Fast incorporation of root-derived lipids and fatty acids into soil - Evidence from a short term multiple 14CO2 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 (CO2)-C-14 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 C-16 (1) and C-18 1, as well as root-derived C-18 (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 C-14 data revealed a very fast incorporation into SOM (rhizosphere k(LIP) 0.82 year(-1), k(FA) 031 year(-1), root-free soil k(LIP) 0.70 year(-1); k(FA) 0.48 year(-1)) after the first C-14 pulse for young plants Thereafter, incorporation rates decreased until the end of the experiment (rhizosphere k(LIP) 0 17 year(-1), k(FA) 0 03 year(-1); root-free less k(LIP) 0.11 year(-1), k(FA) 0 06 year(-1)). The incorporation rates resulting from the C-14 pulse labelling experiment are comparable to turnover rates of total, i.e above ground and root-derived, and lipids from field experiments using C-13 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. (C) 2009 Elsevier Ltd All rights reserved