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

There is growing interest in using stable isotopes to measure the impact of shifting water regimes on terrestrial ecosystems. The analysis of oxygen isotopes (delta O-18) 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 delta O-18 values of cellulose and plant lipids (hexaneextractable compounds) to assess the value of bulk lipids as a proxy for water balance. Using a set of field experiments with three C-3 and three C-4 species, we found significant differences in O-18 enrichment in response to irrigation regime, with a strong linear relationship observed between cellulose and lipid signals. Imposed drought increased lipid delta O-18 values of all species relative to controls and also affected the carbon isotope composition (delta C-13) of cellulose, reflecting increased water- use efficiency in C-3 plants. Lipid extracts did not differ with respect to delta C-13 values, but delta O-18 signals consistently reflected drought effects in C-3 and C-4 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.