Fate of airborne polychlorinated dibenzo-p-dioxins and dibenzofurans in an agricultural ecosystem

The fate of airborne polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in a dairy farm ecosystem is examined. Based on measurements conducted at a dairy farm located in northern Germany, PCDD/F fluxes from the atmosphere into the ecosystem and within the ecosystem are calculated. The results indicate that roughly 15% of the total annual deposition to the farm is captured in the feed crops, while the remaining 85% is transferred directly to soil. The retention efficiency of the different feed crops is variable: grass, accounting for 60% of the cultivated land, contributes 85% of the total PCDD/F flux in feed; maize, accounting for 20%, contributes 15%; while grain, also accounting for 20%, contributes virtually none. While particle-bound deposition accounts for the majority of the atmospheric flux into the ecosystem, the vast majority of this goes directly to soil; gaseous deposition dominates the transfer into crops and, hence, into the food chain. The overall transfer of the annual atmospheric deposition into milk is low, reaching a maximum of 3% for 2,3,7,8-Cl4DD, 1,2,3,7,8-Cl5DD and 2,3,4,7,8-Cl5DF and falling to below 0.1% for many of the other congeners. These low transfer rates are due to a combination of low retention of deposition by crops, incomplete absorption of the PCDD/Fs in the cows and metabolism within the animals. Metabolism represents a sink for PCDD/Fs in the ecosystem, but it is relatively inefficient, removing at most 17% of the total annual atmospheric deposition in the case of the Cl4DD/Fs. A significant fraction of the PCDD/Fs leave the cow barn with the manure, but spreading this manure on the fields only increases the annual input into the soils by about 10%. Although the fluxes must be regarded as estimates due to the limited number of samples analysed, this study presents the first comprehensive overview of PCDD/F behaviour in an agricultural ecosystem.