Complexity of soil organic matter:: AMS 14C analysis of soil lipid fractions and individual compounds

Radiocarbon measurements of different lipid fractions and individual compounds, isolated from soil samples collected on 2 different agricultural long-term study sites, located in the rural area of Rotthalmimster (Germany) and in the city of Halle/Saale (Germany), were analyzed to obtain information about sources and the stability of soil organic matter (SOM). Different lipid compound classes were isolated by automated solvent extraction and subsequent medium-pressure liquid chromatography. Generally, C-14 contents of lipid compound classes from topsoil samples of maize plots at Rotthalmunster are close to the modem atmospheric C-14 content. Lower C-14 values of aliphatic and aromatic hydrocarbons isolated from neutral lipids suggest a contribution of old carbon to these fractions. In contrast, C-14 values of bulk soil (52 pMC) as well as isolated lipid classes from Halle are highly depleted. This can be attributed to a significant contribution of fossil carbon at this site. Extremely low C-14 contents of aromatic (7 pMC) and aliphatic hydrocarbons (19 pMC) reflect the admixture of fossil hydrocarbons at the Halle site. Individual phospholipid fatty acids (PLFA), which are used as a proxy for viable microbial biomass, were isolated by preparative capillary gas chromatography (PCGC) from topsoils at Rotthalmunster and Halle. PLFA C-14 values are close to atmospheric C-14 values and, thus, indicate a clear microbial preference for relatively young SOM. At Rotthalmunster, the C-14 concentration of short-chain unsaturated PLFAs is not significantly different from that of the atmosphere, while the saturated PLFAs show a contribution of sub-recent SOM extending over the last decades. At Halle, up to 14% fossil carbon is incorporated in PLFAs n-Cl 7:0 and cy-C 18:0, which suggests the use of fossil carbon by soil microorganisms. Moreover, it can be concluded that the C-14 age of soil carbon is not indicative of its stability.