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Block 4.2: Ökosysteme
Dienstag, 28.09.2021:
16:00 - 17:30

Chair der Sitzung: Gerhard Gebauer, Universität Bayreuth
Virtueller Veranstaltungsort: Block 4.2 - Meeting Link

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16:00 - 16:30
Session Keynote

Carbon isotope ratio as a measure of photosynthetic water-use efficiency: reconciling a biophysical discrepancy

John Marshall

SLU, Swedish University of Agricultural Sciences

In most plants, d13C of their tissues provides an estimate of the carbon:water exchange, or water-use efficiency, of photosynthesis. This is important because photosynthesis and transpiration represent an important evolutionary trade-off for plants, but also because they link two major parts of the ecosystem carbon and water cycles. The discovery that this trade-off could be quantified with d13C led to a burst of research in fields as diverse as plant breeding, ecosystem ecology, and global climate change. However, there was always some concern that the predicted isotopic composition did not exactly match biophysical theory, which was essentially a Rayleigh distillation model with a strong enzymatic fractionation. The discrepancy favoured overestimates and was approximately as large as the isotope effects under study, leading to some scepticism about the isotopic technique. I will review proposed explanations for the discrepancy, ending with the recent convergence on mesophyll conductance. Mesophyll conductance can now be measured with high precision in the field, offering a mechanistic adjustment leading to water-use efficiency estimates with high precision and accuracy. Eliminating this discrepancy returns d13C to a position of credibility as a measure of the linkage between carbon and water fluxes of plant leaves and canopies.

16:30 - 16:45

Impact of nutrient and water availability on grassland functioning - achieving a process based understanding across scales

Maren Dubbert1, Angelika Kübert2, Youri Rothfuss3, Christiane Werner2

1Zalf, Deutschland; 2Uni Freiburg, Deutschland; 3FZ Jülich, Deutschland

Two important threats to the sustainable functioning of seminatural grasslands in temperate zones are (1) nutrient loading due to agricultural fertilization and pollution, and (2) the increase of extreme drought events due to climate change. These threats may cause substantial shifts in species diversity and abundance and considerably affect the carbon and water balance of ecosystems. The synergistic effects between those two threats, however, can be complex and are poorly understood. Here, we experimentally investigated the effects of nitrogen addition and extreme drought (separately and in combination) on a seminatural temperate grassland, located in Freiburg (South Germany). To study the grassland response, we combined eddy-covariance techniques, open gas exchange systems from the leaf to the plot scale with biomass and beiodiversity assessments. Open gas exchange chambers were connected to an infrared gas analyzer and water isotope spectrometer, which allowed the partitioning of net ecosystem exchange and evapotranspiration. Vegetation parameters were described by species richness, species abundance, and leaf area index. Our results suggest that grassland communities, strongly weakened in their stress response by nitrogen loading, can substantially lose their carbon sink function during drought. While nitrogen addition caused a significant loss in forb species (−25%), precipitation reduction promoted a strong dominance of grass species at season start. Consequently, the grass-dominated and species-poor community suffered from a strong above-ground dieback during the dry summer months, likely caused by lower water use efficiency and weaker drought adaptations of the species community. Over the growing season (April-September), the carbon sequestration of the studied grassland was reduced by more than 60% as a consequence of nitrogen addition. Nitrogen addition in combination with precipitation reduction decreased carbon sequestration by 73%. Eutrophication can severely threaten the resilient functioning of grasslands, in particular when drought periods will increase as predicted by future climate scenarios. Our findings emphasize the importance of preserving high diversity of grasslands to strengthen their resistance against extreme events such as droughts.

16:45 - 17:00

Application of 2-dimensional stable isotope measurements of methane to constrain sources and sinks in a seasonally stratified freshwater lake

Teresa Einzmann1, Moritz Schroll1, Jan F. Kleint1,2, Thomas Klintzsch1,3, Frank Keppler1,4, Markus Greule1

1Institute of Earth Sciences, Heidelberg University, Heidelberg, Germany; 2MARUM – Center for Marine Environmental Sciences and Department of Geosciences, University of Bremen, Bremen, Germany; 3Institute for Plant Nutrition, Justus Liebig University, Giessen, Germany; 4Heidelberg Center for the Environment (HCE), Heidelberg University, Heidelberg, Germany

Supersaturation of methane (CH4) in the oxic surface water layer of lakes has been suggested to play an important role in releasing CH4 to the atmosphere and emissions are predicted to increase with future climate change. Microbial CH4 oxidation in limnic systems as a counteracting sink has the potential to efficiently diminish CH4 effluxes. In this study, sources and sinks of CH4 were investigated in a seasonally stratified, eutrophic lake in southwestern Germany during summer using carbon and hydrogen stable isotope measurements.

In the lake water body, aerobic CH4 oxidation at the oxic-anoxic interface increased in intensity with rising CH4 concentrations over summer. This was accompanied by a strong increase in stable carbon and hydrogen isotope values of the CH4 pool. Incubation experiments with 13C-labeled CH4 revealed CH4 oxidation rates varying between 49-106 nM/d. In the lake sediment, anaerobically produced CH4 was reduced in its concentration through microbial anaerobic CH4 oxidation in sulfate-methane transition zones possible due to high sulfate concentrations in the lake (~2 mmol/l). The decrease in upward migrating sedimentary CH4 was partly accompanied by increasing stable carbon and hydrogen isotope values.

Sources of CH4 were characterized using a novel isotope indicator Δ(2,13) recently introduced by Tsunogai et al. (2020), which is based on dual isotope characterization of CH4 and corrects for isotopic fractionation effects caused by CH4 oxidation. Surface water CH4 showed different Δ(2,13) values if compared with CH4 from the hypolimnion and sediment and was furthermore distinguishable from littoral Δ(2,13) values. In order to investigate the occurrence of oxic CH4 production in the surface water layer, an incubation experiment was performed with 13C-labeled methylphosphonate, a known precursor substrate for aerobic CH4 formation, which showed a strong increase in the stable carbon isotopic composition of CH4 over time. In conclusion, our results strongly indicate internal oxic CH4 production by aerobic organisms as a possible source of excess CH4 in the surface water layer of the lake.


Tsunogai, U. et al. 2020. Dual stable isotope characterization of excess methane in oxic waters of a mesotrophic lake. Limnol. Oceanogr. 65: 2937–2952.

17:00 - 17:15

Belowground C allocation of tropical rainforests in response to drought: an ecosystem 13CO2 labeling approach

lingling shi1, Pratiksha Acharya1, Xujuan Bai1, Niklas Schmuecker1, Nemiah Ladd2, Christiane Werner2, Laura.K Meredith3,4, Michaela Dippold1

1Biogeochemistry of Agroecosystems, Department of Crop Science, Faculty of Agriculture, Georg August University of Göttingen; 2Ecosystem Physiology, University of Freiburg, Freiburg, Germany; 3School of Natural Resources and the Environment, University of Arizona, Tucson, United States of America; 4Biosphere 2, University of Arizona, Tucson, United States of America

Drought affects carbon (C) sources and sinks in forest ecosystems, with potential consequences for belowground C allocation, a vital process of the terrestrial C cycle. However, the extreme drought impacts on the ecosystem are poorly understood, particularly in the tropical rainforests. Within the framework of our large-scale ecosystem manipulation experiment on "Rain Forest Water, Atmosphere, and Life Dynamics" (WALD) at the Biosphere 2 in Arizona, we conducted a whole ecosystem stable isotope labeling with atmospheric 13CO2 to gain in-depth insights into tree belowground C allocations and the C partitioning at the soil–microbe-root interface under ambient conditions and drought stress. In particular, we hypothesized that key drought-adaptation strategies would include i) increased C allocation into subsoil layers that drought down slower than topsoil and ii) increased C investment into rhizodeposits and mycorrhizal fungi. Our data on tree C allocation highlight that drought stress increased the proportion of recently assimilated C translocated into the roots in both the top- and sub-soil with no correlation of C allocation with soil water content or root biomass. In response to drought, the rhizodeposition, and thus allocation of assimilated C into rhizospheres soil, was reduced in topsoil but increased in subsoil. However, we found pronounced plot-specific differences in belowground C allocation, especially between plots with only understory plants vs those with tall trees, suggesting species-specific drought response strategies. However, generally our observations underline that trees attempt to invest assimilated C into the deeper soil layers’ roots and rhizosphere to access subsoil resources. The C investment into deeper soil layers and the absence of any correlation with root biomass suggest that drought adaptation strategies are based on rhizomicrobial mechanisms rather than on C investment into root growth. The upcoming results of 13C incorporation into phospholipid fatty acids will provide further insights into microbial C utilization in the rhizosphere and complete our picture of belowground drought adaptation strategies. In summary, quantification of tree C belowground allocation patterns at the plant-microbe-soil interface will enable us to disentangle belowground drought response strategies of tropical rainforests.

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