Effects of biochar on earthworms in arable soil: avoidance test and field trial in boreal loamy sand

Abstract Biochar is widely studied as a combined soil conditioner in agriculture and a potential carbon sink. The knowledge of the effects of field application of biochar on soil fauna remains limited. Earthworms are a globally common and important faunal group in arable soils and the purpose of our study was to determine the effects of biochar on earthworms under both laboratory and field conditions in a boreal loamy sand. An avoidance test using the earthworm Aporrectodea caliginosa Sav. was conducted for periods of 2 and 14 days with 16 g kg −1 spruce chip biochar. The same biochar was mixed into the top 10 cm of soil at 0 or 30 t ha −1 and its effect on earthworm density and biomass was studied over four and half months in a field experiment where wheat was grown with or without inorganic fertilizer application. In the avoidance test, biochar application did not affect the habitat choice of earthworms in the first 2 d, but after 14 d, they tended to avoid it. The avoidance was possibly the avoidance was possibly caused by a slight decline in soil water potential. Under field conditions the highest earthworm densities and biomasses were measured in biochar amended soils. None of the differences among the treatments studied were, however, statistically significant ( p  > 0.05). The time scale of the study was sufficient for reliably demonstrating the lack of strong toxic effects and immediate avoidance reactions caused by biochar application