Ecotoxicological assessment of a biochar-based organic N-fertilizer in small-scale terrestrial ecosystem models (STEMs)

Abstract

Small-scale Terrestrial Ecosystem Models (STEMs) are valuable tools in soil ecotoxicological approaches. We assessed the effects of topsoil incorporation (0–15 cm) of a biochar-based organic N-fertilizer on the weight and vertical distribution of Eisenia andrei (epigeic earthworm) and on germination and growth of Brassica rapa (turnip) in STEMs. Single-species earthworm avoidance behaviour (ISO 17512-1:2005) and plant germination (based on ISO 11269-2:2012) tests were also performed for contextualization.

A 28 day exposure in STEMs showed no variation in earthworm fresh weight. However, increasing biochar concentrations led to an increased earthworm migration from the biochar amended topsoil to deeper soil layers. Such a migration reached 100% at a lower biochar application rate (25 t/ha) compared to the standard earthworm avoidance tests (50 t/ha), making the STEMs approach potentially more conservative for this endpoint. Plant germination in STEMs significantly declined at 25 t/ha, with no germination at 75 and 100 t/ha. Alongside it, a significant decrease in total plant biomass also occurred in STEMs (10 t/ha) before that in traditional plant germination tests (100 t/ha), suggesting a possible interaction between both species. Further, the coefficients of variation (ratios) for the earthworm and plant endpoints in STEMs were generally comparable to that of the single-species tests. By combining two interacting organisms in a more representative exposure scenario and accounting for an ecologically relevant endpoint (earthworm vertical distribution) that is overlooked in traditional single-species tests, the STEMs approach was suitable for higher-tier biochar ecotoxicological assessment.