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Assessing dynamics in eco-hydrological processes during drought stress across scales in a grassland community by stable isotopes

Subject Area Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
Term from 2017 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 323373178
 
Final Report Year 2023

Final Report Abstract

Semi-natural and managed grasslands are one of the most represented land surface types in central Europe. They feature plant species that display strong resilience properties against extreme hydrological events. The DFG-funded project entitled "Assessing dynamics in eco-hydrological processes during drought stress across scales in a grassland community by stable isotopes” aimed at investigating how a subset of these grassland species dynamically change their water use strategies to both laboratory-simulated and naturally occurring stress periods. To reach this central objective, the project was articulated around three work packages focusing on different scales (plant individual vs. plant community) and including experimental work (in the laboratory and the field) as well as statistical and process-based modeling. Laboratory-controlled experiments highlighted the plastic character of the root water uptake process of an individual Centaurea Jacea individual grown in a soil column with the aerial part enclosed in a dynamically purged plant chamber. More specifically, it could be observed that, based on the online monitoring of the stable isotopic compositions of soil water vs. plant transpired water vapor, the link between measurements of water availability status, root presence, and the estimated root water uptake profile was particularly weak under stress conditions. Furthermore, isotopic data suggested the occurrence of water re-distribution from deeper to shallower soil layers by roots. Finally, the non-isotopic data set (e.g., canopy conductance measurements, atmosphere vapor pressure deficit) shed also light onto acclimation limits of Centaurea Jacea. We assessed the combined effect of long-term drought and nitrogen fertilization in the field using a rainout shelter approach by combining ecophysiological, ecological and ecohydrological methodological approaches from the leaf to ecosystem scale. Notably, both nitrogen fertilization and drought had negative effects on the seasonal carbon balance as well as the water use efficiency of the system, however, the combination of nitrogen fertilization and drought was particularly detrimental reducing the water use efficiency and carbon balance by more than half. We could successfully show that structural changes of the grassland community (e.g., increased dominance of grasses over forbs on N fertilized plots) made the community as a whole very susceptible to drought: 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. In comparison, key species among the two functional plant types (grasses and forbs) were able to maintain homeostasis of plant physiology, indicating a strong interlinkage between form (e.g., species composition and dominance of plant functional groups) and function (e.g. water and carbon cycling) of the studied grassland at the community scale.

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