Living cover crops alter the fate of pesticide residues in soil: influence of pesticide physicochemical properties

Abstract

Abstract. Living cover crops play a key role in reducing nitrogen leaching to groundwater during fallow periods. They also enhance soil microbial activity through root exudates, improving soil structure and increasing organic matter content. While the degradation of pesticides in soil relies primarily on microbial biodegradation, the extent to which cover crops influence this degradation remains poorly quantified. The objective of this study was to evaluate to what extent pesticide residues with contrasting physicochemical properties are affected by living cover crops. We conducted a greenhouse experiment testing two cover crop densities against a bare soil control, and quantified residues (by LC-QTOFMS) of 18 pesticide ingredients (active substances and safeners) in both soil and soil solution. We then related the observed reduction in residues to key physicochemical properties of the pesticide ingredients. Our results show that thin cover crops (0.4 tDM ha−1) reduce pesticide leaching 80 d after sowing relative to bare soil, retaining residues in the topsoil. Moreover, well-developed cover crops (1 tDM ha−1) reduce soil pesticide residues by more than 33 % for compounds with low to high water solubility (s⩽1400 mg L−1) and low to moderate soil mobility (Koc⩾160 mL g−1). This effect is likely due to enhanced pesticide degradation of the retained pesticide in the rhizosphere. These findings confirm previous studies focused on individual compounds, individual cover crop types or individual soil compartments, while providing new thresholds for physicochemical properties associated with significant pesticide degradation. By directly enhancing pesticide degradation within the soil compartment where pesticides are applied, cover crops limit their transfer to other environmental compartments, particularly groundwater.