The data set describes soil ergosterol concentration, which is unique to fungal membranes., from samples from Abisko, Sweden. The samples from Abisko, Sweden, were collected by E.A. Hobbie at a birch forest site and a tundra site. The Abisko sites are described in www.abacus-ipy.org/fieldsites/abisko.html.
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The method , the complete references, and similar results from Alaska are described in Hobbie, J. E., E. A. Hobbie, H. Drossman, M. Conte, J. C. Weber, J. Shamhart, and M. Weinrobe. 2009. Mycorrhizal fungi supply nitrogen to host plants in Arctic tundra and boreal forests: N-15 is the key signal. Canadian Journal of Microbiology 55:84-94. Methods used are in de Ridder-Duine, A. S., Smant, W., van der Wal, A., van Veen, J. A., and de Boer, W. 2006. Evaluation of a simple, non-alkaline extraction protocol to quantify soil ergosterol. Pedobiologia, 50: 293-300.
Bååth, E. 2001. Estimation of fungal growth rates in soil using 14C-acetate incorporation into ergosterol. Soil Biol. Biochem. 33: 2011-2018.
Methods for ergosterol in 2007
Sampling. In 2007, a 10-20 cm deep, 6 cm diameter core was taken at a representative site next to known ectomycorrhizal plants. After collection, soil samples were immediately placed on ice and frozen within six hours. In the laboratory, each core from the 2007 transect was thawed and sectioned into three approximately equal depth organic layers and two approximately equal depth mineral layers for analysis of ergosterol, soil moisture, and soil organic matter.
Ergosterol. This fungal-specific sterol, a component of the cell membrane, estimates living fungal biomass in soils. We modified the saponification methods described by Bååth (2001) and de Ridder-Duine et al. (2006) for HPLC quantification of ergosterol. In the laboratory, thawed samples were sectioned by horizon (e.g., organic and mineral), each layer sub-sectioned by depth, and each subsection sieved (2 mm screen). Duplicate or triplicate sub-samples (0.5 g) of the sieved soil fractions were saponified by heating for 90 min at 70C in 2 ml of methanol and 0.5 ml of 2 M NaOH after brief vortex mixing. After cooling, 1 ml methanol and 3 ml pentane were added and samples vortex-mixed for 20 s. Samples were centrifuged briefly to eliminate emulsions and the upper pentane phase, containing ergosterol, was collected. The pentane addition, extraction and centrifugation was repeated twice more, but with 2 ml pentane per extraction step. The pentane (upper) phase was collected and combined with previous extracts. The combined pentane extracts were dried under nitrogen flow, dissolved in 1.0 ml methanol, filtered through a 0.45 μm pore size PTFE syringe filter (Millipore, Corporation, Billerica, Massachusetts, USA) and injected into a Waters Acquity UPLC system with a binary solvent manager, a sample manager, and a photodiode array detector (PDA) (Waters Corporation, Milford, Massachusetts, USA). Separation was on an Acquity UPLC BEH C18 1.7 micron pore size, 2.1 x 50 mm reverse phase column maintained at 40C. A 5 µl injection of each extract was eluted with 92% methanol-8% water at a flow rate of 0.5 ml min−1. Using Millenium software (Waters) the UV spectrum of the ergosterol peak at ~2 min was compared with standard ergosterol (Fluka Company, St. Louis, Missouri, USA) and was quantified at 282 nm with calibration standards in the range of 0.05 – 20 ppm. Calibration provided linear fit (r2 > 0.999) with standards in all runs with a limit of detection (S/N 3:1) of ~ 30 ppb. Spike recoveries determined by the method of de Ridder-Duine et al. (2006) are 82±4%, in agreement with prior literature values (Bååth 2001). Variance for duplicate or triplicate samples resulting from sample inhomogeneity, extraction, and instrumental sources were generally less than 5% and are calculated as standard deviation for each sample.
Soil moisture was determined gravimetrically on a separate sub-sample by drying to constant weight. Soil organic matter was calculated from the loss in mass of the dry soil after heating at 500 deg C for 12 hours.
Fungal biomass. Several conversion factors have been used to calculate fungal biomass from ergosterol. Montgomery et al. (2000) proposed a factor of 4 mg fungal biomass per µg ergosterol based on a calibration with six species of cultured saprotrophic fungi while Salmanowicz and Nylund (1988) proposed a factor of 3 mg fungal biomass per µg ergosterol based on cultures of three different species of ectomycorrhizal fungi. We followed Clemmensen et al. (2006) and used 3 mg µg-1 as this reflects the fungal taxa most commonly observed at our sampling sites.
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