Soil functional assessment under biochar, organic amendments and fertilizers applications in small-scale terrestrial ecosystem models

Agronomic impact of biochar application to soil is vastly studied, whereas knowledge on possible adverse effects on terrestrial and aquatic ecosystems is still sparse. Understanding of both aspects is essential for a sustainable use of biochar. This study investigated if biochar-based amendments, alone and mixed with a mineral fertilizer, affect soil habitat, productivity, and retention function, with a focus on their possible ecotoxicological effects on soil and aquatic biota. Arable soil was amended with woodchip biochar, biochar-compost, mineral fertilizer (nitrogen, phosphorus, and potassium: NPK), and their combinations, at agriculturally relevant rates. The study was conducted in small-scale terrestrial ecosystem models (STEMs) in two phases: 1) assessment of the effects of the amendments on soil organisms - the earthworm Eisenia andrei and the plant Brassica rapa and 2) assessment of potential toxicity of leachates to the aquatic macrophyte Lemna minor. To complement the effect-based approach a physicochemical characterization of the soil, pore water, and leachates was carried out. Treatments induced low-to-no effects on E. andrei. The number of seeds of B. rapa was statistically higher in the treatment of biochar-compost with NPK. Bait-lamina consumption was reduced over time, particularly in the soil amended with biochar alone. Leachates caused growth inhibition in L. minor, with the fresh weight being the most sensitive endpoint, namely in the soil amended with biochar and NPK: EC50 = 87.5, EC20 = 38.9, EC10 = 24.2 (as % of the leachate dilution). In response to stress, significantly greater dry to fresh weight ratios were observed in L. minor. Focusing on different biological functional groups and endpoints, the STEMs methodology extended to the leachate assessment represents a suitable intermediate approach between laboratory and field. The results suggest a low risk to soil biota upon the application of 40 t ha(-1) woodchip biochar, biochar-compost, in combinations with NPK, whereas the nutrients and contaminants leaching requires further attention.