Soil properties affect vapor-phase adsorption to regulate dimethyl disulfide diffusion in soil.

Dimethyl disulfide (DMDS) is efficacious against nematodes and other soil-borne pathogens known to reduce crop quality and yield. Previous studies reported inconsistent efficacy and suggested that the diffusion of DMDS varied with different soil types. The effect of soil adsorption on gaseous DMDS diffusion through different soil types is poorly understood. To clarify the role and mechanism of soil adsorption in the diffusion of gaseous DMDS in soil, we have studied the diffusion rate constant (R) of gaseous DMDS in soils using a soil column experiment. The adsorption of DMDS at each gas-soil, soil-water and gas-water partition was measured by a batch-equilibrium headspace method. The results showed the DMDS adsorption equilibrium was well-described by the nonlinear Freundlich isotherm and the linear Henry isotherm. R values were strongly negatively correlated with the Henry coefficient (K) values. The K values of dry soil were several orders of magnitude higher than those observed in moist soil within each moisture content range. The K values in dry soil were strongly positively correlated with soil pore size (<2 nm). However, when the soil moisture content ranged from 3 to 12% (w/w), the K values were strongly correlated with specific surface area (SSA). Elevated temperatures promoted the gaseous phase of DMDS (consistent with Henry's Law) and its diffusion through soil. The soil-water partition coefficient (K') ranged from 1.83 to 2.20 μg mL g in tested soils. Our results suggest that the DMDS vapor-phase diffusion in soil was significantly affected by soil adsorption, which in turn depended on the soil's properties especially the SSA and soil moisture content. These findings suggest applicators can reduce the risk of unsatisfactory and inconsistent efficacy results against soil-borne pests by adjusting the DMDS dose and fumigation period according to soil type, moisture conditions, and other environmental factors.