Carbonation of Wood Ash Recycled to a Forest Soil as Measured by Isotope Ratio Mass Spectrometry

In Sweden, large‐scale recycling of wood ash to forests is considered a means of replenishing base cations, particularly Ca, taken up by the trees and removed by logging. To minimize negative effects on the forest ecosystem, slow rates of dissolution of the ash and release of Ca would be preferred. Carbonation of wetted ash [Ca(OH)2] can reduce the rate of Ca dissolution. In this study, field experiments were performed to evaluate carbonation of ash following application to forest soil beneath a spruce [Picea abies (L.) Karst] stand. To accomplish this, a mass spectrometric analytical method was developed that allowed quantitative estimation and isotopic analysis of the ash carbonate concentration. The dynamic range was between 0.07 and 30 μmol of carbonate, measured as CO2 evolved from an acidulated ash sample, which corresponded with between 0.01 and 5 mg of a completely carbonated ash (dried) containing ≈0.30 g g−1 Ca. The method allowed spatially resolved measurements (≈1 mm3) of the amount of carbonate and the 13C/12C and 18O/16O isotopic ratios. Following field application, ash granules (8–11 mm o.d.) increased their degree of carbonation from ≈45 to >80% within 3 d and were >90% carbonated after 3 wk. For the initial 3 d, the rate of carbonation, Rc, was estimated to be 0.70 (± 0.33) μmol mg−1 d−1 carbonate. During the field carbonation process, the outer granule layer could either be enriched or depleted in carbonate relative to the interior parts, which is suggested to depend on the occurrence of rainfall. These results indicate that the rate of carbonation of ash granule Ca in the field is rapid and probably dominates over Ca dissolution, especially under favorable conditions.