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No. 257: Potential environmental impacts when using acidified manure on farmland

Jensen, J., Krogh, P.H., Sørensen, P. & Petersen, S.O. 2018. Potentielle miljøeffekter ved anvendelse af forsuret gylle på landbrugsjord. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 42 s. - Videnskabelig rapport fra DCE - Nationalt Center for Miljø og Energi nr. 257
http://dce2.au.dk/pub/SR257.pdf

Summary

This screening project has as its main purpose to elucidate the potential negative consequences and environmental side effects of using acidic treatment of animal slurry used on agricultural soils in Denmark. In Denmark, approximately 20% of all animal slurry (fluid animal waste) is treated with sulfuric acid prior to amendment on arable soils. The acidification of slurry brings down the pH to 6.0 or lower, which reduces the potential for ammonia evaporation from the slurry.

Provided farmers generally maintain the soil pH within the range of the recommended values, it is concluded that a temporary reduction in soil pH resulting from the use of acidified slurry is unlikely to result in significant impact on the microbial activity or soil dwelling invertebrates like earthworms. Regarding adsorption, mobility and potential leaching of metals and phosphor, data indicates that even a relative limited drop in soil pH of 0.5 units may have a measurable impact. Acidification of the slurry may mobilize phosphor in the slurry and hence an increase of the phosphor loss in soils. Leaching of excess sulphur may furthermore increase the risk of increasing the mobility of phosphor by reducing sulphate and its affinity to iron (Fe) and thereby increase the phosphor loss from the soil. Predicted risk is highest in wetland areas where the water is transported though a matrix with a relative long retention time and a high organic matter content promoting the sulphate reduction. There is, however, a strong need for more quantitative information on these processes. For a number of metal especially nickel and zinc, higher mobility is predicted if soil pH is reduced. Model calculations indicate three and two times levels of nickel and zinc in the pore water, respectively, if the soil pH is reduced with 0.5 units. The effects are, however, depending on the soil specific buffer capacity. In Denmark, zinc is already today a challenge in the recipients as the environmental quality standards are exceed in a number of fresh water systems. Finally, the increased use of lime in order to keep the soil pH at optimum for plant production may in overall terms increase the annual load of metals found in lime. In most cases, this is most likely not posing a risk. One exemption could be cadmium where the extra application may be measurable and potentially undesired.