Plantand Soil 163: 121-130, 1994. © 1994 Kluwer AcademicPublishers.Printed in the Netherlands. Decomposition of tree leaf litters grown under elevated CO2: Effect of litter quality M. E Cotrufo 1, P. Ineson 2 and A. E Rowland 2 1Division of Biological Sciences, Institute of Environmental and Biological Sciences, Lancaster University, Lan- caster, LA 1 4YQ, UK and 2Institute of Terrestrial Ecology, Merlewood Research Station, Grange-over-Sands, Cumbria, LAll 6JU, UK received 12 November1993. Accepted in revisedform 30 March 1994 Key words: decomposition rates, enriched CO2, lignin, litter respiration, microcosms, nitrogen Abstract Ash (Fraxinus excelsior L.), birch (Betula pubescens Ehrh.), sycamore (Acerpseudoplatanus L.) and Sitka spruce (Picea sitchensis (Bong.) Carr.) leaf litters were monitored for decomposition rates and nutrient release in a laboratory microcosm experiment. Litters were derived from solar domes where plants had been exposed to two different CO2 regimes: ambient (350 #L L-1 CO2) and enriched (600 #L L-1 CO2). Elevated CO2 significantly affected some of the major litter quality parameters, with lower N, higher lignin concentrations and higher ratios of C/N and lignin/N for litters derived from enriched CO2. Respiration rates of the deciduous species were significantly decreased for litters grown under elevated CO2, and reductions in mass loss at the end of the experiment were generally observed in litters derived from the 600 ppm CO2 treatment. Nutrient mineralization, dissolved organic carbon, and pH in microcosm leachates did not differ significantly between the two CO2 treatments for any of the species studied. Litter quality parameters were examined for correlations with cumulative respiration and decomposition rates: N concentration, C/N and lignin/N ratios showed the highest correlations, with differences between litter types. The results indicate that higher C storage will occur in soil as a consequence of litter quality changes resulting from higher atmospheric concentrations of CO2. Abbreviations: CHO-soluble carbohydrates, DOC--dissolved organic carbon, HCel-holocellulose, WTREM- weight remaining Introduction The current measured increases in atmospheric con- centrations of CO2 are considered to be the most important long-term change occurring on this planet (Houghton et al., 1990). Whilst the impact of rising CO2 on plant physiology has been investigated exten- sively (Eamus and Jarvis, 1989), little is known about the effect of such change on the global C cycle. Most of the C held in terrestrial ecosystems is in the soil and it is important to evaluate the interaction of high CO2 con- centrations with decomposition processes and, hence, with C stores. Direct effects of rising atmospheric CO2 levels on soil processes are unlikely to be important due to the inherently high soil concentration of CO2 (Van Veen et al., 1991). However, changes in decom- position rates of plant material derived from plants grown under elevated CO2 have been predicted due to changes in litter quality associated with high CO2 lev- els (Lambers, 1993; Norby et al., 1986). Despite these theoretical predictions of the decomposition rates of litter derived from plants grown under elevated CO2 there is very little experimental evidence to support these predictions (Couteaux et al., 1991). Substrate quality has been recognized as one of the most important factors regulating decomposition processes (Swift et al., 1979), and quality parameters such as N concentration, C/N and lignin/N ratios have been correlated with decomposition rates (Melillo et al., 1982; Taylor et al., 1989). The effect of increas-