Biogeochemical cycle and isotope fractionation of copper in plant–soil systems: a review

Copper (Cu) is a bio-essential element and a potentially toxic pollutant in the plant–soil systems. Analysis of stable Cu isotopes can be a powerful tool for tracing the biogeochemical cycling of Cu in plant–soil systems. In this review, we examined the analysis method of stable Cu isotope ratios in plants and soils, and discussed the biogeochemical processes, including redox reactions, mineral dissolution, abiotic and biotic sorption, which fractionate Cu isotopes in plant–soil systems. We also reviewed the variability of the isotopic signature in different plants and plant tissues, as well as different soil types and profiles to discuss the relationship between the biogeochemical transformation of Cu and its isotope fractionation in plant–soil systems. The collected data show that δ 65 Cu values range from − 2.59 to + 1.73‰ in plant–soil systems, and ∆ 65 Cu values range from − 1.00 to − 0.11‰ between the plant and soil. The variation in the ∆ 65 Cu value between the plant and soil is mainly in response to the different uptake strategies during the acquisition of Cu from soils. Cu isotope analyses are proved to be a suitable technique during the biogeochemical transformation of Cu in plant–soil systems, especially during redox reactions. Ultimately, research challenges and future directions for Cu isotope techniques as a proxy for Cu biogeochemical cycles are also proposed. This review is beneficial for soil safety, food safety, and the sustainable development of agriculture and human health. Graphical abstract