Can Foliage- and Root-Specific Plant Lipids be Redistributed with Soil Water?.

Cutin and suberin are used as biomarkers to trace above- and belowground plant tissue contribution to soil organic carbon (SOC). These foliage- and root-specific biomarkers are composed predominantly of long-chain monomers that are considered to be generally hydrophobic. Hydrophobicity is, however, relative, meaning it is a range with no clear cut-offs. In soil, cutin and suberin are not only found in particulate organic matter, but also in the silt and clay fraction (S&C). The main mechanism for C incorporation into this fraction is considered sorption. This, in turn, implies water solubility of the organic molecules.

No published data on the water solubility of cutin and suberin monomers exist. If these biomarkers are truly insoluble, sorption might not be the main mechanism determining cutin and suberin presence in the S&C fraction. Furthermore, water insolubility would also indicate that these biomarkers are mostly representative of in-situ detritus input, with transfer in soil only possible in the form of suspended organic matter.

The goal of this work was to investigate whether cutin and suberin constituents are present in soil pore water (SPW). By capitalizing on an on-going study that collected SPW under aspen and conifer forests in Utah, we freeze-dried the solutions, and extracted the samples with base hydrolysis to identify cutin and suberin monomers. Overall, we found almost no foliage- and root-specific monomers in SPW. Only ferulic acid and 8or10, 16-dihydroxy hexadecanoic acid were present in small amounts. Both of these compounds have a log octanol-water partitioning coefficient of less than 3, which was the lowest among all the biomarkers investigated, meaning these compounds were the most water soluble from the monomers examined. Furthermore, 8or10, 16-dihydroxy hexadecanoic acid was the most dominant constituent of cutin with one of the highest concentrations in soil. We conclude that cutin and suberin in soil are mostly derived from in-situ detritus input, and redistributed suspended organic matter.