Rapid Degradation of Soil Organic Matter Sampled from Anoxic Surface Horizons of Fens after Contact with Oxygen – Implications for Sampling and Sample Pretreatment.

The composition of soil organic matter (SOM) in anoxic H horizons of two fens with different parent material and peat-forming plants (limestone weathering residues, Carex spec.; granite, Sphagnum spec.) was investigated after having been subject to two different sampling and sample pretreatment routines. One subsample was transported, homogenized, and dried traditionally under the presence of oxygen; for another subsample the contact with oxygen during transport and sample pretreatment was strictly ruled out. For all subsamples, the C speciation was assessed by ¹³C CPMAS NMR. Furthermore, the concentration of different non-cellulosic polysaccharides was determined by gas chromatography after hydrolysis with trifluoroacetic acid (TFA), derivatisation of their neutral sugar monomers by reduction to alditols, and subsequent acetylation.

Compared to the anoxic sample pretreatment, oxic pretreatment resulted in a reduced contribution of O/N-alkyl-C to the SOM pool of the samples. The alkyl-C/O-alkyl C ratio was strongly increased, indicating increased microbial degradation. Additionally, the ratio (galactose + mannose)/(arabinose + xylose) (GM/AX) increased, indicating an increased contribution of microbial sugars and a reduced contribution of plant-derived sugars. Results of a laboratory incubation of the samples under oxic conditions at 20°C for different time intervals ranging from 1.5 to 96 h showed a consistent decrease of the soil mass, the C/N ratio, and the amount of total sugar, particularly of glucose. Concomitantly, the alkyl-C/O-alkyl C ratio and the GM/AX ratio increased. The results indicate a rapid microbial degradation of organic matter in anoxic peat after access of oxygen. Appropriate sampling and processing of such samples should consider this by preventing any access of oxygen during the entire sample pretreatment procedure.