Land use effects on the composition of organic matter in soil particle size separates. III. Analytical pyrolysis

Abstract

SummarySoil samples from the A horizon of an Eutrochrept under spruce forest and permanent grass were fractionated into clay‐, silt‐ and sand‐size separates. Humic acids extracted from each fraction were analysed by pyrolysis‐gas chromatography‐mass spectrometry. Protection of functional groups by simultaneous pyrolysis and methylation yielded pyrolysates in which methyl esters of fatty acids, aliphatic dicarboxylic acids, abietic acids, phenolic acids and benzenecarboxylic acids were represented. However, methylation was not complete, and unmethylated compounds were also present. Spectra showed differences in humic acid composition between size separates as well as across land use regimes. The abundance of lignin‐derived pyrolysis products increased with decreasing particle size, and was greater in soil under spruce than in soil under grass. Also, the lipid components differed, with hexadecanoic and docosanoic acid methyl esters being the dominant compounds in humic acids from soil under spruce and hexadecanoic and octadecanoic acid methyl esters in the humic acids from grassland. A good correlation was found between previous 13CNMR and wet chemical data and pyrolysis data, indicating that pyrolysis‐methylation can be used for fast detailed chemical characterization of humic acids extracted from size separates.