Experimental design

The experiment was set up in a two-way factorial design with the factors beech and ash (absence: ―-― and presence: ―+‖), resulting in the following treatments with four replicates each: (a) two beech seedlings (BB), (b) two ash seedlings (AA), (c) a mixture with one beech and one ash seedling (BA or AB, depending on target tree species), and (d) an unplanted control (Co), resulting in rhizotrons without (B-: Co and AA) and with beech (B+: BB and BA), as well as rhizotrons without (A-: Co and BB) and with ash (A+: AA and AB).

6.3.3 Sampling

After After 475 days rhizotrons were harvested. They were opened in horizontal position and a sampling grid was used to identify locations for sampling, i.e., at ES and the surrounding of these sites (SS; see Fig. 6.1). Samples from the depth layers II, III, IV and V of the central compartment were analyzed. Further, as we were not interested in effects of soil depth we pooled the data from the four layers. In addition

Table 6.2. Isotopic signatures of the used soil, labeled ash litter and of the soil-litter-mixture in manipulation sites at the start of the experiment and at the end after 422 d of litter incubation (means ± 1 SE).

Start End

Soil Litter Soil-litter mixture

Soil-litter mixture

Difference*

[%]

δ¹³C [‰] -26.20 ± 0.10 146.80 ± 0.32 69.00 ± 0.60 -17.44 ± 1.86 88.25 δ¹⁵N [‰] 1.60 ± 0.16 13139.30 ± 59.10 6153.80 ± 0.40 577.38 ± 124.88 81.23

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to soil samples, plant shoots and roots from each of the soil layers were taken for measuring plant biomass. Details on root biomass distribution along the soil depth gradient as well as on gas emissions are presented elsewhere (Fender et al., 2013).

Plants

At harvest shoot length and root collar diameter of seedlings was measured. Roots were separated from soil, washed and cleaned from adhering soil particles. To obtain overall plant biomass fine root biomass estimated from ES for mycorrhizal analysis were combined with plant biomass data from SS. Whenever possible three intact root strands of ca. 7 cm length from each tree species per compartment and soil depths were taken and digitalised on a flat-bed scanner for image analysis carried out using WinRhizo 2005c software (Régent Instruments Inc., Québec, QC, Canada) to determine specific fine root area (SRA; cm² g^-1 dry matter), specific fine root length (SRL; cm g^-1 dry matter) and total fine root surface. Thereafter, samples were oven-dried (70°C, 48 h), weighed and milled for measurement of organic carbon (Corg), total nitrogen (Ntotal) as well as δ¹³C and δ¹⁵N signatures (Delta C, Finnigan MAT, Bremen, Germany).

Mycorrhiza

Colonization of roots at ES by mycorrhiza-forming fungi was determined. Fine roots were stored in Falcon tubes with moist tissue paper at 4°C until analysis. Fine roots of beech were analyzed with a stereomicroscope (Leica M205 FA, Leica Microsystems, Wetzlar, Germany). The percentage of EM fungi colonization was calculated using the following equation: determining the colonization by AM fungi roots were stained with lactophenole-blue (Schmitz et al., 1991) and stored at room temperature in 50% glycerol until microscopic inspection at 200x magnification. AM fungi colonization was calculated with the magnified intersection method of McGonigle et al. (1990) using a 10x10 grid. The abundance of vesicles, arbuscles and hyphae was calculated as percentage of mycorrhizal structures of the total number of intersections. The percentage of

vesicles was taken as relative colonization rate of AM fungi and used for further calculations.

Soil properties

Soil pH was measured in a suspension of 4 g soil and 10 ml H2O with a Vario pH meter (WTW GmbH, Weilheim, Germany). Soil water content was measured gravimetrically after drying at 105°C for 24 h. Nitrate and ammonium concentrations were measured by extracting soil samples in 0.5 M K₂SO₄ solution (1:3 wet soil mass-to-solution ratio). Samples were shaken for 1 h and filtered through Sartorius folded filters (Sartorius Stedim, Aubagne, France). Nitrate and ammonium concentrations of filtered extracts were analyzed using continuous flow injection colorimetry (SAN⁺ Continuous Flow Analyzer, Skalar Instruments, Breda, The Netherlands). Nitrate was determined by copper cadmium reduction method (ISO method 13395) and ammonium was quantified by Berthelot reaction method (ISO method 11732). Corg, Ntotal as well as δ¹³C and δ¹⁵N values were measured after grinding soil samples with a disc mill. Samples were analyzed with a coupled system consisting of an elemental analyzer (NA 1500, Carlo Erba, Mailand) and a mass spectrometer (Delta C, Finnigan MAT, Bremen, Germany).

Microbial respiration

Basal respiration (BAS), microbial biomass (C_mic), and specific respiration (qO₂) were measured by substrate-induced respiration (SIR), i.e., the respiratory response of microorganisms to glucose (Anderson and Domsch, 1978). Before measurement, roots were removed and soil samples were sieved (2 mm). Measurements were done using an automated O2 microcompensation system (Scheu, 1992). BAS of microorganisms reflected their averaged oxygen consumption rate without the addition of glucose within 10-30 h after attachment of the samples to the analysis system. Subsequently, 4 mg glucose g^-1 soil dry weight was added as aqueous solution to the soil samples. The mean of the three lowest hourly measurements within the first 10 h was taken as the maximum initial respiratory response (MIRR).

Cmic (µg C g^-1) was calculated as 38 x MIRR (µl O2 g^-1 soil dry weight h^-1) according to Beck et al., (1997). Microbial specific respiration qO2 (µl O2 mg^-1 Cmic h^-1) was calculated as BAS/Cmic.

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Fatty acid analysis

Before extraction of lipids, soil samples were sieved (2 mm) and root and litter pieces were removed. Lipid extraction followed Frostegård et al. (1991). Bacterial biomass was estimated using the following PLFAs: a15:0, i15:0, i16:0, 16:1ω7, i17:0, cy17:0 and cy19:0; the PLFA 18:2ω6,9 was used as fungal biomarker (Ruess and Chamberlain, 2010). A gas-chromatography-combustion-isotope-ratio-monitoring-mass spectrometer (GC-C-IRM-MS) using Thermo Finnigan Trace GC coupled via a GP interface to a Delta Plus mass spectrometer (Finnigan, Bremen, Germany) was used to determine the isotopic composition of individual PLFAs.

Fatty acid identification was verified by GC-MS using a Varian CP-3800 chromatograph coupled to a 1200L mass spectrometer and a fused silica column (Phenomenex Zebron ZB-5MS, 30 m, 0.25 µm film thickness, ID 0.32 mm) and helium as carrier gas.

Pyrosequencing

DNA 16S rRNA as well as the 16S rDNA were co-isolated to capture the active and the present microbial community; 2 g soil were extracted from control, beech and ash treatments using the RNA PowerSoil^TM Total RNA Isolation Kit and DNA Elution Accessory Kit (MO BIO Laboratories Inc., Carlsbad, CA, USA). Residual DNA contaminations in RNA extracts were removed using the TURBO DNA-free™ Kit (Ambion Applied Biosystems, Darmstadt, Germany). RNA was concentrated using the RNeasy MiniElute Kit (QIAGEN, Hilden, Germany). The nucleic acid concentration was estimated using a NanoDrop ND-1000 spectrophotometer (Peqlab Biotechnologie GmbH, Erlangen, Germany).

The V2-V3 region of the 16S rRNA was reverse transcribed using the SuperScript^TM III reverse transcriptase (Invitrogen, Karlsruhe, Germany). As template 100 ng of the DNA-free RNA were applied. The resulting cDNA as well as the extracted DNA was amplified in triplicate using the Phusion^® Hot Start High-Fidelity DNA polymerase (FINNZYMES, Espoo, Finland) as described by Nacke et al., (2011).

The following barcoded primer set was used for reverse transcription and amplification, containing the Roche 454 pyrosequencing adaptors (underlined):

V2for

5‘-CTATGCGCCTTGCCAGCCCGCTCAGAGTGGCGGACGGGTGAGTAA-3‘ and V3rev 5‘-CGTATCGCCTCCCTCGCGCCATCAGCGTATTACCGCGGCTGCTG-3‘ modified from Schmalenberger et al., (2001).

The PCR products were treated and purified as described by Nacke et al., (2011). All kits were used as described in the manufacturer´s instructions. The Göttingen Genomics Laboratory determined the sequences of the partial 16S rRNA genes using a Roche GS-FLX 454 pyrosequencer (Roche, Mannheim, Germany) according to the manufacturer´s instructions for amplicon sequencing.

Sequences shorter than 300 bp were removed from the dataset. To minimize the bias introduced by pyrosequencing due to decreasing read precision at the end of the reads denoising was carried out using Denoiser 0.91 (Reeder and Knight, 2010).

OTU determination was performed using uclust OTU picker 1.2.22q (Edgar, 2010) at genetic divergence of 3%, 5% and 20% according to Schloss and Handelsman (2005). The resulting datasets have been deposited in the GenBank short-read archive under accession number SRA050002.

Soil animals

Soil not needed for other analysis was taken to extract soil animals by heat (Kempson et al., 1963). Animals were conserved in saturated NaCl solution and kept at -10°C until analysis. The gamasid mite Hypoaspis aculeifer (G. Canestrini, 1884) was taken for stable isotope analysis as it occurred in sufficient numbers for the analysis. Twenty adult mites were weighed into tin capsules and dried at 40°C for 24 h. Samples were analyzed as described above.