Flushing or mixing? Stable water isotopes reveal differences in arctic forest and peatland soil water seasonality

Understanding the relative importance of different water sources that replenish soil water storage is necessary to assess the vulnerability of sub-arctic areas to changes in climate and altered rain and snow conditions, reflected in the timing and magnitude of water infiltration. We examine spatiotemporal variability and seasonal origin of soil water at the soil-vegetation interface in Pallas catchment, located in northern Finland. The field study was conducted from May 2019 to June 2020 over two snowmelt seasons and one summer growing season. We sampled soil cores up to a 1-m depth and stem water of dominant tree species at four sites located in forests and forested peatlands for stable water isotopes. Seasonal rainfall variation and late snowmelt events were well identifiable in the well-drained soils of forested areas, while this input signal is heavily attenuated in wetter, forested peatland areas. Spatiotemporal variability of soil water in two forest sites was similar, whereas soil water storage in peatland sites was controlled by the extent of hydrologic connectivity to the adjoining water pools. A mixture of both summer and winter precipitation was present in peatlands during the entire study period, while forest mineral soils showed an ephemeral response to water input and got nearly fully flushed twice during the hydrological year. Meltwater signal in forest soils was dominant after snowmelt and in early spring but became displaced by isotopically enriched rainfall during the summer. Seasonal evolution of soil water pools was not reflected in tree stem dynamics but offset between soil and stem water isotopic signals was less pronounced in forested peatlands. This field data set uncovered seasonal changes of soil water isotopic signal at high depth-resolution, quantifying the importance of snowmelt water in replenishing and sustaining soil water storage in sub-arctic conditions.