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Einflüsse rapider Transportwege (preferential flow) auf die Umsetzung von gelösten und partikulären Kohlenstoffformen (DIC, DOC und POC). Neuentwicklungen mit kombinierten Modellierungen und stabilen Isotopenuntersuchungen (Pref-Carb-Flow)

Fachliche Zuordnung Hydrogeologie, Hydrologie, Limnologie, Siedlungswasserwirtschaft, Wasserchemie, Integrierte Wasserressourcen-Bewirtschaftung
Bodenwissenschaften
Förderung Förderung von 2014 bis 2018
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 246172612
 
Erstellungsjahr 2018

Zusammenfassung der Projektergebnisse

Contributions of carbon dioxide (CO2) outgassing from stream surfaces to atmospheric CO2 concentrations is currently only insufficiently captured on the global scale. With estimated numbers of often more than 50 % of total stream carbon exports, these leave large uncertainties to global carbon budgets and terrestrial carbon cycling. Data analyzes from the literature and the Global River Chemistry database (GLORICH) showed that the global river average pCO2 of 3100 ppmV is more often exceeded by contributions from small streams when compared to rivers with larger catchments (> 500 km2). With more than 96 % of the global number of streams and rivers being headwaters, these running waters have the potential to contribute large amounts of CO2 to the atmosphere. Recent estimates indicate that headwaters contribute 36 % (i.e., 0.93 Pg C yr^–1) of total CO2 outgassing from running waters globally. In this work, recent numbers were reviewed and challenged. A primary finding is that cumulative contributions of headwater streams are still underrepresented in the global carbon cycle. River CO2 outgassing is typically quantified by means of the gas transfer velocity (k), that particularly for headwaters are often insufficiently estimated by empirical models that in turn are based on stream hydraulic geometry (i.e., slope, stream depth and width). In this work, an inverse model that computes the integral CO2 outgassing upstream of a measurement point was applied and further developed. The model relies on the principle, that initial 13C/12C ratios of carbon sources and the intensity of CO2 outgassing control the isotope ratio of dissolved inorganic carbon (DIC) in stream water. To gain better knowledge on carbon cycling and transport, data from the 1.78 km2 large Uhlirska catchment in the Czech Jizera Mountains were collected. With only a few key parameters (pCO2, DIC and δ13CDIC), we were able to show that 80 % of inorganic carbon that enters the stream along its course was lost to the atmosphere in as CO2 before reaching the exit point of the catchment. Lateral HCO3- and CO2 export contributed approximately 7 and 12 %, respectively. As a novelty, groundwater isotope data (δ13CDIC) were incorporated into the model to better constrain initial CO2 values that are used to calculate the upstream CO2 losses. These results confirm the role of headwaters as important CO2 sources in the global carbon cycle and the importance of carbon isotopes and thus CO2 contents in groundwater for the determination of CO2 fluxes. It also highlights the potential of this new integral approach to better assess CO2 outgassing from the large number of acidic, humic-rich headwaters and thus to improve global scale estimations. Moreover, decreases of anthropogenic acidification were responsible for most of the increased dissolved organic carbon (DOC) concentrations reported in Europe since 1990. Due to the absence of carbonates and the low buffering capacity of soils the investigated Uhlirska Catchment was strongly affected by atmospheric acid depositions during the 1980s. In this study Sulfate (SO4^2–) analyses allowed to establish mass-balances (deposition input minus surface water export) and to investigate different water compartments such as soil water at the hillslope and the valley as well as shallow and deep groundwater. In the study catchment, the mixing of sulfate-rich groundwaters and DOC-rich soil waters during runoff generation was identified as the main reason for observed negative correlation between SO4^2– and DOC than declining acid depositions. Mass-balances showed that even 30 years after peak acid rain depositions the stream SO4^2– export exceeds current imports by a factor of 2.2 even though the catchment lacks natural SO4^2– sources. This can be explained by storage and gradual release from wetlands and local groundwater that both must have formed SO4^2– reservoirs and thus present environmental memories of legacy pollutant SO4^2–. All findings demonstrated that headwaters are important, but so far underestimated components of the global carbon cycle. It also stresses the importance of including groundwater data in future investigations and models to assess SO4^2– and CO2 dynamics in headwaters. Other than unexpected results with the sulfate export, the project went very well.

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