The effects of biochar and compost amendments on copper immobilization and soil microorganisms in a temperate vineyard

• Biochar and biochar–compost did not reduce available Cu in a temperate vineyard.
• Compost and biochar–compost did not affect Cu phytoextraction by cover crops.
• Compost and biochar–compost significantly increased microbial abundance and activity.
• Treatments significantly changed microbial community structure in soil.

The use of copper (Cu) fungicides in agriculture has led to Cu accumulation in European topsoils. This study is the first to investigate the in situ efficacy of biochar and biochar–compost as Cu immobilizers, reducing Cu uptake by plants and increasing microbial abundance and activity, in a temperate vineyard topsoil (0–10 cm). After application of biochar, compost and biochar–compost in April 2011, plant and soil samples were taken in November 2011, April 2012, August 2012, and November 2012. Similar amounts of exchangeable Cu fractions (CuDTPA) in all treatments showed that there was no significant effect on Cu immobilization in soil. In contrast, cover crops grown between vine rows were observed to take up a significant amount of Cu (38.7 mg Cu kg−1), reducing soil Cu concentrations over time. Treatments with biochar and/or compost initially increased total carbon, with compost and biochar–compost additionally increasing extractable organic carbon in soil. Compost and biochar–compost significantly increased microbial biomass, phospholipid fatty acids (PLFAs), enzyme activities (phosphatase, arylsulfatase) and bacterial taxa abundances (Actinobacteria, α-Proteobacteria, β-Proteobacteria, Firmicutes, Gemmatimonadetes). A high abundance of gram+ Actinobacteria in all treatments suggested that they are adapted to heavy metals, likely due to their specific cell membrane structures. Additionally, each treatment was characterized by a specific microbial community composition. Compost and biochar–compost increased the relative abundance of Firmicutes, while control and biochar increased Acidobacteria,Gemmatimonadetes and Actinobacteria. In conclusion, biochar and/or compost were not viable Cu remediation options, but compost and biochar–compost provided ecosystem services by reinforcing the microbial community.