Co-composted biochar compost for enhancing juvenile growth of hop plants in copper-contaminated soils

Abstract 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 Five treatments were tested: unamended control, lime-treated soil, hop bine compost without biochar, and hop bine compost co-composted with 5 vol% or 20 vol% biochar. Compost amendments were applied into the planting holes during replanting of a copper-contaminated hop garden. 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, leaf damage was likely due to molybdenum deficiency. 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. Graphical Abstract