Effects of ash application on nutrient and heavy metal fluxes in the soil and soil solution in a Norway spruce plantation in Denmark

Wood ash application is used in forests to counteract nutrient losses and acidification due to intensified harvesting methods, where harvest residues are removed. The aim of the study was to investigate the effect of ash application at doses of 0, 3, 4.5 and 6 Mg ha−1 with three replicates on soil and soil solution chemical properties in a mature Norway spruce plantation in Denmark from 2014 to 2017. Un-replicated extreme scenarios of 15 and 30 Mg ha−1 were included. Soil solution from 15 to 60 cm depth were sampled each month over a period of 2 years and 9 months after ash application and soil samples were obtained after 2.5 years. The fluxes of nutrients and heavy metals through the soil were calculated based on water fluxes modelled with CoupModel. Geochemical modelling was applied on selected soil solution data to investigate changes in heavy metal complexation and mobility. Application of ash to forest soil increased both soil pH and nutrient concentrations in the O-horizon but not in the mineral soil. No changes were detected in the soil solution chemistry or in the speciation of heavy metal at doses ranging up to 30 Mg ha−1, and there was therefore no change in the mobility of metals caused by ash application. The un-replicated extreme scenario suggested that the O-horizon concentration of Cd, Co, Cr, Cu and Zn could increase proportionally with increasing ash application dose, while no indications of changes in the mineral soil below 5 cm could be seen. Among the experimentally replicated treatments, only the Cd concentration at dose 6 Mg ha−1 exceeded the Danish legislation limits, in the O-horizon of 0.5 ppm.