Impacts of elevated atmospheric CO2 on litter quality, litter decomposability and nitrogen turnover rate of two oak species in a Mediterranean forest ecosystem

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Abstract
Elevated CO2 may affect litter quality of plants, and subsequently C and N cycling in terrestrial ecosystems, but changes in litter quality associated with elevated CO2 are poorly known. Abscised leaf litter of two oak species (Quercus cerris L. and Q. pubescens Willd.) exposed to long‐term elevated CO2 around a natural CO2 spring in Tuscany (Italy) was used to study the impact of increasing concentration of atmospheric CO2 on litter quality and C and N turnover rates in a Mediterranean‐type ecosystem. Litter samples were collected in an area with elevated CO2 (>500 ppm) and in an area with ambient CO2 concentration (360 ppm). Leaf samples were analysed for concentrations of total C, N, lignin, cellulose, acid detergent residue (ADR) and polyphenol. The decomposition rate of litter was studied using a litter bag experiment (12 months) and laboratory incubations (3 months). In the laboratory incubations, N mineralization in litter samples was measured as well (125 days). Litter quality was expressed in terms of chemical composition and element ratios. None of the litter quality parameters was affected by elevated CO2 for the two Quercus species. Remaining mass in Q. cerris and Q. pubescens litter from elevated CO2 was similar to that from ambient conditions. C mineralization in Q. pubescens litter from elevated CO2 was lower than that from ambient CO2, but the difference was insignificant. This effect was not observed for Q. cerris. N mineralization was higher from litter grown at elevated CO2, but this difference disappeared at the end of the incubation. Litter of Q. pubescens had a higher quality than Q. cerris, and indeed mineralized more rapidly in the laboratory, but not under field conditions.