The rhizospheric transformation and bioavailability of mercury in pepper plants are influenced by selected Chinese soil types

Abstract

Understanding and prediction of mercury (Hg) phytoavailability in vegetable-soil systems is essential for controlling food chain contamination and safe vegetable production as Hg-contaminated soils pose a serious threat to human health. In this study, four typical Chinese soils (Heilongjiang, Chongqing, Yunnan, and Jilin) with varied physicochemical properties were spiked with HgCl₂ to grow sweet pepper (Capsicum annuum L.) in a pot experiment under greenhouse condition. The chemical fractionation revealed a significant decrease in exchangeable Hg, while an increase in organically bound Hg in the rhizosphere soil (RS) compared to bulk soil (BS). This observation strongly highlights the vital role of organic matter on the rhizospheric Hg transformation irrespective of contamination levels and soil properties. Stepwise multiple linear regression (SMLR) analysis between Hg concentration in plants, Hg fractions in RS and BS, and soil properties showed that Hg in plant parts was significantly influenced by soil total Hg (THg) (R² = 0.90), soil clay (R² = 0.99), amorphous manganese oxides (amorphous Mn) (R² = 0.97), amorphous iron oxides (amorphous Fe) (R² = 0.70), and available Hg (R² = 0.97) in BS. Nevertheless, in the case of RS, Hg accumulation in plants was affected by soil THg (R² = 0.99), amorphous Mn (R² = 0.97), amorphous Fe oxides (R² = 0.66), soil pH, and organically bound Hg fraction (R² = 0.96). Among all the evaluated soils (n = 04), metal (mercury) concentration in terms of plant uptake was reported highest in the Jilin soil. Based on SMLR analysis, the results suggested that the phytoavailability of Hg was mainly determined by THg and metal oxides regardless of the rhizospheric effect. These findings facilitate the estimation of Hg phytoavailability and ecological risk that may exist from Hg-contaminated areas where pepper is the dominant vegetable.

Keywords: Mercury fractionation; Pepper; Risk assessment; Soil properties; Stepwise multiple linear regression.