An evaluation of land suitability for forest fertilization with biofuel ash on organic soils in Sweden

The nutrients removed from the forest when branches and tree tops are harvested for fuel can be returned to the site by bringing back the wood-ash from the burning. In Sweden, this compensation measure is not carried out to any appreciable extent, mostly because there is no economic incentive to the landowner. The ash contains all the elements required for tree growth except for nitrogen (N), the most important element limiting growth on mineral soils. Since ash, so far, is brought back only to mineral soils, increased forest growth cannot be expected. In contrast, for organic soils, N is often abundant whereas the amounts of other mineral nutrients in peat are small. This means that the peatland forests provide an opportunity for ash amendment in order to increase forest production. Old fertilization trials using wood-ash show that the growth increase can be very large. The aims of this study were (i) to calculate the area of peat-covered land that, with respect to stand growth responses, could be regarded as most suitable for biofuel ash (wood-ash and peat-ash) fertilization and (ii) to assess the amount of biofuel ash needed for fertilizing this area. Most of the area calculations were based on data from the Swedish National Forest Inventory (SNFI) 1997-2001. Sites were selected using existing knowledge about ash fertilization effects on tree growth and with the aid of registrations made in SNFI regarding peat thickness, site productivity, drainage, condition of drains, dominating field vegetation, and stage of stand development. Additional calculations were made to estimate the area of abandoned peat harvesting fields ready for after-use by afforestation. According to the selection criteria used, the most suitable sites for biofuel ash fertilization are drained, mid-rotation or old peatland forests where the field vegetation is dominated by shrubs or low sedge plants indicative of medium-low site productivity. The selected area comprises 190,000 ha. Most are located in North and North Central Sweden (90,000 ha), whereas South Central and South Sweden accounted for 30,000 and 70,000 ha, respectively (Figs. 1 and 2). To these areas, 2000-3000 ha of abandoned peat fields ready for afforestation should be available within a period of about 5 years. The present annual production of biofuel ash in Sweden, of acceptable quality for forest fertilization, is 250,000300,000 tonnes. If it were desired to fertilize all the sites (190,000 ha) identified in this study with 5000 kg per ha, it would require 3-4 years of the current annual production of bio-ash. (c) 2005 Elsevier B.V. All rights reserved.