Stable water isotopes reveal the effects of land use on ecohydrological partitioning in a drought-sensitive mixed land-use catchment

Ecohydrological fluxes in the critical zone are characterised by complex interactions between soils, plants, and the atmosphere. Vegetation and land use therefore play a crucial role in forming the interface between the soil and atmosphere, and spatial variation in land management within a catchment can have a dominant influence on water partitioning. Consequently, in drought sensitive environments there is a need for careful assessment of the water use of different land use types, likely resilience to future climate change and implications for groundwater recharge and runoff.

We monitored isotopes in precipitation, soil water and groundwater over the growing season of 2021 (March – October) under 8 plots with contrasting land cover in the drought-sensitive Demnitzer Millcreek Catchment (DMC) in NE Germany. These encompassed traditional arable crops, cropping schemes with water conservation measures (e.g. syntropic crops and agroforestry), grasslands and contrasting forests. The isotope monitoring was complemented with a flux tower, forest sap flow monitoring, pasture lysimetry and soil moisture measurements to provide hydroclimatic and ecohydrological context.

The growing season of 2021 being relatively wet compared to the drought years of 2018-20. Nevertheless, ET was high and soil moisture declined from May onward with only a large (60mm) event in June substantially replenishing the soil water storage before more general re-wetting in autumn. Soil moisture availability was generally highest under grassland and syntropic crops and lowest under forests. In general, precipitation isotopic composition varied during the summer, and was largely tracked by soil water isotopes, though at all sites variations were increasingly damped and lagged with depth. Soil water isotopes generally plotted close to the local meteoric water line, with limited effects of soil evaporation showing differences in interception and transpiration mainly explained the differences in soil moisture between sites. Estimates of soil water ages showed that the upper soils of arable sites had the greatest variability in isotopic composition and most rapid turnover of water, whilst soil water under trees had more limited isotopic variability, was much older and showed low levels of groundwater recharge.

Our study illustrated the advantage of monitoring the spatial variability of natural stable water isotope abundance in soil-vegetation systems to understand heterogeneity in water partitioning. Further, conservation measures like syntropic agriculture were tentatively shown to be a useful adaptation against dryer climatic conditions as this site was able to retain the highest proportion of precipitation in the soil for crop growth. However, monitoring over multiple growing seasons with contrasting hydroclimatic conditions is needed for a fuller assessment.