A new peer-reviewed study published in the Journal of Soil Science and Plant Nutrition examines how different compost and biochar formulations influence earthworm behavior and microbial respiration in copper-contaminated soils. The extensive use of copper-based fungicides in hop and orchard systems has led to long-term copper buildup, creating toxic conditions for soil organisms. The researchers tested whether composts containing co-composted biochar could reduce these harmful effects.
The study compared three composts made from hop bines: a biochar-free compost and two co-composted biochar treatments containing either 5% or 20% biochar by volume. These amendments were applied to both artificially copper-spiked soils and long-term contaminated topsoils collected from hop gardens and an apple orchard. Additional treatments included lime, fresh biochar, and combinations of fresh biochar with compost.
Across almost all soil types, earthworms showed a strong attraction to compost-amended soils, even when copper concentrations were high. This attraction exceeded the worms’ natural avoidance response to copper, meaning compost improved habitat preference. However, the study found no evidence that biochar-containing composts reduced copper bioavailability. Instead, the positive effects on worms appeared linked to improved physical and chemical soil conditions created by organic matter.
Microbial respiration also increased substantially after compost application, particularly with the biochar-free compost. Because biochar is more resistant to decomposition, composts containing biochar had lower proportions of degradable carbon, resulting in weaker stimulation of microbial activity. In contrast, the non-biochar compost delivered the strongest short-term enhancement of both basal and substrate-induced respiration.
Importantly, fresh biochar applied alone or in combination with compost produced weaker responses than co-composted biochar composts, suggesting that co-composting helps reduce potential negative microbial effects associated with raw biochar.
Overall, the findings indicate that biochar-free compost is sufficient—and often superior—for mitigating copper-related soil stress, at least in controlled short-term conditions. The authors note that long-term field studies are still required to evaluate how these amendments influence soil biodiversity, microbial communities, and earthworm species that naturally inhabit mineral soils.
Reference
Görl, J., Lohr, D., Meinken, E. et al. Effects of Co-Composted Biochar Compost on Earthworm Avoidance Behavior and Microbial Respiration in Copper-Polluted Soils. J Soil Sci Plant Nutr (2025). https://doi.org/10.1007/s42729-025-02827-9
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