Fast incorporation of root-derived lipids and fatty acids into soil – Evidence from a short term multiple pulse labelling experiment

Plant-derived lipids are regarded to be mainly incorporated into soil via above ground biomass. The contribution of root-derived lipids to soil organic matter (SOM) is not easily accessible, so this incorporation pathway is mostly underestimated, whereas it is common knowledge that the contribution of rhizodeposits to SOM is of major importance for the turnover of organic carbon. Not only the contribution, but also the incorporation rates and turnover time of exclusively root-derived lipids remain unknown. We determined for the first time the incorporation rates of rhizodeposit-derived lipids into soil planted with Lolium perenne, using a multiple 14 CO 2 pulse labelling experiment carried out under controlled laboratory conditions. Additionally, we accessed differences in lipid composition between the rhizosphere and root-free soil to evaluate the direct contribution of root-derived lipids to SOM. The lipid composition in the rhizosphere clearly showed a greater abundance of microbial lipids like C16:1 and C18:1, as well as root-derived C18:2+3 fatty acids (FAs) than the initial and root-free soil. The incorporation rates of total lipids (k Lip) and FAs (k FA) based on 14C data revealed a very fast incorporation into SOM (rhizosphere: k Lip 0.82year−1; k FA 0.31year−1; root-free soil: k Lip 0.70year−1; k FA 0.48year−1) after the first 14C pulse for young plants. Thereafter, incorporation rates decreased until the end of the experiment (rhizosphere: k Lip 0.17year−1; k FA 0.03year−1; root-free loess: k Lip 0.11year−1; k FA 0.06year−1). The incorporation rates resulting from the 14C pulse labelling experiment are comparable to turnover rates of total, i.e. above ground and root-derived, and lipids from field experiments using 13C labelling approaches. The fast allocation of root-derived lipids to substrate beneath, and distant from, roots gives new insights into the carbon incorporation of OM on a molecular level. This implies that incorporation of lipid compounds, especially into soil deeper than the uppermost few centimetres or the ploughing layer, is mainly due to root-derived OM, in contrast to common knowledge.