Evidence that saprotrophic fungi mobilise carbon and mycorrhizal fungi mobilise nitrogen during litter decomposition

Commentary Evidence that saprotrophic fungi mobilise carbon and mycorrhizal fungi mobilise nitrogen during litter decomposition With improvements in molecular techniques, identification of taxa in mycorrhizal ecology has expanded from fruitbodies to mycorrhizal roots to extraradical hyphae (Anderson & Cairney, 2004). These molecular techniques are, in general, equally applicable to saprotrophic fungi, although this important functional group has received relatively little focus in community studies (Allmer et al ., 2006). Only a few studies have examined the spatial patterns of ectomycorrhizal fungi in soil profiles, and no studies have examined similar patterns for saprotrophic fungi. In this issue of New Phytologist (pp. 611– 620), Lindahl et al. reported on the spatial patterns of ectomycorrhizal and saprotrophic fungi from soil profiles in a Pinus sylvestris forest in Sweden, and compared those patterns with patterns of bulk carbon:nitrogen ratios, 15 N content and radiocarbon (as a proxy for age). As expected, each of these parameters increased with depth in soil profiles. The authors also reported a striking separation of the ectomycorrhizal and the saprotrophic communities, with the surface litter layer strongly dominated by saprotrophic fungi and the deeper horizons strongly dominated by ectomycorrhizal fungi. This physical separation implies that these fungal types also play separate roles in the carbon and nitrogen cycles by exploiting discrete pools of litter. '… these observations are the best evidence to date that the ectomycorrhizal transfer of 15 N-depleted N may be a primary driver for 15 N enrichment in soil profiles' Inputs to the soil profile The two overlapping fungal communities (ectomycorrhizal and saprotrophic) surveyed by Lindahl et al. primarily consume carbohydrates for their metabolic requirements. Their relative abundance through the soil profile partly reflects the pattern of fresh carbon input through two main sources. Saprotrophic fungi, because of their extensive capabilities to degrade lignin, predominate where above-ground litterfall is the primary input. In contrast, the peak of relative abundance of ectomycorrhizal taxa in the F and H layers presumably reflects a related peak in below-ground carbon input via roots. The unexpected increase in the C:N of organic matter from the F layer to the lower H layer may reflect these inputs, but could also indicate that soil N is sufficiently available in the lower organic horizon to be mobilised and removed by mycorrhizal fungi. Thus, carbon inputs into boreal forest soils can be spatially separated into a discrete above-ground input to the litter surface via litterfall …