Plant and soil lipid modification under elevated atmospheric CO2 conditions: II. Stable carbon isotopic values (δ13C) and turnover

Future enrichment of atmospheric CO2 and its effect on ecosystems were studied using grassland free air CO2 enrichment (FACE) experiments. Plant waxes have been shown to be directly modified under elevated CO2 concentration. Lipids, as major components of plant waxes, are important constituents of plant surfaces and their position at the plant/atmosphere interface makes them susceptible to environmental change. The main focus of this study was to improve knowledge about modifications to stable carbon isotopic (δ13C) values of individual lipids within plant biomass and soils as a result of the increased atmospheric CO2 concentration, implying an addition of 13C labelled CO2. The isotopically labelled biomass facilitates turnover time determination of lipids in soils due to the direct comparison of identical plants grown under ambient and 13C-depleted atmospheric conditions. We demonstrate which lipids were influenced by modified CO2 concentration and how the lipid isotopic values of plant biomass and soil were influenced under elevated vs. ambient CO2 conditions. Most plant carboxylic acids and alkanes were uniformly depleted in 13C by ca. 6‰ when compared to plant biomass bulk isotope values. In soil, short chain carboxylic acids (< C20), derived mainly from microbial sources, revealed a lower depletion in isotope value than plant-derived long chain acids ( C20). The isotopic differences between individual compounds in soil under ambient vs. elevated CO2 conditions varied significantly between 2 and 6‰ for individual acids and 0–6‰ for individual alkanes. This argues against plant/soil turnover determinations for individual compounds. Preferably, weighted mean average isotopic values of the most abundant lipids provide reliable calculation of replaced carbon proportions and turnover times. Carboxylic acids were turned over fastest in grassland soil, followed by bulk carbon, whereas alkanes exhibited the slowest turnover times. This is in contrast to previous studies of arable soil, but confirms observations made on peaty soil indicating that alkanes may be part of the relatively stable carbon fraction in soils. The turnover of total organic carbon, carboxylic acids and long chain alkanes was observed to be significantly greater in soil under Lolium perenne (ryegrass) than in soil under the leguminose plant Trifolium repens (white clover).