Elevated atmospheric CO(2) fuels leaching of old dissolved organic matter at the alpine treeline

• Published in: Global biogeochemical cycles 2008-04, Vol.22 (2), p.GB2004
• Main Authors: Hagedorn, Frank, van Hees, Patrick A. W., Handa, I. Tanya, Haettenschwiler, Stephan
• Format: Article
• Language: English
• Subjects: Environmental Chemistry; Miljökemi
• ISSN: 1944-9224; 0886-6236
• DOI: 10.1029/2007GB003026

Dissolved organic matter (DOM), the mobile form of soil organic matter (SOM), plays an important role in soil C cycling and in nutrient transport. We investigated the effects of 5 years of CO(2) enrichment (370 versus 570 mu mol CO(2) mol(-1)) on DOM dynamics at the alpine treeline, including the analysis of fast-cycling components such as low molecular weight organic acids (LMWOAs), dissolved organic carbon (DOC) biodegradability, and the decomposition of (14)C-labeled oxalate. Concentrations of DOC in canopy throughfall were 20% higher at elevated CO(2), probably driven by higher carbohydrate concentrations in leaves. In the organic soil layer, 5 years of CO(2) enrichment increased water-extractable organic C by 17% and soil solution DOC at 5 cm depth by 20%. The (13)C tracing of recently assimilated CO(2) revealed that the input of recent plant-derived C (< 15% of total DOC) was smaller than the CO(2)-induced increase in DOC. This strongly suggests that CO(2) enrichment enhanced the mobilization of native DOC, which is supported by significant increases in dissolved organic nitrogen (DON). We mainly attribute these increases to a stimulated microbial activity as indicated by higher basal and soil respiration rates (+27%). The (14)C-labeled oxalate was more rapidly mineralized from high CO(2) soils. The concentrations of LMWOAs, but also those of "hydrophilic'' DOC and biodegradable DOC (6% of total DOC), were, however, not affected by elevated CO(2), suggesting that production and consumption of "labile'' DOC were in balance. In summary, our data suggest that 5 years of CO(2) enrichment speeded up the cycling of "labile'' DOM and SOM in a late successional treeline ecosystem and increased the mobilization of older DOM through a stimulated microbial activity. Such a "priming effect'' implies that elevated CO(2) can accelerate the turnover of native SOM, and thus, it may induce increasing losses of old C from thick organic layers.