Long-term amendment of urban and animal wastes equivalent to more than 100 years of application had minimal effect on plant uptake of potentially toxic elements

Urban waste products such as sewage sludge and organic household waste compost have a potential as fertilizers in agriculture, but evidence from historical field experiments performed in England and Germany with sewage sludge as fertilizer pointed towards negative effects of sludge-derived heavy metal accumulation on soil microbiota and plants. However, sewage sludge has improved in quality during the last half century and source separation of organic household waste has been shown to decrease the concentration of heavy metals in composts made from organic waste. This alongside a growing awareness on the benefits of an increased degree a recycling of nutrients from urban areas to agricultural soils led to the establishment of the CRUCIAL field trial in 2003 where different urban and animal wastes have been applied each year. In the present study we investigated the effect of the different animal and urban waste treatments in CRUCIAL on oat yield after ten years of experimentation on concentrations of heavy metals in oat grain harvested in the field and pea plants grown in a pot trial using soil from the CRUCIAL field trial. We found that all animal and urban waste fertilizers were suitable for fertilization purposes, although some of these will result in unbalanced nutrient inputs if used as single sources. Furthermore, we found that of the elements studied only Zn and Cu were significantly elevated in soil concentration in the urban waste treatments. In oat grain Cd was significantly elevated in a sewage sludge treatment corresponding to more than 200 years’ legal application, but even in this extreme case the concentration of Cd in grain did not exceed the relevant EU limit for Cd content. In pea plants only the concentrations of Zn and Mo were significantly higher in plants grown in soil that had received large amounts of urban wastes compared to an unfertilized control and the build-up of Zn could be regarded as a beneficial side-effect of using sewage sludge as a fertilizer due to the status of Zn as an essential element. In conclusion, our data indicates that contemporary urban waste products can be used for supplementary fertilization purposes in agriculture with no substantial risks related to potentially toxic elements.