Depth-related variations in organic matter at the molecular level in a loamy soil: reference data for a long-term experiment devoted to the carbon sequestration research field

Three lipid fractions, namely the freely extractable fraction and those associated with humin and humic acid fractions, were obtained from the loamy soil of a carefully maintained long-term experiment located on Deffend ORE field, Poitiers, France. The analyses showed differences in molecular distribution, suggesting different sources and diagenetic states of the source material. Despite a major input of plant material to the soil organic matter, intensive bacterial activity was suspected. Most distributions suggested an increase in the microbial/terrestrial lipid ratio from the free to humin to humic fractions. Molecular evidence of fungal activity, especially in the top layer, was also found in the distributions of n-alkanes and n-alkanoic acids. In the surface horizon A(1) alkanes were the major compounds, followed by n-alkanoic acids and sterols. The degraded horizons, poorer in organic matter, i.e. the A(2) and B horizons, were dominated by long-chain (> C-20) n-alkanoic acids with a strong even-over-odd predominance and C-24 and C-26 n-alkanols. Sterols had been removed from these horizons through degradation. A comparison of humic acid and humin composition on the basis of Py(methylation)-GC-MS showed that the two fractions produce partly similar pyrolysis products. Most prominent were molecules from plant and microbial carbohydrates, lignin building blocks and linear aliphatic (carboxylic acids, esters) and nitrogen compounds. The investigation showed that while low-molecular-weight soil lipids were highly dominated by compounds derived from the overlying vegetation, pyrolysis data from the corresponding high-molecular-weight fractions reflected the incorporation of microbial biomarkers into the humic-type fractions.