Copper phytoremediation is influenced by the production time of vermicompost used as organic amendment

Phytoremediation is an efficient and low-cost alternative to mitigate the impacts caused by heavy metals in the environment. The efficiency of phytoremediation of copper-contaminated soil can be enhanced by the addition of organic amendments, such as vermicompost. However, different vermicomposting time results in different chemical, physical, and biological properties of the vermicompost, which may consequently influence its efficiency as a phytoremediation amendment. The objective of this study was to evaluate the effect of cattle manure vermicomposting time on the efficiency of vermicompost in enhancing the phytoremediation of copper-contaminated soil. Black oat (Avena strigosa Schreb.) was cultivated in a greenhouse for 60 days in 5-kg pots containing an Acrisol with 889 g kg⁻¹ sand. The soil was collected from the surface layer of a vineyard with a copper (Cu) content of 104 mg kg⁻¹ (Mehlich-1) due to the long-term use of Bordeaux mixture (copper sulfate + calcium oxide) as a fungicide. Manure was obtained from feedlot cattle raised for meat production. The four treatments consisted of manure without vermicomposting and vermicomposts produced after 30, 60, or 120 days of vermicomposting. The experiment was conducted in a completely randomized design with four replicates. The amendment dose added to the soil was equivalent to 155 kg ha⁻¹ of P, with adjustments of N and K levels using urea and KCl. At plant flowering, Cu toxicity on the functioning of the photosynthetic apparatus was evaluated by determining the chlorophyll index, effective quantum yield of photosystem II (Y(II)), initial fluorescence (F₀), and electron transport rate (ETR) using a pulse-amplitude-modulated fluorometer. Subsequently, roots and shoots were collected to determine dry mass and Cu concentrations in plant tissues. Overall, the results show that shorter vermicomposting times led to lower Cu concentrations absorbed by plants, reduced phytotoxic effects on the photosynthetic apparatus, and greater biomass production. Plants grown in the manure treatment (without vermicomposting) accumulated 443 and 19 mg kg⁻¹ of Cu in roots and shoots, respectively, and produced 362 mg plant⁻¹ of root dry mass and 504 mg plant⁻¹ of shoot dry mass. In contrast, plants grown with vermicompost produced after 120 days accumulated 489 and 34 mg kg⁻¹ of Cu in roots and shoots, respectively. As a consequence, phytotoxicity was intensified and biomass production was lower in shoots (331 mg plant⁻¹; p = 0.002) and roots (224 mg plant⁻¹; p = 0.007). Longer vermicomposting time of cattle manure results in an amendment that increases Cu uptake by black oat, which reduces plant growth and decreases total Cu accumulation in the plant.