Uptake of an amino acid by ectomycorrhizal and saprotrophic fungi in a boreal forest

Soil Biology & Biochemistry 40 (2008) 1964–1966 Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier. com/locate/soilbio Short communication Uptake of an amino acid by ectomycorrhizal fungi in a boreal forest Kathleen K. Treseder a, * , Claudia I. Czimczik b , Susan E. Trumbore b , Steven D. Allison a, b a b Department of Ecology and Evolutionary Biology, University of California,, Irvine, CA 92697, USA Department of Earth System Science, University of California, Irvine, CA 92697, USA a r t i c l e i n f o a b s t r a c t Article history: Received 9 October 2007 Received in revised form 19 February 2008 Accepted 20 February 2008 Available online 5 May 2008 We assessed the degree to which ectomycorrhizal fungi exploit organic nitrogen in situ. In an Alaskan boreal forest, we identiﬁed pairs of sporocarps from ﬁve taxa of ectomycorrhizal fungi. We added 13 C- labeled alanine to the soil surrounding one sporocarp within each pair; the second served as an un- labeled control. Peak rates of 13 C-respiration from alanine were higher in the labeled sporocarp plots than the controls, indicating that the 13 C-alanine was detectably respired from the soil. ‘‘Reference’’ plots adjacent to the sporocarps served as an indication of background 13 C-respiration rates released by the soil community as a whole. Ectomycorrhizal sporocarps displayed higher 13 C-respiration rates than their reference plots. Thus, the sporocarps and associated mycorrhizal mycelium appeared to contribute sig- niﬁcantly to the release of alanine-derived 13 CO 2 , conﬁrming the hypothesis that ectomycorrhizal fungi may access soil amino acid pools under natural conditions. O 2008 Elsevier Ltd. All rights reserved. Keywords: Alanine Organic nitrogen Nutrient uptake Decomposition Soil carbon dioxide respiration Sporocarp Isotopic labeling Ectomycorrhizal fungi have long been recognized as an impor- tant mechanism for uptake of N by plants. These fungi colonize root tips of certain plant species and extend hyphae and rhizomorphs into the soil for several meters or more (Agerer, 1992, 2001). Ni- trogen and other nutrients are absorbed by the hyphal network, and a portion is then translocated to the plant in exchange for C (Smith and Read, 1997). Altogether, host plants could acquire as much as 61–86% of their total N budget from these symbionts (Hobbie and Hobbie, 2006). Numerous culture- and greenhouse-based studies have dem- onstrated that ectomycorrhizal fungi can use as a sole N source a wide array of organic N compounds including amino acids, pro- teins, chitin, and DNA (e.g., Melin and Mikola, 1948; Abuzinadah and Read, 1986; Leake and Read, 1990). In addition, amino acid transporters have been identiﬁed in three ectomycorrhizal species (Abuzinadah and Read, 1988; Chalot and Brun, 1998; Wipf et al., 2002). These studies have established the physiological capacity of ectomycorrhizal fungi to take up organic N. Less is known about the extent to which ectomycorrhizal fungi acquire organic N under ﬁeld conditions (Jones et al., 2005; Ho¨gberg and Read, 2006). In our study, we measured the respiration of 13 C by ectomycorrhizal sporocarps in situ after injections of 13 C-labeled alanine into the surrounding soil. We hypothesized that ectomycorrhizal * Corresponding author. Tel. : þ1 9498247634; fax: þ1 9498242181. E-mail address: treseder@uci.edu (K.K. Treseder). 0038-0717/$ – see front matter O 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio. 2008.02. 010 sporocarps would release detectable levels of 13 C derived from al- anine, owing to the uptake and use of this amino acid by the fungus. To test our hypothesis, we ﬁrst selected fresh sporocarps of ﬁve ectomycorrhizal taxa (Table 1) in an Alaskan black spruce forest near Delta Junction, AK (site characteristics detailed in Treseder et al., 2004, 2007) in August 2006. Each taxon was represented by two sporocarps: one to be exposed to 13 C-alanine, and one to re- main unlabeled as a control. We established two 10 cm 10 cm plots for each sporocarp. One plot was centered on the sporocarp. The other was located 15–40 cm away, contained no sporocarps of any species, and served as a ‘‘reference plot’’. Both plots received simultaneous injections of 13 C-alanine or deionized water, depending on the treatment. Respired CO 2 from each plot was sampled for 13 C analysis in order to calculate the isoﬂux (mg 13 C m 2 h 1 ) of alanine-C released, following Czimczik et al. (2005). Essentially, isoﬂux is the product of the respiration rate and the 13 C: 12 C ratio of CO 2 . The isoﬂux from the sporocarp plot was a mixture of CO 2 respired from the sporocarp and from other microbes and plant roots present in the plot. The reference plots provided an estimation of the isoﬂux produced by these other sources. C from labeled alanine was respired by the soil community within hours of injection, given that peak isoﬂuxes from labeled plots were higher than those for non-labeled control plots (Table 1; paired t-test, P ¼ 0.009). Speciﬁcally, peak isoﬂuxes were higher in labeled plots than in control plots for each of the ﬁve pairs. In addition, sporocarp plots displayed greater isoﬂuxes than did their