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Compound-specific stable isotope analysis as a tool to characterize the role of microbial community structure in C cycling K. Denef, P. Boeckx, O. Van.

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Presentation on theme: "Compound-specific stable isotope analysis as a tool to characterize the role of microbial community structure in C cycling K. Denef, P. Boeckx, O. Van."— Presentation transcript:

1 Compound-specific stable isotope analysis as a tool to characterize the role of microbial community structure in C cycling K. Denef, P. Boeckx, O. Van Cleemput Laboratory of Applied Physical Chemistry (ISOFYS) Ghent University (Belgium)

2 Soil organic carbon Ecosystem Management Global Change (climate, elevated GHG) ? Microbial community FungiBacteria

3 Soil organic carbon Ecosystem Management Global Change (climate, elevated GHG) Microbial community Fungi Bacteria

4 Possible reasons for fungal- induced C sequestration Fungal alteration of soil physical structure –Aggregate formation (Bossuyt et al., 2001) –Aggregate stabilization: Glomalin (Wright et al., 1999) –Fungal-induced macroaggregate-C protection (Frey et al., 2003) –Preferential protection of fungal-derived C in microaggregates within macroaggregates (Simpson et al., 2004) Differences in “physiology”: more uncertainties –C utilization efficiency? (Thiet et al., 2006) –Stability of fungal- vs. bacterial-derived OM: unknown

5 Molecular markers for fungi vs. bacteria (Compound-specific analysis: CSA) Microbial communities distinguishedMolecular marker Phospholipid fatty acids (PLFA): living structures Gram + bacteriai14:0, i15:0, a15:0, i16:0, i17:0, a17:0 Gram – bacteriaMonounsaturated (16:1w7, 18:1w7, cy17:0, cy19:0) Actinomycetes10Me-FAs Fungi saprotrophic18:1w9c, 18:2w6,9 Fungi mycorrhizal16:1w5 Amino sugars (AS): microbial residues Bacterial residuesGalactosamine Muramic Acid Fungal residuesGlucosamine From Glaser, 2006; Drissner et al. (2006)

6 STRUCTURE (CSA) Microbial community FUNCTION (CSSIA) Research Objective I. METHODOLOGY Carbon cycling Grassland management intensity Elevated CO 2 II. APPLICATIONS GC-c-IRMS ( 13 C-PLFA) LC-c-IRMS ( 13 C-AS)

7 II. APPLICATIONS: CO 2 pulse-labeling approach Soil biota 13 C 13 C-PLFA GC-c-IRMS 13 CO 2 Roots + exudates 13 C I. Impact of elevated CO 2 (Giessen FACE, Germany since 1998) 1. Ambient CO 2 (350 ppm) 2. Elevated CO 2 (450 ppm) II. Impact of grassland management (Merelbeke, Belgium since 2000): 1.N-fertilization level kg N ha -1 yr kg N ha -1 yr kg N ha -1 yr -1 2.Mowing frequency 1.5 times per year 2.3 times per year OBJECTIVES Investigate elevated CO 2 and grassland management impacts on root-C assimilating microbial communities Activity niche differentiation Link stimulated fungal pathways to C stabilization mechanisms (aggregation; fungal-derived OM)

8 Measurements (ongoing) Aboveground plant material: 13 C Roots: 13 C Root-associated soil: bulk 13 C & 13 C-PLFA Bulk soil: bulk 13 C & 13 C-PLFA Physical fractions (aggregate size fractions): – 13 C fractions – 13 C-PLFA –AS concentrations Expected stimulated fungal/mycorrhizal pathways: * less intense management * elevated CO 2 Expected niche dominance of fungal activity: * macroaggregates Expected preferential stabilization of fungal products: * microaggregates within macroaggregates 24 h after pulse-labelingSeveral times during/after pulse-labeling

9 First results FACE pulse-labeling Mol% PLFA-C (0-7.5 cm) - 10h after start pulse-labeling In collaboration with Müller et al Gram+Gram- Act Fungi Enhanced saprotrophic fungal abundance

10 First results FACE pulse-labeling In collaboration with Müller et al G+G-ActFungi G+G-ActFungi

11 First results FACE pulse-labeling In collaboration with Müller et al Root-derived mol% PLFA-C (0-7.5 cm) - 10h after pulse-labeling G+G- ActFungi Saprotrophic fungi AM fungi

12 First results FACE pulse-labeling In collaboration with Müller et al G+G-ActFungi G+G-ActFungi C-assimilating community shifts over time? Different preferential OM sources?

13 Possible reasons for fungal- induced C sequestration Fungal alteration of soil physical structure –Aggregate formation (Bossuyt et al., 2001) –Aggregate stabilization: Glomalin (Wright et al., 1999) –Fungal-induced macroaggregate-C protection (Frey et al., 2003) –Preferential protection of fungal-derived C in microaggregates within macroaggregates (Simpson et al., 2004) Differences in “physiology”: more uncertainties –C utilization efficiency? (Thiet et al., 2006) –Stability of fungal- vs. bacterial-derived OM: unknown

14 2. 13 C-substrate incubation approach OBJECTIVES Determine formation rates of fresh plant-residue-derived fungal vs. bacterial amino sugars Investigate impact of substrate quality on fungal and bacterial activity and turnover Determine inherent biochemical stability of fungal vs. bacterial amino sugars + 13 C substrate (uniformly labeled) 90% sand 4% POM 4% silt 2% clay Soil biota 13 C Wheat substrateC/N  13 C Grains12.7 ± ± 3.3 Leaves37.5 ± ± 2.3 Roots41.0 ± ± 6.9 Stems57.2 ± ± 1.0 HWE Leaves81.3 ± ± C-CO2 GC-c-IRMS 13 C-PLFA 13 C-Amino sugars GC-c-IRMSLC-c-IRMS Gas-IRMS

15 3. 13 C-substrate incubation approach Expected results 1.Fungal:bacterial activity ( 13 C-PLFA) greater for lower quality substrate soils 2.Different fungal vs. bacterial AS formation rates  estimates for fungal vs. bacterial turnover rates 3.No differences in “inherent” stability of fungal vs. bacterial AS; stability controlled by clay-OM interactions & physical protection

16 Summary 13 C-PLFA analysis: –Structure of the active C-metabolizing community and how affected by land-use/management/global change –Trace C sources (roots vs. residue/fresh vs. native OM) –But limited to group-level (species?) 13 C-Amino Sugar analysis: –Fate of microbial residues –Quantify formation/turnover rates –Investigate stabilization mechanisms

17 Funding Agency FWO – Fund for Scientific Research Vlaanderen (Belgium) Collaboration (FACE research) Dr. Christoph Müller (Justus-Liebig University, Giessen, Germany) Masters students Mihiri Wilasini (Physical Land Resources, UGent) Undergraduate thesis students Heike Bubenheim (Justus-Liebig University, Giessen, Germany) Charlotte Decock (UGent) IRMS technicians Jan Vermeulen Katja Van Nieuland


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