Soil under stress – how drought changes grasslands

Grasslands cover almost 40 per cent of the Earth’s terrestrial surface and play an important role in the global water cycle. Nevertheless, so far, the impact of climate change on these vital ecosystems has been only insufficiently understood. A new study conducted in the context of an international project supported by the Austrian Academy of Science and headed by Michael Bahn of the Forschungsgruppe Funktionelle Ökologie at the University of Innsbruck’s Institute of Ecology in Austria now provides new insights into the future of grassland ecosystems. 

Based on years of research, the results of this study, which were published in the specialist journal Science in January, highlight the effects of drought, rising temperatures and raised CO₂ concentration levels on the availability of soil water and the utilisation of water by plants. “We simulated the changes expected in three key factors in future climate scenarios: warming, raised atmospheric CO₂ concentrations and drought. We examined the effects both one by one and in various combinations,” Bahn explains. Using rainwater marked by stable isotopes, when the drought was over, the team were able to conduct a detailed analysis of the movement and storage of water in the space provided by pores in the soil, also during evaporation.

Immediate positive effects

In raised CO₂ concentration conditions, the rhizosphere of plants remains moister since the plants use water more efficiently. In contrast, warmth results in a general loss of moisture in the soil. If, in a future warmer climate, there is recurrent drought, soil characteristics will strongly change. “In these scenarios, the water mixes with the soil more quickly because it above all flows through the large, fast-draining pores while less of it penetrates the small, slowly draining pores. This is also where the older water stays longer,” explains Jesse Radolinski, a co-author of the study. This has an impact on hydrological connectivity, which is essential for the availability of water for plants. 

The researchers stress that this restriction of water flows in the soil has far-reaching consequences for the functioning and resilience of grassland ecosystems. Plants are forced to make more economical use of water, which could in the long run inhibit their ability to grow and regenerate. “But at the same time, our study shows that raised CO₂ concentrations in the atmosphere can have positive effects in the short term, such as faster recovery after drought phases. Nevertheless, these effects are superimposed by the negative impacts of progressive warming and drought on the soil characteristics,” Bahn notes.

Recurrent drought disturbs mixing

The crucial insight of the study is that recurrent phases of drought permanently disturb hydrological connectivity in the pore system of the soil, which has especially strong effects in a warmer climate with higher CO2 concentration levels. “It used to be assumed that soil water is mixed through well when it rains, but our results show that after recurrent droughts in a future climate, this mixing is severely restricted. This has considerable impacts on water use by plants and ecosystem dynamics,” Radolinski explains.

“The study shows that interaction between the soil and plants can be far more complex than hitherto assumed. This has considerable impacts on the ability of ecosystems to survive in drought periods and recover from them,” Bahn states, summing up. The results of the study once again underscore the need to develop strategies to strengthen the resilience of ecosystems to climate change and promote global efforts to combat global warming.

(University of Innsbruck/wi)

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