Operationally Extractable Copper and Early-Stage Phytotoxicity in Vineyard Soils: Implications for Cu Risk Assessment and Soil Management

Abstract

Long-term use of Cu-based fungicides in humid vineyards promotes Cu accumulation in topsoils, but plant risk depends on soil properties and on the Cu pools that are operationally extractable rather than on total Cu alone. In this study, soils were collected from vine rows and inter-row areas in 58 vineyards in Galicia, NW Spain. Soil physicochemical properties and total Cu were determined, and six chemically defined Cu fractions were assessed using deionized water, CaCl₂, NH₄OAc, NH₄Cl, EDTA, and a rhizosphere-simulating organic acid extractant. Early-stage phytotoxicity was evaluated using standardized germination and seedling growth bioassays with Sinapis alba, Lepidium sativum, and Sorghum saccharatum. Total Cu concentrations indicated moderate enrichment typical of long-term vineyard soils affected by Cu fungicide inputs, with a mean of 109 mg kg^–1 and a range of 16.4–301 mg kg^–1. Water and CaCl₂ extracted only approximately 1–2% of total Cu, EDTA extracted approximately 50%, and NH₄OAc, NH₄Cl, and the rhizosphere method extracted approximately 5–7%. Despite Cu enrichment, all species showed high germination rates under the short-term assay conditions. Early seedling responses were more associated with soil pH, organic matter (OM), texture, and selected extractable Cu fractions than with total Cu alone. S. alba showed predominantly stimulatory early growth responses, suggesting that this species may be a promising candidate for further evaluation as a cover crop in Cu-enriched vineyard soils. These results support the combined use of chemical extractions and early-stage plant bioassays for Cu risk assessment, although longer-term studies are required to evaluate chronic phytotoxicity and agronomic performance.