Effects of elevated atmospheric CO2 on soil organic carbon dynamics in a mediterranean forest ecosystem

Elevated atmospheric CO 2 has the potential to change the composition and dynamics of soil organic matter (SOM) and consequently C and N cycling in terrestrial ecosystems. Because of the long-lived nature of SOM, long-lasting experiments are required for studying the effect of elevated CO 2 on soil organic matter dynamics. Therefore, the study of ecosystems that have been exposed to long-term enhanced CO 2 concentrations is highly desirable for better understanding feedback mechanisms between litter production, litter quality, soil organic matter decomposability and the atmospheric CO 2 level.This work deals with the effect of enhanced atmospheric CO 2 on chemical composition and C and N mineralization in a leaf litter-soil organic matter continuum around a mineral CO 2 spring in a Mediterranean woodland ecosystem. Leaf litter from Quercus cerris L., Quercus pubescens Willd. and Smilax aspera L., and soil samples from the forest floor (F and HA layers) and 0-10 cm mineral soil were taken at elevated and ambient CO 2 concentrations, and analyzed for chemical composition (C, N, lignin, cellulose, polyphenols). C and N mineralization in plant litter and soil samples were determined using litterbag and laboratory incubation methods.Elevated CO 2 affected neither chemical composition nor elemental ratios of leaf litter. The C mineralization rate during litter decomposition was not affected by elevated CO 2 , in accordance with the absence of a CO 2 effect on litter quality . Leaf litter produced at high CO 2 had a higher N mineralization during the initial stage of decomposition period. This difference, however, disappeared at the end of the incubation. Q. pubescens had a higher litter quality than Q. cerris , and subsequently in vitro faster C and N mineralization rates, but litter decomposition under field conditions did not differ significantly between the two species.Total C contents in the forest floor were higher at elevated CO 2 , but not so in the 0-10 cm mineral soil. For the three layers, total N contents and C/N ratios were not affected by elevated CO 2 . Total C and N pool sizes in the forest floor were doubled by elevated CO 2 , but such effects were not seen in the 0-10 cm mineral soil. The C mineralization rates of the three soil layers of the areas exposed to elevated CO 2 did not differ from those of the areas under ambient conditions. Although N immobilization in the F and HA layers from the elevated CO 2 plots was lower, that of the 0-10 cm A horizon was not affected by high CO 2 .The increase in the organic carbon pool of the forest floor in the absence of an effect of elevated CO 2 on litter quality and decomposability can be explained by increased biomass production under elevated CO 2 . Under elevated CO 2 soil N pools also increased, but the rate of N immobilization in forest floor was lower than that under ambient CO 2 . This study of long-term CO 2 effects casts some doubt on the common view that elevated CO 2 changes litter quality of plants, and thereby slows down decomposability of litter and N release. Because species composition has a strong influence on C and N cycles than elevated CO 2 , effects of increasing atmospheric CO 2 on species composition may be more important to feedbacks between CO 2 concentration and soil organic matter than the CO 2 effect on litter quality of a given species.