Soil and tree responses to the application of wood ash containing charcoal in two soils with contrasting properties

Mixed wood ash (MWA), a mixture of fly ash and charcoal generated in power plants, could be used to replenish nutrients and organic matter in intensively managed soils. However, the response of the system will be determined by the soil properties, which will modify the interactions between soil components, microorganisms and plants.The study involved two young Pinus radiata plantations growing on soils that differed in texture, soil organic matter (SOM) and aeration. The soils were treated with MWA alone (a single dose of 4.5 tonnes ha−1 or three annual doses of 4.5 tonnes ha−1) or with MWA supplemented with P fertilizer (4.5 Mg MWA plus 45 kg P ha−1). Analysis of the MWA revealed that the organic compounds in the ash were strongly aromatic. Although MWA is less reactive than fly wood ash, application of MWA enhanced the long-term nutrient availability and positively affected tree nutrition (mainly for P and K) and growth. Significant tree growth responses were recorded 3 yr after the first application. Supplementation with mineral P fertilizer was not required with the larger amounts of MWA (×3 doses). Loss of elements by leaching was low, even in the sandy soil. The SOM content of the sandy soil increased in response to both treatments; this was attributed to the addition of charcoal but also to the higher SOM turnover in the litter. Microbial activity was enhanced in both soils, especially in the sandy soil.In conclusion, the soil response to the application of MWA is greatly influenced by the soil properties; the nutrient and SOM dynamics are directly affected by interactions between the ash and soil components and indirectly by soil biological activity and plant growth.

► MWA was added to two forest plantations growing on soils with different properties.
► Loss of elements by leaching was low, possibly due to charcoal contained in the MWA.
► Additional mineral P fertilizer was not required when large amounts of MWA were applied.
► SOM and nutrient dynamics after MWA application were influenced by soil properties.
► The forest response to MWA application was determined by soil properties.