Humification in subalpine Rendzinas: chemical analyses, IR and 13C NMR spectroscopy and pyrolysis-field ionization mass spectrometry

Abstract

Litter decomposition and humification in different horizons of two subalpine Rendzinas of the Bavarian Alps, differing mainly in morphology and soil climate, were studied by using chemical degradation methods, IR and 13C NMR spectroscopy, as well as pyrolysis-field ionization mass spectrometry. The Tangelrendzina, classified as a Lithic Borofolist, has a thick, peat-like organic surface layer directly overlying the parent material. The specific soil climate is cold and wet due to northerly aspect. The Moderrendzina, classified as a Lithic Rendoll, has a southerly exposure, a warmer soil climate, and a pronounced humic A horizon. Both soils are derived from dolomite debris. In both soils (L to Oh/Ah horizons) similar mechanisms are responsible for litter decomposition and humification, including loss of polysaccharides, increase of alkyl and carboxyl C proportions, and degradation of lignin. The aromatic C proportion remains nearly constant in the Tangelrendzina, indicating similar decomposition rates for aromatics and total organic C or an equilibrium between decomposition and inputs (roots). In the Moderrendzina, it decreases due to higher mineralization rates. The decrease in lignin subunits with increasing soil depth is more pronounced than the decrease in total aromatics. Water-soluble organic substances containing decomposed lignin fragments are leached from the acid surface horizons and precipitated in deeper layers by Ca ions. Intermediate substances of lignin biodegradation like demethylated lignin dimers dominate the pyrolysis products from the Oh, ca horizon of the Tangelrendzina. Products typical of advanced lignin degradation, producing furanoid structures during pyrolysis, accumulate in the Ah layer of the Moderrendzina. This shows that the humification process in the two profiles differs only in intensity and not in the specific pathways of transformation.