People: Georg Guggenberger (PI), Robert Mikutta (co-PI), Olga Shibistova, Norman Gentsch
Aims and objectives
The accessibility of soil organic matter (SOM) in cryoturbated soils to microbial decomposition is strongly affected by freezing/thawing and subduction of organic-rich topsoil horizons to the subsoil. However, the chemical composition of SOM and its involvement into additional stabilization processes such as occlusion within aggregates and/or formation of mineral-organic associations may strongly alter the magnitude and the kinetics of the response of SOM in cryoturbated soils to changing environmental conditions [1, 2, 3]. IP 2, therefore, aims at elucidating the origin, structure, and degree of decomposition of SOM together with the identification and quantification of stabilization processes such as protection by soil minerals by a series of geochemical analyses. IP 2 will provide SOM quality criteria along the permafrost gradients, thus focussing directly to objective 2 of the CryoCARB JRP (Identify the major SOC stabilization mechanisms in cryoturbated soils).
By comparison with microbial processes and community structure studied in WP3, we will contribute to assess the effects of SOM quality to GHG emission, and to the biomass and composition of the microbial community. The SOM quality criteria will be also used to develop a vulnerability index of SOM that will be combined with soil carbon quantity and quality data along ecoclimate and permafrost gradients by WP1 to adress the SOM vulnerability to climate change at the landscape level.
For our analyses we will use aliquots of the same soil samples investigated by the other IPs.
Fractionation of SOM into functionally different pools
Fractionation of soil into density fractions yields distinct SOM pools, differing in origin, structure, turnover, and function [4, 5]; the so-called light fraction is considered to be largely particulate OM while the heavy fraction contains mainly OM associated with minerals, i.e., mineral-organic associations. Density fractionation will be carried out by floating the light fraction in Na polytungstate at a density of 1.6 g cm-3 .
Analysis of the structural composition of SOM using biomarker analysis
Bulk soil and physical soil fractions will be analyzed for the following parameters:
Lignin analysis will be carried out by the CuO oxide oxidation method . The comparative analysis of the yield of lignin monomers released upon CuO oxide oxidation informs about the speed of decomposition as compared to other biomolecules, whereas the ratio of lignin-derived acids versus aldehydes inform about the degree of oxidative alteration of the lignin macromolecules.
Non-cellulosic sugars will be released from soil polysaccharides by TFA . The concentration of the sugar monomers as compared to lignin-decomposition products reflects preferred decomposition of one or the other biomolecule. Ratios of individual sugar monomers inform about decomposition of plant-derived polysaccharides and resynthesis of microbial polysaccharides during decomposition of SOM.
Amino sugars and muramic acid will be released from living and dead microbial cell walls by HCl hydrolysis . The sum of amino sugars and muramic acid provide information about the accumulation of microbial residues in SOM, while the ratio of glucosamine to muramic acid informs about the relative contribution of fungal to bacterial residues to SOM.
Estimation of C and N turnover
On selected samples of the soil fractions the turnover of carbon will be estimated by 14C AMS measurements, which are based either on the decay of natural 14C in SOM fractions older than about 50 years or using the ‘bomb’ 14C spike for younger fractions . These analyses will complement the radiocarbon dating on bulk SOM carried out by IP 1. For estimating the nitrogen turnover in proteins we will analyze amino acid enantiomers in order to make use of the time-dependent racemization of amino acids , which allows, together with the 14C analysis, a relative age dating of proteinaceous organic compounds .
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 Mikutta et al., 2010, Geochim Cosmochim Acta 74: 2142-2164