The interactions between biochar and earthworms, and their influence on soil properties and clover growth: A 6-month mesocosm experiment

A six-month mesocosm experiment was conducted to investigate the joined effect of biochar and earthworms on soil properties and plant (white clover) growth in two contrasting soils – a dystric Cambisol and a sil-andic Andosol, both soils with pH-H2O < 6. Treatments were (i) biochar amendment (1% weight basis), (ii) a positive control (lime added at the liming equivalence of the biochar application), and (iii) a negative control (no amendment). Each treatment had two variants: with or without earthworms (Aporrectodea caliginosa). Soil chemical and biological properties were measured before the start of the experiment and after 6 months of incubation. Earthworms were associated with higher ammonium-N and nitrate-N concentrations, lower pH, higher fungi:bacteria ratio, higher abundance of Collembola, and higher clover biomass in mesocosms. The influence of biochar on plant productivity was overshadowed by earthworm activity, yet a significant positive effect of biochar on clover biomass was observed in the absence of earthworms; this effect was not related to the liming potential of biochar. Synergistic effects of biochar and earthworms were observed for increasing abundance of Collembola and soil fungal biomass. The interaction between biochar and earthworms was soil-type specific – for example, on their own, biochar and earthworms increased clover growth more in the Cambisol, while the positive synergistic effect of biochar and earthworms on soil biochemical processes and clover growth was more evident in the Andosol. Combined use of biochar and earthworms has good productivity potential for acidic soils, and can be part of sustainable soil management.