Decomposition, nitrogen and phosphorus mineralization from beech leaf litter colonized by ectomycorrhizal or litter-decomposing basidiomycetes

The decomposition and the nitrogen and phosphorus mineralization of fresh beech (Fagus sylvatica L.) leaf litter are described. Leaves were buried for up to 6 months in plant containers in which Scots pine (Pinus sylvestris L.) seedlings were cultivated at a low rate of nutrient addition. The saprotrophic abilities of three ectomycorrhizal fungi, Thelephora terrestris Ehrh.: Fr., Suillus bovinus (L.: Fr.) O. Kuntze and Paxillus involutus (Batsch: Fr) Fr., were compared with the degradation caused by the litter-decomposing basidiomycete, Lepista nuda (Bull.: Fr.) Cooke. Uninoculated leaves were included as controls. The investigation was performed at two different pH values since substrate pH is supposed to have an effect on the activities of extracellular enzymes of ectomycorrhizal fungi. The enzyme expression might also be largely influenced by the substrate they colonised. The mycorrhizal fungi caused only a low decomposition rate of the litter compared with that of L. nuda, and nitrogen was released only by L. nuda. Leaves colonized by mycorrhizal fungi showed no net release of nitrogen; on the contrary, a small accumulation of N in the litter was observed. It therefore seems likely that the ectomycorrhizal fungi studied do not have the ability to decompose efficiently the lignocellulose matrix of the relatively recalcitrant beech leaf litter. The degradation of this matrix seems to be essential for the fungi to gain access to the leaf nitrogen pool of fresh beech litter. A direct release of nitrogen from organic compounds by ectomycorrhizal fungi seems therefore to be confined to the older litter layers. The beech leaf litter contained an important fraction of easily mineralizable phosphorus. P was not a growth limiting factor in the cultivation system, and could therefore accumulate in the leaf litter colonized by the ectomycorrhizal mycelium.