Beyond the cellulose: Oxygen isotope composition of plant lipids as a proxy for terrestrial water balance

Abstract

There is growing interest in using stable isotopes to measure the impact of shifting water regimes on terrestrial ecosystems. The analysis of oxygen isotopes (δ180) of plant cellulose has been widely used for that purpose, but its application is limited by cellulose's short life in most soils and sediments. Here we compare δ18O values of cellulose and plant lipids (hexane-extractable compounds) to assess the value of bulk lipids as a proxy for water balance. Using a set of field experiments with three C3 and three C4 species, we found significant differences in 18O enrichment in response to irrigation regime, with a strong linear relationship observed between cellulose and lipid signals. Imposed drought increased lipid δ18O values of all species relative to controls and also affected the carbon isotope composition (δ13C) of cellulose, reflecting increased water-use efficiency in C3 plants. Lipid extracts did not differ with respect to δ13C values, but δ18O signals consistently reflected drought effects in C3 and C4 species, regardless of variation in productivity and abundance of oxygen-containing functional groups. These results show that oxygen isotope composition of plant lipids can be used as a proxy for changing water regimes.