The wood-decaying fungus Hygrophoropsis aurantiaca increases P availability in acid forest humus soil, while N addition hampers this effect Ann-Mari Fransson a, * , Inger Valeur b , Ha ˚kan Wallander c a Department of Ecology, Soil-Plant Research, Ecology Building, Lund University, SE-223 62 Lund, Sweden b Department of Ecology and Environmental Research, Swedish University of Agricultural Sciences, SE-750 07 Uppsala, Sweden c Department of Ecology, Microbial Ecology, Ecology Building, Lund University, SE-223 62 Lund, Sweden Received 11 February 2003; received in revised form 2 April 2004; accepted 27 April 2004 Abstract We evaluated the influence of the brown rot fungus Hygrophoropsis aurantiaca on P solubility in the humus layer of a podzolic forest soil. This fungus is known to exude large amounts of oxalic acid that may stimulate weathering of minerals and increase dissolution of humus, which in turn may increase P availability in the soil surrounding the fungus. Humus was inoculated using small wooden pieces colonised by the fungus. The presence of the fungus resulted in elevated concentration of PO 4 2 in the humus solution. In a second experiment birch seedlings grown in the same humus were able to utilise the PO 4 2 mobilised by the fungus to increase their internal P content. The factor determining this increased P uptake and the increased available P might be oxalate produced by fungus. The acid may directly dissolve P or change organic forms of P making it more susceptible to reaction with phosphatases. This fungal effect on P solubility diminished when N was added to the soil in the form of a slow release N fertilizer (methyl urea), or when a soil with a higher soil N concentration was used. We found a strong correlation between NH 4 þ concentration and total organic carbon in the soil solution at high NH 4 þ concentrations, suggesting the dissolution of humus as a result of the high NH 4 þ content in the solution. This study demonstrates that the wood-decaying fungus H. aurantiaca influences nutrient turnover in forest soil, and thereby nutrient uptake by forest trees. An intensified harvest of forest products such as whole tree harvesting may decrease the active biomass of the wood decomposers and may thereby change the availability of P and the leaching of N. q 2004 Elsevier Ltd. All rights reserved. Keywords: Betula pubescens; P uptake; Decomposition; Oxalic acid; Wood residues; NO 3 2 leaching; Hygrophoropsis aurantiaca; Acid humus 1. Introduction Nitrogen limits tree growth on most boreal forests soils. Anthropogenic input of N has resulted in increased forest productivity, which may lead to depletion of other nutrients (Mohren et al., 1986; Dralle and Larsen, 1995; Thelin et al., 1994). In some studies N fertilisation resulted in develop- ment of P deficiency symptoms in white spruce (Picea glauca (Moench) Voss) seedlings (Teng and Timmer, 1995). Finding better ways to maintain nutrient availability in forest soil is of vital importance. Intensified harvesting, such as whole-tree harvesting, where branches and foliage are removed, is becoming a more common practice in modern forestry. This biomass is used to support the increasing demand for renewable energy sources. This practice inevitably leads to less decomposable wood on the forest floor, and is likely to influence the composition and biomass of the decomposer community. A better understanding of the relative importance of these bacteria and fungi to nutrient availability is necessary to evaluate the influence of whole-tree harvesting on the long-term sustainability of forest soils. Apart from being involved in nutrient cycling in forests, decomposer organisms can also compete with the trees for nutrients and interact with mycorrhiza (Lindahl et al., 1999). Decomposer organisms may also affect nutrient transform- ations in forests. Wood-decaying fungi produce large amounts of oxalic acid; this is especially true for those causing brown rot (Akamatsu et al., 1994). Oxalic acid has been suggested to contribute to the decomposition process by withdrawing Ca from the cell wall (Dutton and Evans, 1996) and to fracture hemicellulose and depolymerize 0038-0717/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2004.04.027 Soil Biology & Biochemistry 36 (2004) 1699–1705 www.elsevier.com/locate/soilbio * Corresponding author. Tel.: þ 46-46-222-4481; fax: þ46-46-222-4423. E-mail address: ann-mari.fransson@planteco.lu.se (A.-M. Fransson).