How does preferential flow influence carbon transport and changes between DIC, DOC and POC? Advances with combined modeling and stable isotope approaches (Pref-Carb-Flow)
Soil Sciences
Final Report Abstract
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.
Publications
- (2016), Carbon fluxes in an acid rain impacted headwater catchment, EGU General Assembly 2016, 17-22 April 2016, Vienna, Austria
Marx, A., S. Hintze, J. Jankovec, M. Sanda, J. Dusek, T. Vogel, R. van Geldern, and J. A. C. Barth
- (2016), Hydrochemical and isotopic tracing of runoff generation in the small mountainous catchment Uhlirska (Czech Republic), using the NETPATH approach, Acta Hydrologica Slovaca, 17(2), 190-198
Vitvar, T., M. Sanda, A. Marx, E. Hubert, J. Jankovec, and J. A. C. Barth
- (2016), Modeling stable isotope and organic carbon in hillslope stormflow, EGU General Assembly 2016, 17-22 April 2016, Vienna, Austria
Dusek, J., T. Vogel, M. Dohnal, A. Marx, J. Jankovec, M. Sanda, J. Votrubova, J. A. C. Barth, and Milena Cislerova
- (2017), A review of CO2 and associated carbon dynamics in headwater streams: A global perspective, Rev. Geophys., 55, 560-585
Marx, A., J. Dusek, J. Jankovec, M. Sanda, T. Vogel, R. van Geldern, J. Hartmann, and J. A. C. Barth
(See online at https://doi.org/10.1002/2016RG000547) - (2017), Acid rain footprint three decades after peak deposition: Longterm recovery from pollutant sulphate in the Uhlirska catchment (Czech Republic), Sci. Total. Env., 598, 1037-1049
Marx, A., S. Hintze, M. Sanda, J. Jankovec, F. Oulehle, J. Dusek, T. Vogel, T. Vitvar, R. van Geldern, and J. A. C. Barth
(See online at https://doi.org/10.1016/j.scitotenv.2017.04.109) - (2017), Dynamics of dissolved organic carbon in hillslope discharge: Modeling and challenges, J. Hydrol., 546, 309-325
Dusek, J., T. Vogel, M. Dohnal, J. A. C. Barth, M. Sanda, A. Marx, and J. Jankovec
(See online at https://doi.org/10.1016/j.jhydrol.2016.12.054) - (2017), Small streams make big contribution to carbon cycle, Eos, 98
van Geldern, and J. Barth
(See online at https://doi.org/10.1029/2018EO078009) - (2017), Three decades after peak acid deposition: Environmental memories of legacy pollutant sulfate in the northern Czech Republic, EGU General Assembly 2017, 24-28 April 2017, Vienna, Austria
Marx, A., S. Hintze, M. Sanda, J. Jankovec, F. Oulehle, J. Dusek, T. Vogel, R. van Geldern, and J. A. C. Barth
- (2017), Using oxygen-18 to study DOC transport in a macroporous forest soil, EGU General Assembly 2017, 24-28 April 2017, Vienna, Austria
Dusek, J., T. Vogel, M. Dohnal, J. A. C. Barth, M. Sanda, A. Marx, and J. Jankovec
- Modeling subsurface fluxes of dissolved organic carbon at the hillslope scale, EGU General Assembly 2017, 24-28 April 2017, Vienna, Austria
Dusek, J., T. Vogel, M. Dohnal, J. A. C. Barth, M. Sanda, A. Marx, and J. Jankovec
