Source determination of lipids in bulk soil and soil density fractions after four years of wheat cropping

Preservation of soil organic matter (SOM) is strongly affected by occlusion within aggregates and by association of SOM with minerals. Protection of organic carbon (C) due to adsorption to mineral surfaces can be assessed by investigation of SOM in soil density fractions. Apart from the physical properties the preservation of SOM is affected by its chemical composition. While for bulk organic C this was demonstrated for numerous soils, SOM density fractions have been scarcely studied regarding their molecular composition. Lipids as a compound class that can derive from plants, microorganisms and contamination by products from incomplete combustion or fossil carbon were not investigated in density fractions so far. We hypothesized that molecular proxies deriving from lipid composition yield a large potential to elucidate the sources of organic matter entering soil, and in combination with density fractions they enable the identification of incorporation and preservation pathways of SOM. We determined distribution patterns of aliphatic hydrocarbons and fatty acids as two representative groups for total lipids in soil density fractions. The fatty acids showed a predominant input of plant-derived poly unsaturated short chain and saturated long chain fatty acids in free particulate organic matter (fPOM). The microorganism-derived compounds such as unsaturated short chain fatty acids were largely abundant in fPOM and especially in occluded particulate organic matter (oPOM 1.6). The proportion of plant-derived components like long chain fatty acids increased with increasing density of the fractions, whereas the abundance of short chain fatty acids decreased in the same direction as indicated by the ratio of long chain vs. short chain fatty acids. The main portion of soil lipids (60% of total lipids) was recovered in the mineral (Min) fraction, which denotes the strongest protection of lipids adsorbed to mineral surfaces. For the aliphatic hydrocarbons the contribution of plant- and microorganism-derived components was the largest in fPOM. Short chain alkanes as part of the aliphatic hydrocarbons showed contamination of soil by an incompletely burned plant biomass or fossil carbon. These contaminants were the most abundant in fPOM and subsequently attributed to particles with a low density, which derived probably from soot. However, a large contribution of fossil C was found in the Min fraction as well, which is thought to be attributed to degraded soot particles being adsorbed to minerals. We demonstrated at the molecular level that the incorporation of individual C sources varies with the density fractions. It was found that microorganism-derived compounds were most abundant in fPOM and oPOM 1.6 fractions, whereas plant-derived long chain biopolymers were enriched in mineral dominated fractions (oPOM 2.0 and especially Min). Thus, preservation of plant-derived lipids in soil is strongly attributed to the association with minerals. Based on lipid composition in density fractions we evaluated several molecular proxies, which help to elucidate the sources of SOM.