The concentrations of fatty acids in organo‐mineral particle‐size fractions of a Chernozem

Abstract

SummaryFatty acids, the most abundant class of soil lipids, indicate pedogenetic processes and soil management. However, their quantitative distribution in organo‐mineral particle‐size fractions is unknown. The concentrations of n‐C10:0 to n‐C34:0 fatty acids both in whole soil samples and in the organo‐mineral particle‐size fractions of the Ap horizon of a Chernozem were determined (i) to evaluate the effects of long‐term fertilization and (ii) to investigate their influence on the aggregation of organo‐mineral primary particles. Quantification by gas chromatography/mass spectrometry (GC/MS) showed that long‐term fertilization with nitrogen, phosphorus and potassium (NPK) and farmyard manure (FYM) led to larger concentrations (25.8 µg g−1) of fatty acids than in the unfertilized sample (22.0 µg g−1). For particle‐size fractions of the unfertilized soil, the fatty acid concentrations increased from the coarse silt to the clay fractions (except for fine silt). Fertilization with NPK and FYM resulted in absolute enrichments of n‐C21:0 to n‐C34:0 fatty acids with a maximum at n‐C28:0 in clay (×2.2), medium silt (×2.0), coarse silt (×1.8) and sand (×2.9) compared with the unfertilized treatment (the factors of enrichment are given in parentheses). New evidence for the aggregate stabilizing function of n‐C21:0 to n‐C34:0 fatty acids was shown by the characteristic pattern in size‐fractionated, disaggregated and aggregated samples. Highly significant correlations of fatty acid concentrations with organic C concentrations and specific surface areas are interpreted as indicators of (i) trapping of fatty acids in organic matter macromolecules and (ii) direct bonding to mineral surfaces. This interpretation was supported by the thermal volatilization and determination of fatty acids by pyrolysis‐field ionization mass spectrometry (Py‐FIMS).