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In arid and semi-arid areas biological soil crusts are main contributors to C and N-cycles and the origin of organic matter. Nevertheless systematic studies on the spatial distribution of total organic carbon (TOC) and nitrogen (N) and a characterization of crust organic matter composition are missing. To describe the spatial distribution of TOC and N we examined three soil depths and three relief positions along a steep rainfall gradient. In addition the molecular composition of organic matter was characterized by Pyrolysis-field ionization mass spectrometry.

TOC and N concentrations decreased with increasing depth, the effects of the relief followed no clear trend. Surprisingly the amount of TOC and N decreased with increasing rainfall. Stable organic matter compounds were reduced with increasing rainfall. Topcrusts (0-2 mm) showed a relative enrichment in bacteria, as indicated by proportionally larger contents in N-acetylmuramic acid (m/z 167 + 276) than the subcrusts (2-40 mm). These were enrichment in cyanobacteria, as indicated by proportionally larger contents of hexadecadienoic acid (m/z 252).

We conclude that the spatial distribution of TOC and N is related to sampling depth and annual precipitation. Organic matter composition and the main biomass contributors in crusts are successfully identified by pyrolysis-field ionization mass spectrometry. (C) 2013 Elsevier Ltd. All rights reserved.