LIPID BIOMARKER RECORDS OF ECOHYDROLOGICAL RESPONSE TO GLOBAL CHANGE: VARIATION IN THE ISOTOPIC COMPOSITION OF N-ALKANES IN PLANTS EXPOSED ELEVATED CO2 AND WARMING

Long chain lipids constituting plant leaf waxes (i.e., n-alkanes) are useful biomarkers used to reconstruct ecohydrological changes of terrestrial landscapes. There remains some uncertainty about the direct linkage between environmental drivers, such as pCO₂ and climate, and the isotopic signals recorded by plants in n-alkanes. For example, the offset between the δ¹³C value of n-alkanes and that of bulk leaf tissue is generally assumed to be constant such that discrimination against ¹³C of atmospheric CO₂ during plant photosynthesis, a commonly used index of plant water-use efficiency, and the relative composition of C3 and C4 vegetation can be determined directly from n-alkane biomarker information. We investigated the composition, abundance and isotopic composition of n-alkanes in the above ground biomass of three plant species (a C4 grass, a C3 grass, and a C3 forb) exposed to experimentally elevated atmospheric CO₂ and warming treatments in a natural grassland ecosystem in Wyoming. The Prairie Heating and CO₂ Enrichment (PHACE) experiment, conducted in a native C3/C4 mixed-grass prairie ecosystem, was a fully factorial and replicated study involving two CO₂ treatments (ambient [490ppm] and elevated [630ppm] CO₂) and two warming treatments (ambient and warmed [+3.0C night, +1.5C day]). Above ground biomass harvests by species were conducted annually at peak biomass in late July between 2006 and 2013. For the current study, we selected biomass samples from only two years; a wet year (2009) and a dry year (2010). N-alkanes were isolated from total lipid extracts and analyzed for composition and abundance using GC-FID and isotopic composition using GC-C-IRMS. The δ¹³C values of n-alkanes are then compared against that of bulk above ground biomass to illustrate how changes in atmospheric CO₂ concentration, precipitation variability and temperature influence the fidelity of the biomarker record of environmental change. We also report how the abundances of n-alkanes per unit of above ground biomass also record important information about changes in terrestrial environments.