Wood ash applications to temperate forest ecosystems-potential and drawbacks

The objectives of the present work were (a) to quantify the effects of wood ash on forest ecosystems through a meta-analysis approach associated with a detailed review of the literature (mainly composed of work carried out in Nordic countries) and (b) to extrapolate the effects on forest growth to other contexts (i.e. warm temperate countries) by identifying the cases for which wood ash applications can be beneficial to forest production. Three databases were built regarding the effects of wood ash on soil (151 observations; 33 experimental field trials), on nutrient concentrations of tree foliage (68 observations; 28 trials) and on annual stem biomass growth rate (70 observations; 27 trials). We obtained information on the wide variability of ash properties due to differences in burnt compounds, combustion processes and ash conditioning. Two important properties of wood ash are its high pH value and neutralizing capacity. These properties result in biochemical modifications of forest soils limed with ash. In the short term, soil solution composition was dramatically modified. Intense peaks of the K, Na or SO4 concentrations were observed, resulting from the dissolution of salts contained in ash. At the same time, Ca and Mg concentrations increased as the carbonate pool of wood ash started to dissolve. The consequence of this dissolution process was an increase in the pH in all the soil phases. These modifications increased the activity of the soil microflora and some isolated peaks in the mineralization of soil organic matter may be observed in mineral soils. In the longer term, that is to say after the first year following ash application, only the effect on the acidity status of the soil remained significant. The effects of ash addition on forest ecosystems usually increased with the dose and were more pronounced with loose ash compared to aggregated ash. The addition of wood ash into forest ecosystems increased the foliar Ca status of trees. Some modifications of other nutrients, like P or K, were also observed but only for a few years after treatment. For most stands growing on mineral soils of Nordic countries, this treatment did not result in an increase in tree growth, probably because of the absence of N in the ash. For stands growing on organic soils of the same area, this input, associated with a long-lasting increase of soil organic matter mineralization, was sufficient to improve tree growth significantly (median=+59% compared to the control). For soils located in warm temperate regions, similar responses are expected for organic soils. For mineral soils, the wood ash application is expected to be suitable for stands showing deficiencies in K, Ca or Mg. Ash may contain high amounts of toxic heavy metals such as Cd. The bioavailability of most of these elements appeared to be very low in a forest context. No contamination of food chains has been observed, except possibly via some species of fungi, and heavy metals remain in the forest litter or in the topsoil. Based on all the reviewed results, several guidelines for wood ash application into forest ecosystems are proposed. Wood ash application should be restricted to acidic soils. Applications should consist of low doses of a stabilized ash form. Wood ash should be applied to adult stands rather than onto seedlings.