Co-Composted Biochar Compost for Improving Juvenile Growth After Replanting Copper-Contaminated Hop Gardens

Background and Aims The long-term application of copper-based fungicides in hop cultivation has led to substantial copper accumulation in the topsoil, potentially impairing the early growth of newly planted hop plants and affecting soil biota. To reduce copper bioavailability in soil, co-composted biochar compost was evaluated as a remediation strategy. Methods Two biochar composts, produced by co-composting 5 and 20 vol% biochar with chopped hop bines, and a biochar-free hop bine compost were applied into the planting holes during replanting of a copper-contaminated hop garden. A limed treatment and an unamended control were included. Remediation effects were assessed over two growing seasons based on visual damage ratings, copper concentrations in leaves and roots, biomass production, and soil respiration. Results During both growing seasons, leaf chlorosis and necrosis were observed, but were associated with copper toxicity only in the year of planting. In the second year, Mo deficiency was the primary cause of leaf damage. Biochar compost, particularly the one co-composted with 5 vol% biochar, significantly reduced these symptoms and increased biomass production by about 30%. In contrast, liming and biochar-free compost were less effective. Soil respiration was significantly enhanced by up to 81% with biochar-free compost showing the strongest effect due to higher microbial degradability. Conclusions Based on these findings, co-composted biochar compost can be recommended for farmers to improve juvenile growth after replanting copper-contaminated hop gardens. However, biochar-free compost also showed beneficial effects, particularly on microbial respiration, and may serve as a cost-effective alternative on less challenging sites.