How will the future of our soils - and thus also water availability - be shaped by the effects of impending climate change? An international study led by Jesse Radolinski and Michael Bahn from the Department of Ecology at the University of Innsbruck shows how drought, warming and increased atmospheric CO₂ concentrations are changing existing hydrological processes in soils and challenging the resilience of ecosystems. The findings were published in the journal Science.
Grasslands cover almost 40 per cent of the Earth's surface and are key to the global water cycle. Nevertheless, the impact of climate change on these important ecosystems is still poorly understood. A new study, carried out as part of an international project funded by the Austrian Academy of Sciences and led by Michael Bahn from the Functional Ecology research group at the University of Innsbruck, now provides new insights into the future of grassland ecosystems. The results of this study illustrate how drought, climate warming and rising CO₂ concentrations affect the availability of soil water and plant water use. 'We tested for the effects of warming, increased atmospheric CO₂ concentrations and drought not only individually but also in combination, as expected to occur in the near future,' explains Michael Bahn. Using rainwater labelled with stable isotopes, the team was able to obtain a detailed understanding of the movement and storage of water in the pore space of the soil and its loss from the system through seepage and evapotranspiration.
Positive effects in the short term
Under increased CO₂ concentrations, the main plant rooting horizon remains more moist, as the plants use water more efficiently. By contrast, warming leads to a reduction of soil moisture. Recurrent drought, especially under a future warmer climate, leads to major changes in the soil structure, with consequences for the fate of water in soil. 'In this most extreme scenario, the water in the soil is less well mixed, as it flows primarily through the large, fast-draining pores and penetrates less into the smaller, slow-draining pores. The older water also sticks around there for longer,' explains Jesse Radolinski. Thus, recurrent drought in a future climate impairs hydrological connectivity, which is essential for the water availability of plants. The researchers also emphasize that this restriction of water flow in the soil has far-reaching consequences for the functionality and resilience of grassland ecosystems as a whole. Plants are forced to use water more efficiently, which in the long-term could limit their ability to grow and regenerate. 'At the same time, we also observed that elevated CO₂ can have positive effects in the short term, such as faster recovery after periods of drought. However, these effects are overshadowed by the negative effects of increasing warming and drought on overall water availability and hydrological connectivity,' says Bahn.
Unique experiment setting
'Our experiment is unique, as we have been able to experimentally simulate the conditions of a future climate since 2014 and can thus analyze the longer-term effects,' emphasizes Bahn. The researchers used a unique experimental facility, which they designed at the agricultural research center in Raumberg-Gumpenstein in Styria. It comprises 54 test plots with infrared heaters and CO₂ fumigation systems as well as automated roofs which can shield the rain. This made it possible to simulate a range of realistic climate change scenarios and analyze the interactions between soil water and plants in detail.
Repeated drought disrupts soil water mixing
The main finding of the study is that the hydrological connectivity in the soil pore system is strongly disrupted by recurring droughts occurring in a warmer climate with elevated CO2. 'Soil water is generally assumed to be well mixed following rainfall, but our results show that this mixing will be severely limited after repeated drought in a future climate. This has significant implications for water utilisation by plants,' explains Radolinski.
'Our study shows that the interactions between soil and plants could be much more complex than previously assumed. This has significant implications for the ability of ecosystems to resist and recover from periods of drought,' summarizes Bahn. The results of the study once again emphasize the need to develop strategies strengthening the resilience of ecosystems to climate change and to advance global climate protection efforts.
Publication: Drought in a warmer, CO2-rich climate restricts grassland water use and soil water mixing. Jesse Radolinski, Michael Bahn et al. Science, 16 Jan 2025, Vol 387, Issue 6731, pp. 290-296. DOI: 10.1126/science.ado0734
