Stable water isotope seasonality at the soil-vegetation interface in cold climate

Mixing and transport mechanisms of water in unsaturated shallow soil govern the partition of infiltrating water into the plant available water in soil water storage and groundwater recharge and modify the distribution of soil solutes and contaminants in subsurface. Consequently, they play a major role in the regulation of eco-hydrological processes at the soil–vegetation–atmosphere continuum. In sub-arctic regions, where both current and predicted warming rates are highest, the water cycle is undergoing marked changes and a limited understanding of storage and movement of water in soil has been recognized as one of the biggest knowledge gaps in addressing this issue. Stable isotopes of water are frequently used to explore water fluxes at the soil-vegetation interface, as they have proved to be a potent tool for tracing the origin and variability of waters that occupy different soil and plant compartments. We used a combination of field experiments and surveys that utilize stable isotopes of water as both natural and artificial tracer to assess the main drivers of spatiotemporal variability of water fluxes at the soil-vegetation interface in a sub-arctic catchment. First, soil coring and xylem sampling campaign was performed to quantify the variability of soil water isotopes under different land covers and in different seasons, and further identify under which conditions is soil water isotopic composition reflected in the stem water. Afterwards, an irrigation experiment using deuterated water was carried out on a forested hilltop to understand how infiltrating water gets redistributed in subsurface and how sub-arctic forest till soil and vegetation respond to massive infiltration events. The studies were conducted at the Pallas catchment, located in Northern Finland.We found that seasonal rainfall variation and late snowmelt events were clearly represented in forest till soils, while the water input signal was heavily attenuated in forested peatlands. However, the seasonal evolution of soil water pools was not reflected in tree stem dynamics. In addition, the main infiltration mechanisms in shallow till soil were delineated through an inspection of interplay between soil water fluxes of different mobility. We further observed how a large snowmelt event can cause an isotopic homogenization of all water fluxes at the soil–vegetation interface.Our results highlight the unique role of snowmelt in replenishing and sustaining soil water storage and modifying isotope dynamics at the soil–vegetation interface.