earthworm growth stages along a naturally occurring soil salinity gradient

Abstract

One major soil health challenge of the Northern Great Plains of North America is the natural occurrence of soluble salts in the soil. Salinity impacts on plant production are well understood, but we stand to learn more about the effects of salinity on soil biological characteristics, especially for sulfate-based salts. We conducted a field survey for three years to quantify the abundance and growth stages of earthworms across a naturally occurring salinity gradient. The gradient was approximately 150 m in length and traversed electrical conductivity values < 1 to 6 mS/cm and associated organic matter content from 6.9 to 4.8 %. We recovered earthworms and cocoons from intact soil cores (20 cm diameter) to 15 cm depth. Most of the recovered earthworms were Aporrectodea trapezoides (Dugés, 1828); however, Aporrectodea tuberculata (Eisen, 1874) and Octolasion tyrtaeum (Savigny, 1826) were also observed in low abundance. Juvenile earthworms were abundant and contributed the most individuals to the total counts in non-saline soils; they were also the group that exhibited the steepest decline in response to increasing salinity levels. Mature earthworm and cocoon counts were generally stable across salinity levels. Though we observed some inter-year variability in earthworm observations, we conclude that the occurrence of juvenile earthworms may serve as a soil health indicator in a salinity context.

Keywords

• dryland salinity
• endogeic
• mollisol
• soil health
• sulfate salts