Controls on dissolved organic matter leaching from forest litter grown under elevated atmospheric CO2

The aim of our study was to identify controls on initial dissolved organic matter (DOM) leaching from decomposing forest litter and to estimate how it is affected by increasing atmospheric CO 2 . Using microcosms, we measured initial C mineralization and leaching rates of DOC, DON and biodegradable DOC from litter of eight tree species from CO 2 enrichment experiments in a 100 year-old broadleaf forest and a 30 year-old treeline ecosystem. Over 11 weekly leaching cycles, between 2.5% (Pinus uncinata, Fagus sylvatica) and 15% (Carpinus betulus) of litter C were leached as DOC, corresponding to 9–36% of the total mass loss. Significantly less, 0.9% (Pinus) to 4.5% (Tilia platyphyllos) of litter N was leached as DON. Leaching of DOC was not correlated to C mineralization, which ranged between 12% (Fagus) and 32% (Tilia) of litter C. While C mineralization increased with decreasing litter C/N ratios and lignin contents, DOC leaching particularly the initially leached DOC was significantly related to concentrations of non-structural carbohydrates (NSC) and phenolics. DOC leached after the third leaching cycle did not correlate with any of the measures of litter quality, but with the molar UV absorptivity of DOC, suggesting that DOC production is linked to lignin degradation. Previous CO 2 enrichment increased NSC and phenolics in the litter and decreased lignin contents, which resulted in significantly greater initial C mineralization ( + 5 % ) and DOC leaching rates ( + 16 % ) . However, these CO 2 effects were only significant during the initial leaching phase and much smaller than the differences between tree species. Initially leached DOC was less biodegradable when its parent litter was grown under elevated than under ambient CO 2 (38% vs. 42% of DOC across all species, P 0.05). Therefore, leaching of ‘refractory’ DOC was increased under elevated CO 2 , which will rather accelerate DOC inputs into mineral soils than further stimulate microbial activity. In summary, our study shows (1) that initial DOM leaching is controlled by other factors than C mineralization; and (2) that CO 2 enrichment of forests can stimulate initial mineralization and leaching of C from litter by altering its quality, but these effects will be short-term and much smaller than any change in species composition.