Boric acid as alternative reference substance for earthworm field tests

Boric acid was applied in an earthworm field test according to ISO 11268-3 as a possible alternative for the currently used reference substances that may no longer be available in the near future. The test site was a pasture with a silt- and clay-dominated soil, a pH of 5.7 and an organic content of 2.8%. In addition to a water-only control, two separate treatments of boric acid were applied: single applications at rates of 187 and 750 kg/ha. In order to investigate the acute impact of this compound, biological sampling was undertaken 4 weeks after application by hand sorting combined with formaldehyde extraction. The earthworm community consisted of seven species commonly found at German grassland sites. In the control plots, mean abundance was very high: 984 individuals per square metre before application and 390 individuals per square metre 4 weeks following application of boric acid. In the treated plots, abundance and biomass were reduced by more than 50% compared to the control plots, respectively. In general, juvenile earthworms as well as the epigeic species were affected most strongly. On the basis of this study, boric acid has the potential to be an alternative reference substance in assessing the acute impact on earthworm species in field tests.