Enhanced Mobilization of Soil Heavy Metals by the Enantioselective Herbicide R-Napropamide Compared to its S-Isomer: Analyses of Abiotic and Biotic Drivers

Abstract

Chiral herbicides applied to agricultural soils are typically mildly to moderately contaminated with heavy metals (HMs), necessitating a thorough investigation into their effects on soil HMs availability. This study evaluated the effect of the chiral herbicide napropamide (NAP) on HMs bioavailability in different soil types, including weakly alkaline clay in Northeast China, neutral sandy loam in Zhejiang, and weakly acidic clay loam in Sichuan, China. The results demonstrate significant differences in the availability of HMs (Cd, Pb, Zn, and Ni) in the soil following enantiomer treatments, with variation ranges of 4.57-45.67%, 5.03-96.21%, 2.92-52.30%, and 10.57-29.79%, respectively. Overall, R-NAP enhanced the bioavailability of HMs more effectively than S-NAP, specifically by significantly activating available iron 3.33-191.97% and markedly affecting soil pH and cation exchange capacity. Additionally, R-NAP influenced biotic processes by enriching dominant microbial communities, such as Chitinophaga, Niabella, and Promicromonospora, and by constructing more stable microbial networks. Notably, bioavailable Fe plays a dual regulatory role, affecting both the abiotic and biotic processes affected by soil NAP. In summary, although R-NAP is commonly used in agriculture, it poses a greater risk of HMs contamination in crops, highlighting the need for careful application and management. This study provides a fundamental theoretical basis for the judicious use of chiral herbicides in agricultural soils with mild-to-moderate HMs contamination.