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

• Published in: Global biogeochemical cycles 2008-05, Vol.22 (2)
• Main Authors: Hagedorn, Frank, Van Hees, Patrick AW, Handa, ITanya, Haettenschwiler, Stephan
• Format: Article
• Language: English
• ISSN: 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 sub(2) enrichment (370 versus 570 [mu]mol CO sub(2) mol super(-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 super(14)C-labeled oxalate. Concentrations of DOC in canopy throughfall were 20% higher at elevated CO sub(2), probably driven by higher carbohydrate concentrations in leaves. In the organic soil layer, 5 years of CO sub(2) enrichment increased water-extractable organic C by 17% and soil solution DOC at 5 cm depth by 20%. The super(13)C tracing of recently assimilated CO sub(2) revealed that the input of recent plant-derived C (-15% of total DOC) was smaller than the CO sub(2)-induced increase in DOC. This strongly suggests that CO sub(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 super(14)C- labeled oxalate was more rapidly mineralized from high CO sub(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 sub(2), suggesting that production and consumption of labile DOC were in balance. In summary, our data suggest that 5 years of CO sub(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 sub(2) can accelerate the turnover of native SOM, and thus, it may induce increasing losses of old C from thick organic layers.