Micro vertical changes in soil pH and base cations over time after application of wood ash on forest soil

Application of wood ash to forest soils is used to counteract acidification and to reintroduce nutrients otherwise lost by harvesting, particularly in silvicultural systems with intensified biomass removal. pH governs many essential processes like nutrient availability for plant uptake, heavy metal mobility and ecosystem functioning in forest soil, but changes in soil pH on a micro scale after ash application have not been reported. The aim of this study was therefore to investigate micro vertical changes in soil pH and base cations over time after application of wood ash on forest soil. A column experiment with repacked O- and A-horizon forest soil was constructed and treatments of 0, 3 and 9 t wood ash ha−1 were applied on top of the soil in replicates of three. Soil cores were extracted from the columns monthly over one year for pH micro sensor measurements of O- and A-horizon (0-5 cm soil depth) and at four occasions (62, 152, 278 and 363 days after treatment) additional soil cores were extracted for base cation analysis. Results showed that ash application creates strong pH gradients ranging from pH 11 to 4.5 within the top 1-1.5 cm of the soil, with the highest variation within the first 50 days after application. Application of ash increases the nutrient availability. If micro scale variation is not considered, mean pH values can be a misleading indicator for the extreme conditions that can exists in the top part of the soil after ash application.