Soil fluxomics and tracing SOM decomposition at the molecular level

Soil organic matter (SOM) represents a vast store of carbon, exceeding that of the atmosphere by at least 3-fold. Global change effects on the formation and breakdown of SOM therefore can have strong repercussions on the global C cycle and on the Earth’s climate. However much of the controls of SOM decomposition have remained elusive given the lack of approaches to trace gross rates of SOM decomposition on the molecular level.

Organic matter from plants, microbes and soils typically consists of regular polymers (polysaccharides, proteins, peptidoglycan etc.) and irregular polymers (lignin, humic acids). The breakdown of these compounds by extracellular enzymes yields biologically available compounds (e.g. sugars, amino acids, amino sugars) small enough to be taken up and metabolized by microbial cells. The extracellular cleavage of regular polymers by hydrolytic enzymes and of irregular polymers by oxidative enzymes is considered to be the major bottleneck in organic matter decomposition. While much has been learned about the makeup of the extracellular enzyme complement by meta-genomic approaches, the assessment of their activity in situ has been hampered by the lack of suitable approaches. The only way of estimating in situ rates of extracellular reactions is based on isotope pool dilution assays (IPD), in which the compound produced during organic matter breakdown is isotopically enriched and the isotope dilution thereof is measured over time. Currently IPDs only exist for the extracellular protein and cellulose breakdown, pioneered by our working group but not for most of the other naturally occurring polymers such as peptidoglycans, nucleic acids, lipids and lignin. In this project we develop a universal IPD platform for the simultaneous measurement of gross rates of organic matter depolymerization processes using state-of-the-art High Resolution Orbitrap LC/MS and apply it to investigate major controls of SOM decomposition, such as effects of environmental factors and organic matter quality, across a wide range of soils. At the same time we trace the metabolism of these breakdown products, following isotopomers of carbon (13C, 12C) and nitrogen (15N, 14N) through the soil microbial food web and into soil mineral N and respiratory CO2.

Publications:

S. Leitner, W. Wanek, B. Wild, I. Haemmerle, L. Kohl, K.M. Keiblinger, S. Zechmeister-Boltenstern, A. Richter (2012) Influence of litter chemistry and stoichiometry on glucan depolymerization during decomposition of beech (Fagus sylvatica L.) litter. Soil Biology and Biochemistry 50, 174-187.
W. Wanek, M. Mooshammer, A. Blöchl, A. Hanreich, A. Richter (2010) Determination of gross rates of amino acid production and immobilization in decomposing leaf litter by a novel isotope pool dilution technique. Soil Biology and Biochemistry 42, 1293-1302.
M. Mooshammer, W. Wanek, J. Schnecker, B. Wild, S. Leitner, F. Hofhansl, A. Blöchl, I. Hämmerle, A.H. Frank, L. Fuchslueger, K.M. Keiblinger, S. Zechmeister-Boltenstern, A. Richter (2012) Stoichiometric controls of nitrogen and phosphorus cycling in decomposing beech leaf litter. Ecology 93(4), 770-782.
Wild, B., Schnecker, J., Barta, J., Capek, P., Guggenberger, G., Hofhansl, F., Kaiser, C., Lashchinsky, N., Mikutta, R., Mooshammer, M., Santruckova, H., Shibistova, O., Urich, T., Zimov, S.A., and Richter, A. (2013). Nitrogen dynamics in Turbic Cryosols from Siberia and Greenland. Soil Biology & Biochemistry 67, 85-93

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decomposing leafs on the ground — © Pixabay/ Filmbetrachter