Relationship between the Vertical Distribution of Pesticides, Including Neonicotinoids, in Soils and Their Uptake by Brassica rapa L. var. peruviridis

Developing a method for estimating the pesticide concentrations taken up by crops based on their concentrations in soils can be used to prevent contamination of succeeding crops by pesticide residues. To determine the soil sampling depth needed in the field, we investigated the relationship between vertical distributions of pesticides, including four neonicotinoids, in 0–30 cm deep soil and pesticide concentrations in a leafy vegetable (komatsuna, Brassica rapa L. var. peruviridis). Pesticide concentrations in komatsuna in a field experiment showed a strong positive correlation not with total extractable (using water and acetone) concentration but with water-extractable concentration in topsoil (0–10 cm depth). Furthermore, artificial field and pot experiments clearly indicated that substances in <10 cm deep soil were difficult for komatsuna to take up. The results suggest that considering the sorption and narrow-range vertical distribution of pesticides in topsoil is important to accurately estimate their concentrations in komatsuna.