Seasonal variations in soil–plant interactions in contrasting urban green spaces: Insights from water stable isotopes

We used stable isotopes to help assess ecohydrological partitioning in different types of urban green space in the city of Berlin. We focused particularly on the role of the near-surface of soils as a crucial interface that determines shallow subsurface (“non-Hortonian”) flows and water cycling in cities. Grassland soils tended to be wetter than soils under continuous urban tree cover, and areas around individual trees in parks or on streets. This is consistent with greater interception losses and transpiration from trees. Soil water isotopes showed distinct seasonality in response to precipitation inputs, mixing with resident soil water and the effects of evaporative fractionation. Effects of fractionation were most marked under individual trees and grasslands, where the effects of evaporation on the isotopic composition of percolating water from the upper 0.1 m of the soil was also most pronounced. Isotope dynamics showed that under all land covers, the upper soils had rapid water turnover and were dominated by younger water (<2 months old). At depths of 0.4 m, more damping was evident with water being ∼ 6 months old. Isotopes in plant water also show seasonal variations which were more marked in grasses than trees. Mixing models revealed that grass most likely recycled shallow, younger soil water in transpiration, whilst trees were more dependent on deeper, older sub-soil and groundwater sources. These preliminary results highlight the urgent need for a much greater understanding of water movement and cycling in the shallow critical zone of urban green spaces. This is essential to manage future resilience to climate change and understand the trade-offs between partitioning urban water between evapotranspiration and groundwater recharge.