Wood fuel quality of two Salix viminalis stands fertilised with sludge, ash and sludge–ash mixtures

This study assessed the effects of stand structure and fertilisation with wood ash and/or sludge on wood fuel quality of Salix viminalis. The relative proportions of bark and wood in 1-, 2- and 3-year-old shoot populations were determined. The concentrations of essential elements (N, P, K) and heavy metals (Cu, Zn, Cd, Ni) in bark and wood were used to assess the wood fuel quality in harvestable shoot biomass. Controlled field experiments were conducted on two newly harvested commercial short-rotation willow coppice fields. Five treatments were applied: sewage sludge at the maximum legally permitted amount; ash; two sludge–ash mixtures supplying the maximum and twice the maximum permitted sludge–ash amount; and a control receiving mineral nutrients only.The proportion of bark in the willow stands was decreasing with the age of the shoot population. The shoot population with few large stems, compared to that with many small stems, had a lower proportion of element-rich bark in the harvestable shoot biomass, meaning better quality of the wood fuel. Overall, wood fuel quality in terms of mineral concentrations was influenced by the age of the shoot population at harvest, stand structure, management practices (e.g. planting density, fertilisation) and site conditions (soil type, element availability). Our results imply that harvestable shoot biomass of willows grown as few large stems have better wood fuel quality, compared to harvestable shoot biomass of many small stems. Increased length of cutting cycle improves the wood fuel quality.