Research SpeciesBlackwell Publishing Ltd. level patterns in 13C and 15N abundance of ectomycorrhizal and saprotrophic fungal sporocarps Andy F. S. Taylor1, Petra M. Fransson1, Peter Högberg2, Mona N. Högberg2 and Agneta H. Plamboeck3 1Department of Forest Mycology and Pathology, Swedish University of Agricultural Sciences, PO Box 7026, SE-750 07 Uppsala, Sweden; 2Section of Soil Science, Department of Forest Ecology, Swedish University of Agricultural Sciences, S-901 83 Umeå, Sweden. 3Center for Stable Isotope Biogeochemistry, Department of Integrative Biology, University of California, Berkeley, CA 94720, USA. Abstract Author for correspondence: • The natural abundance of 13C (δ13C) and 15N (δ15N) of saprotrophic and ecto- Andy Taylor mycorrhizal (ECM) fungi has been investigated on a number of occasions, but the Tel: +46 18 672797. significance of observed differences within and between the two trophic groups Fax: +46 18 673599 Email: [email protected] remains unclear. • Here, we examine the influence of taxonomy, site, host and time upon isotopic Received: 24 February 2003 data from 135 fungal species collected at two forest sites in Sweden. Accepted: 15 May 2003 • Mean δ13C and δ15N values differed significantly between ECM and saprotrophic doi: 10.1046/j.1469-8137.2003.00838.x fungi, with only a small degree of overlap even at the species level. Among ECM fungi, intraspecific variation in δ15N was low compared with interspecific and intergeneric variation. Significant variation due to site, year and host association was found. • At broad scales a number of factors clearly influence δ13C and δ15N values making interpretation problematic. We suggest that values are essentially site-specific within the two trophic groups, but that species-level patterns exist potentially reflecting ecophysiological attributes of species. The species is therefore highlighted as the taxonomic level at which most information may be obtained from fungal δ13C and δ15N data. Key words: fungal diversity, functional groups, nutrient cycling, ectomycorrhizal (ECM) fungi, saprotrophic fungi, stable isotopes. © New Phytologist (2003) 159: 757–774 fungi have the potential to assimilate many of the major Introduction nitrogen (N)- and phosphorus (P)-containing organic Ectomycorrhizal (ECM) fungi, obligate root symbionts of molecules in plant, microbial and animal detritus (Leake most boreal forest trees, and saprotrophic macromycetes & Read, 1997). This may occur either directly via the contribute most to the observed fungal diversity within boreal production of catabolic extracellular enzymes or indirectly forest ecosystems (Bills et al., 1986; Såstad & Jenssen, 1993; via combative interactions with saprotrophic fungi (Lindahl Renvall, 1995; Väre et al., 1996). Traditionally, these fungi et al., 1999). In addition, some fungi formally regarded as have been regarded as two distinct functional groups within saprotrophs are now known to be ECM fungi (see Agerer & ecosystems (Dighton, 1995; Leake & Read, 1997) with Beenken, 1998; Erland & Taylor, 1999; Kõljalg et al., 2000). saprotrophic fungi obtaining carbon (C) and nutrients from Direct in situ observation of fungi is difficult because of the the degradation of organic compounds (Swift et al., 1979) small size of the vegetative structures and the opaque nature and ECM fungi facilitating the uptake of nutrients by their of the growing medium. Consequently, most knowledge con- autotrophic host plants in return for fixed C as photosynthate cerning the involvement of these fungi in ecosystem processes (Smith & Read, 1997). However, over the past two decades is derived from laboratory investigations. One method that the distinction between the two groups has become less well has, in recent years, been investigated as a potential indirect defined. There is now considerable evidence that some ECM method for assessing the functional roles of the fungi in © New Phytologist (2003) 159: 757–774 www.newphytologist.com 757 758 Research ecosystems is the analysis of the stable isotopes 13C and 15N capabilities, carbon demand and habitat preferences (Tyler, in fungal material. 1985; Leake & Read, 1997; Cairney, 1999). All of these Cycling of C and N through different components of factors could be expected to affect the δ15N and δ13C values ecosystems create small but measurable differences in the of the sporocarps produced by individual ECM species. It is isotope ratios of 13C : 12C and 15N : 14N (Dawson et al., 2002). also well established that there are distinct successions of These differences have been used extensively to investigate saprotrophic fungi colonizing plant debris (Renvall, 1995), with plant ecology and the pathways of C and N cycling through each fungal species or group of species capable of utilizing ecosystems (Farquhar et al., 1989; Fung et al., 1997; Högberg, different chemical components of the plant material (Tanasaki 1997) and this approach has also been used as a tool to et al., 1993). As these components may vary with respect to investigate ECM and saprotrophic fungal ecology (Hobbie 15N (Högberg, 1997) and 13C abundance (Gleixner et al., et al., 1999a, 2001; Högberg et al., 1999b; Henn & Chapela, 1993), it seems inevitable that the 15N and 13C signatures of 2001). Direct analysis of soil mycelia is rarely practical and sporocarps will reflect a differential substrate usage (Gebauer nearly all of these studies have involved determining the & Taylor, 1999). natural abundance of 15N and 13C in the sporocarps of If differences in the 15N and 13C abundance of sporocarps macromycetes. One exception to this is the study by Högberg are a reflection of the physiological diversity among fungal et al. (1996), where the 15N abundance in the mantles of species, then the data may be most informative at the taxonomic beech (Fagus sylvatica) mycorrhizas was determined. Some level of the species. Support for this idea comes from the study general patterns are becoming apparent from these studies. by Högberg et al. (1999b), which demonstrated that by Ectomycorrhizal fungi are generally more depleted in 13C and analysing 13C abundance data from mycorrhizal fungi at the more enriched in 15N than saprotrophic fungi (Hobbie et al., species level, host specificity between trees and their associated 1999b; Högberg et al., 1999b; Kohzu et al., 1999; Henn & ECM fungi could be examined in situ for the first time. Chapela, 2001). Among the saprotrophs, litter fungi are more The present study examines the 15N and 13C natural enriched in 15N than wood decomposers, with both groups abundance in sporocarps of ECM and saprotrophic fungi to enriched relative to their substrates (Gebauer & Taylor, 1999; determine (1) how site, host and time influence 15N and 13C Kohzu et al., 1999). natural abundance and (2) if species-specific patterns exist. In Another general pattern is emerging in which ECM fungi addition, inter-yearly variation was also examined. The data is are considerably enriched in 15N relative to their host plants derived from extensive collections of ECM and saprotrophic (Gebauer & Dietrich, 1993; Högberg et al., 1996, 1999a; fungi at sites in central and northern Sweden. The results Taylor et al., 1997; Michelsen et al., 1998; Hobbie et al., show that potentially important ecological information 1999b). Given that > 95% of the host root tips are usually may be lost when 15N and 13C natural abundance data are colonized by ECM fungi (Dahlberg et al., 1997; Fransson interpreted above the level of the species and that using the et al., 2000; Taylor et al., 2000), most of the N taken up by the data to distinguish between saprotrophic and ECM fungi tree would have to pass through the fungi. It could therefore must be done with caution. be expected that the 15N signatures of the fungi and the host plant would be similar. Fractionation during transfer of N Materials and Methods from the soil through the fungal tissue to the host plant has been suggested as the main reason for the observed differences Sites between the host and fungal 15N abundances (Hobbie et al., 1999b; Högberg et al., 1999a). The study was conducted at Stadsskogen in Uppsala, central In many of the field investigations cited above, fungi are Sweden (59°52′N, 17°13′E, 35 m above sea level) and at often split into large ecological groups (ECM, litter or wood Åheden in the Svartberget Research Forest, 60 km north-west decomposers), within which species are considered to carry of Umeå, northern Sweden (64°14′N, 19°46′E, 175 m above out similar functions within the ecosystem. It therefore sea level). Both forests have 150-yr-old Scots pine (Pinus follows that within a functional group, species are expected sylvestris L.) as an overstorey with Norway spruce (Picea abies to have similar isotope signatures and conclusions concerning (L.) Karst.) approaching the role of a codominant. At the significance of observed values are therefore often made at Stadsskogen, understorey deciduous tree species included these very broad functional levels, regardless of the taxonomic aspen (Populus tremula L.), birch (Betula pendula Roth), diversity of the organisms involved. There is, however, con- willow (Salix caprea L.), and alder (Alnus incana (L.) Moench). siderable evidence, both physiological and ecological, which At Åheden, birch was the only understorey tree species. strongly suggests that this assumption of ecological equiva- lency within fungal functional groups is invalid (Smith & Sampling Read, 1997). Among ECM fungi, much variation exists among species, Sampling was conducted in an area of c. 1 ha at Åheden in even within the same genus, with regard to enzymatic August 1997 and at five plots, each with an area of 0.25 ha at www.newphytologist.com © New Phytologist (2003) 159: 757–774 Research 759 Stadsskogen between August and October 1997. Between using and Tukey’s family error (5%) test. Temporal one and eight mature sporocarps from each fungal species effects were examined by calculating mean values for each of were sampled at each site (see Appendix 1). Nomenclature the six sampling periods based on all sporocarps collected at primarily follows that of Hansen & Knudsen (1992, 1997).