Bacterially mediated carbon-iron coupling drives differential effects of herbicide enantiomers on soil heavy metal bioavailability

Abstract

In China, heavy metal (HM) contamination of farmland soil is severe. However, the differential effects of herbicides, particularly their chiral configurations, on the bioavailability of soil HMs and their underlying mechanisms remain unclear. Therefore, in this study, we applied different configurations of the typical herbicide Napropamide (NAP) to various types of soils contaminated with composite HMs, including cadmium (Cd), nickel (Ni), lead (Pb), and zinc (Zn), to demonstrate enantiomeric differences in the influence of herbicide isomers on HM bioavailability. Interestingly, we noticed notable enantiomeric variations in the dissolved organic carbon (DOC) levels within these systems. These differences vanished once the systems underwent γ-irradiation sterilization. This suggests a deep-rooted connection between DOC and HMs, facilitated by soil carbon (C)-related bacterial functional groups such as cellulolysis, aromatic compound degradation, and chitinolysis. These functional groups, which are influenced by NAP, play a role in differentially regulating the availability of soil HMs. When NAP isomers coexisted, the soil DOC content increased, as did iron reducing bacteria, leading to the formation of iron (Fe) oxides. The Mantel test results suggested that the DOC-driven C-Fe coupling was a crucial factor in the impact of NAP on soil HM bioavailability. The enantiomeric differences in soil Zn and Ni bioavailability induced by R- and S-NAP were associated with variations in the complexity of soil C- and Fe-related bacterial networks and key species such as Mesorhizobium silamurunense. This study is the first to reveal the underlying mechanism by which herbicide isomers affect soil HMs from a microbially-driven C-Fe coupling perspective, providing a more comprehensive theoretical basis for the scientific application of herbicides and the mitigation of soil HM contamination.