Temporal changes in the efficiency of biochar- and compost-based amendments on copper immobilization in vineyard soils

Copper (Cu)-based fungicides have been an important tool against disease in viticulture since the 19th century. However, their prolonged use can lead to Cu accumulation in the soil and negatively affect soil microbiology and plant growth. The application of biochar (BC)-based amendments is a promising mitigation strategy, due to BC’s longevity in the soil and its potential to complex Cu. This study investigated temporal changes in the efficiency of various compost- and BC-based amendments to immobilize Cu in a calcareous and a slightly acidic Austrian vineyard soil. The immobilization of both historically accumulated Cu and freshly spiked Cu (250 mg kg⁻¹) was studied. The soils were treated with six combinations of amendments containing compost and BC, with and without surface modification, as well as an additional lime treatment for the acidic soil. After treatment, the soils were incubated for 6 weeks and 3 years, after which the 0.01 M CaCl₂-extractable Cu was measured. The amendments were not effective in reducing the mobility of the historically accumulated Cu in the calcareous soil, with pure compost doubling the soluble Cu. Pure wood-chip BC was the only organic amendment that led to a reduction (by 20%) of soluble Cu after 6 weeks in the acidic soil; however, after 3 years, the same amendment reduced soluble Cu by 40% and all other tested amendments were also effective in reducing the mobility of the historically accumulated Cu. The lime treatment achieved the greatest reduction in Cu mobility (56%). Freshly spiked Cu was strongly immobilized in both unamended soils, with 0.06% and 0.39% extractable after 6 weeks in the calcareous and slightly acidic soil, respectively. The amendments did not effectuate additional Cu immobilization in the calcareous soil, but in the acidic soil, the soluble Cu was further reduced to between 25% and 50% of the unamended control by the tested organic amendments and to 6% by the lime treatment after 6 weeks of incubation. Overall, the acidic soil exhibited a stronger response to the amendments than did the calcareous soil, suggesting the amendments’ effect on the soil pH was an important factor for Cu immobilization in this study. These results show the importance of developing site-specific remediation strategies for Cu accumulation in agricultural soils