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A novel field experiment in Austria reveals that compounding climate conditions – namely drought, warming, and elevated atmospheric carbon dioxide (CO₂ ) – could fundamentally reshape how water moves through soils in temperate grasslands. The findings provide new insights into post-drought soil water flow, in particular. Soil water, though a minuscule fraction of Earth's total water resources, plays a critical role in sustaining terrestrial life on Earth by regulating biogeochemical cycles, surface energy balance, and plant productivity. Soils also govern the fate of precipitation, directing it back to the atmosphere via evapotranspiration or into surface and groundwater systems, depending on soil water storage and flow properties, such as soil texture and structure. However, droughts – expected to become more frequent and severe under change – could disrupt these crucial processes. Atmospheric warming may increase evapotranspiration and soil water loss, while elevated atmospheric CO₂ could reduce transpiration by narrowing plant stomata and conserving soil moisture. Thus, the combined effects of warming and elevated CO₂ can produce complex, albeit poorly understood, hydrological outcomes. Grasslands, which cover 30-40% of Earth's land surface, depend heavily on shallow soil water, making them ideal for studying rootzone ecohydrological dynamics.

 

Jesse Radolinski et al. conducted a novel deuterium (²H) labeling field experiment in a temperate grassland in Austria to examine how elevated atmospheric CO₂, warming, and recurring drought – individually and in combination – affect soil water. Radolinski et al. induced experimental drought conditions and then applied ²H-labeled rainfall under ambient and simulated future climate scenarios. According to the findings, elevated CO₂ increased rootzone moisture, while warming reduced soil moisture, with soil water remaining well mixed under most conditions. However, combined summer drought, warming, and elevated CO₂ drove grassland plants to conserve water by reducing transpiration, which restricted soil water flow to large, rapidly draining pores, limiting mixing with smaller pores. The findings suggest that future drought conditions could fundamentally alter soil water dynamics by limiting post-drought soil water flow and grassland vegetation water use.

Severe droughts are becoming hotter, longer, and increasingly devastating to ecosystems as climate change accelerates, according to a new study, which reports that temperate grasslands, including in parts of the United States, are facing the worst effects. The findings provide a global quantitative understanding of multiyear droughts (MYDs) – prolonged events lasting years or decades – and offer a benchmark for understanding their global trends and impacts. As droughts become more frequent worldwide, the likelihood of MYDs rises, posing severe threats to both natural ecosystems and human systems. These events deplete soil moisture and reduce streamflow, potentially causing irreversible damage, with consequences that include widespread crop failures, increased tree mortality, diminished ecosystem productivity, and reduced water supplies. Recent MYDs, like the Western U.S. drought (2000–2018) and Chile's ongoing drought (since 2010), have drawn significant concern due to their devastating impacts. However, less prominent MYDs, with fewer perceived consequences, may have gone unnoticed, highlighting the need for comprehensive knowledge of their occurrence and effects. To address this need, Liangzhi Chen and colleagues mapped the global distribution of 13,176 MYDs from 1980 to 2018 using the standardized precipitation evapotranspiration index (SPEI), which measures drought severity based on the balance between precipitation and potential evaporative demand. Chen et al. also assessed these events’ impacts on vegetation through remotely sensed greenness data. The findings show that over the last 40 years, MYDs have occurred on nearly every continent and have become increasingly widespread, hotter, and drier, with global land affected by these events increasing by 49,279 ±14,771 square kilometers annually. What’s more, the authors report that the ecosystem impacts of MYDs varied significantly, with temperate grasslands exhibiting greater declines in greenness compared to subtropical and tropical forests. While grasslands demonstrated low resistance to drought, they often displayed rapid recovery following drought events, highlighting their unique resilience dynamics. In a related Perspective, David Hoover and William Smith discuss the study and its findings in greater detail.