Tracing water and nitrogen uptake in mature forests using stable isotopes

Trees are powerful mediators within ecosystem water and nutrient cycles. Through their roots, they take up these essential resources from the soil, distributing them in the system and upward into the canopy to maintain transpiration and photosynthesis.

However, understanding and predicting real-world dynamics remains challenging: tree species identity, species mixture, site and soil conditions may shape tree water and nutrient uptake fundamentally, particularly in mature forests. Moreover, in the face of climate change, access to resources in deeper, less drought-prone soil layers is crucial for buffering drought impacts and maintaining forest functioning. Studies targeting tree resource uptake in mature forests are still scarce; but they are emerging, with stable isotopes as a central tool.

We investigated root water uptake depth and subsoil water and nitrogen uptake in mature temperate forests of north-western Germany, using ²H, ¹⁸O and ¹⁵N as tracers. Native European beech, non-native Douglas fir, and native but drought-sensitive Norway spruce were studied, revealing tree species-specific uptake strategies and influences of species mixture. Furthermore, we found consistent site effects: on well-drained, sandy soils, the trees integrated more resources from deeper layers than on loamy soils. Notably, transit times from soil to canopy were slower for nitrogen than for water, highlighting the biotic and abiotic interactions that decouple nitrogen from water.

We conclude that species-specific traits in interaction with soil characteristics are crucial for understanding and predicting water and nutrient fluxes in forests. Our findings underscore the importance of belowground processes when assessing forest functioning and resilience.