The complexity of wood ash fertilization disentangled:Effects on soil pH, nutrient status, plant growth and cadmium accumulation

Wood ash is a by-product from energy production that can be recycled to forests to regain nutrients and prevent acidification. However, low concentrations of nitrogen (N) in wood ash may reduce its potential positive effect on plant growth. In addition, wood ash can have a high content of toxic heavy metals such as Cd, thus there are concerns that it may increase Cd accumulation in plants. We grew Deschampsia flexuosa (Wavy hair-grass) in pots in acidic nutrient poor forest soil fertilized with different concentrations of wood ash (corresponding to field application of 0, 1.1, 3.3, 11 and 33 t ha−1). Additionally, to disentangle the pH and nutrient effects of wood ash application, we included treatments with either CaO, to simulate pH effects of wood ash, or potassium (K) + phosphorus (P) fertilizer to mimic the nutrient effects. After 4.5 months of growth, we measured soil pH, plant biomass, Cd accumulation in shoots and N concentration in the various compartments of the system. Wood ash addition stimulated plant growth, whereas CaO and K + P addition resulted in more moderate increases in biomass. Despite the low concentration of N in the wood ash, plant uptake of N increased in wood ash amendments, probably because wood ash stimulated mineralization of soil organic N. Plant Cd content significantly increased at the highest dose of wood ash. Our results suggest that addition of wood ash significantly stimulates plant growth due to the combined effect of increased pH, elevated nutrient levels and increased N mineralization. Furthermore, despite the rather high Cd content in used wood ash (16.3 mg kg−1), wood ash amendments up to 11 t ha−1 did not result in significantly increased plant uptake of Cd.