How changing environmental conditions affect soil water isotopic composition in pre-Alpine grassland soils

Alpine and pre-Alpine grasslands offer crucial ecosystem services like fodder production, biodiversity support, carbon sequestration, and water retention. However, changing environmental conditions like rising temperatures threaten these grassland soils, potentially disrupting their functionality. Understanding how climate change and farming practices impact soil functions and eco-hydrological processes is vital for developing effective strategies to sustainably manage these grasslands.

For this study, we conducted soil water isotope and soil water balance measurements from 2018-2019 in the grassland lysimeters of the TERENO Pre-Alpine observatory along an elevation gradient. Several lysimeters were translocated from the higher-elevation site Graswang (860 m a.s.l., control site) to the lower lying site at Fendt (600 m a.s.l., climate-change site). This gradient represents a 2°C temperature rise along with a 400 mm precipitation decrease at the climate-change site. Our study aimed to explore how elevated temperature and reduced precipitation affect soil hydrological and soil water isotopic composition seasonally, annually and with regard to soil depth.

We did not find significant differences in the isotopic composition at 0.1m soil depth among the different lysimeter groups. Differences in soil water isotopic composition between the lysimeter groups become more pronounced at deeper soil layers, which are typically less affected by daily temperature fluctuations.

Despite higher temperatures at the climate-change site, soil water isotopes closely followed the Local Meteoric Water Line, indicating minimal evaporation. However, the line-conditioned excess (lc-excess) significantly differed between the control and the climate-change site across depths. In contrast, no differences were found between the Fendt and Graswang climate-change site’s isotopic values at any depth. This suggests a stronger influence of actual evapotranspiration at the climate-change site visible in the lc-excess values. Overall, this research enhances our understanding of climate change's impact on water cycling through pre-Alpine grassland soils at varying altitudes. This insight could help to manage these grasslands sustainably in the face of climate change.