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

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. In this paper we (1) monitored pesticide residue levels in soil and soil solution under two different cover crop densities and (2) correlated the observed reductions with physicochemical properties of the active substances. Our results show that thin cover crops (0.4 tDM ha-1) reduce pesticide leaching 80 days after sowing compared to bare soil, retaining the residues in the microbiologically active topsoil. In addition, well-developed cover crops (1 tDM ha-1) reduce soil pesticide contents 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 probably due to enhanced pesticide degradation of the retained pesticide in the rhizosphere. These results confirm previous studies on individual compounds, individual cover crop types and 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.