We studied carbon and nitrogen allocation in mycorrhizal and non-mycorrhizal Scots pine (Pinus sylvestris L.) seedlings grown in a semi-hydroponic system with nitrogen as the growth limiting factor. Three ectomycorrhizal fungi were compared: one pioneer species (Thelephora terrestris Ehrh.: Fr.) and two late-stage fungi (Suillus bovinus (L.: Fr.) O. Kuntze, and Scleroderma citrinum Pers.). By giving all plants in each treatment the same amount of readily available nitrogen, we ensured that the external mycelium could not increase the total nitrogen content of the plants, thereby guaranteeing that any change in carbon or nitrogen partitioning was a direct effect of the mycorrhizal infection itself. Carbon and nitrogen partitioning were measured at an early and a late stage of mycorrhizal development, and at a low and a high N addition rate. Although mycorrhizal seedlings had a higher net assimilation rate and a higher shoot/root ratio than non-mycorrhizal seedlings, they had a lower rate of shoot growth. The high carbon demand of the mycobionts was consistent with the large biomass of external mycelia and the increased belowground respiration of the mycorrhizal plants. The carbon cost to the host was similar for pioneer and late-stage fungi. Above- and belowground partitioning of nitrogen was also affected by mycorrhizal infection. The external mycelia of Scleroderma citrinum retained 32% of the nitrogen supplied to the plants, thus significantly reducing nitrogen assimilation by the host plants and consequently reducing their growth rate. By contrast, the external mycelia of T. terrestris and Suillus bovinus retained less nitrogen than the mycelia of Scleroderma citrinum, hence we attributed the decreased growth rates of their host plants to a carbon drain rather than a nitrogen deficiency.
summary Activities of extracellular enzymes were determined in beech (Fagus sylvatica L.) leaf litter colonized by the litter decomposer Lepista nuda (Bull.: Fr.) Cooke or by vegetative mycelium from the mycorrhizal fungi Thelephora terrestris Ehrh.: Fr. or Suillus bovinus (L.: Fr.) O. Kuntze. Organic matter (OM) was buried for up to 6 months in plant containers in which mycorrhizal or non‐mycorrhizal Pinus sylvestris L. seedlings were cultivated at a low rate of nutrient addition. After different periods of colonization the activities of phosphomonoesterase and protease, two enzymes involved in nitrogen and phosphorus mobilization, were determined in colonized and uncolonized litter. The activities of cellulase, β‐xylosidase, β‐glucosidase and polyphenol oxidase were investigated as indicators of the decomposition capacity of the fungi in the litter. Low activities of all enzymes tested were found in the uncolonized beech leaves. Phosphomonoesterase activity was high in litter colonized with L. nuda or S. bovinus, and was intermediate in the T. terrestris treatment. For all other enzymes the activities in the OM inoculated with the white‐rot litter decomposer were considerably larger than those detected in litter colonized by ectomycorrhizal basidiomycetes. Cellulase activity was low in the control as well as in the mycorrhizal treatments. β‐Xylosidase and β‐glucosidase were detected in the litter with mycorrhizal mycelium, whereas polyphenol oxidase activity was only clearly increased in the S. bovinus treatment. These results demonstrate the low lignocellulase activity of both mycorrhizal fungi. This feature reduces the capacity of the mycorrhizal fungi to exploit fresh beech leaf litter, whose endogenous nitrogen is associated with or shielded by refractory compounds. The results are discussed in relation to the role of ectomycorrhizal fungi in nutrient cycling processes m temperate forest ecosystems.
S U M M A R YThe 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 m which Scots pine (Pxnus 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.: Ft.) O. Kuntze and Paxillus involutus (Batsch: Fr) Ft., were compared with the degradation caused by the iittet-decomposing basidiomycete, Lepista nuda (Bull.: Ft.) 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 ectomyccrrhizal fungi studied do not have the ability to decompose efficiently the ligDOcellulose matrix of the relatively recalcitrant beech Jeaf 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 m the cultivation system, and could therefore accumulate m the leaf litter colonized by the ectomycorrhizal mycelium.
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