Almond trees as bioindicators of trace metal pollution in a Mediterranean agricultural island

Mediterranean agricultural soils are exposed to increasing diffuse contamination by potentially toxic elements (PTEs), particularly in insular environments where agriculture, urbanization and tourism coexist. Despite the widespread distribution of almond (Prunus dulcis Mill.) orchards across the Mediterranean Basin, field-based studies assessing trace element behavior in this species remain scarce. This study evaluates trace element dynamics in almond-based agroecosystems on the island of Mallorca (Spain), integrating soil physicochemical characterization, plant tissue analysis and multivariate statistical approaches to assess the bioindicator potential of almond trees. Topsoil samples and almond stem and leaf tissues were collected across the main almond-growing areas of the island. Soils were moderately alkaline (mean pH ≈ 8.2) with high carbonate contents (∼40 %), strongly limiting trace element mobility and bioavailability. Total PTE concentrations in soils were generally low to moderate (e.g., Pb mean 24.3 mg kg^-1; Zn mean 68.6 mg kg^-1), reflecting predominantly natural background levels. Trace element concentrations in almond tissues were also low, with Bioconcentration Factor (BCF) well below unity for all elements (e.g., <0.4), indicating that Prunus dulcis does not behave as a metal accumulator under Mediterranean calcareous conditions. Soil-plant relationships differed markedly between organs. In stems, Pb and Cd were more related to soil contents, whereas Cd showed decoupling due to limited translocation. Mercury exhibited a distinct behaviour, with strong foliar enrichment (mean 28.3 μg kg^-1 in leaves vs. 16.2 μg kg^-1 in stems), consistent with diffuse atmospheric deposition. Overall, almond trees are not reliable indicators of soil contamination through bioconcentration, but their leaves may serve as effective bioindicators of atmospheric Hg pollution in Mediterranean agricultural and insular environments.