Importance of winter climate and soil frost for dissolved organic carbon (DOC) in boreal forest soils and streams : - implications for a changing climate

Sammanfattning: Dissolved organic carbon (DOC) is a defining feature of surface waters in a large part of the world and it influences a variety of physical, chemical and biological processes in aquatic ecosystems. Riparian soils exert a major control on stream water chemistry in many northern latitude regions and provide a local source of DOC to adjacent streams. As the winter climate in northern regions is predicted to be particularly affected by climate change, it is important to investigate the sensitivity of DOC in riparian soils and the potential implications for adjacent streams’ chemistry in a changing winter climate. The effects of a changing winter climate on riparian soil DOC production and export to streams has received little attention to date, and this is the focus of my thesis. In this thesis I first evaluate the hydro-climatic drivers of inter-annual variation in spring snowmelt DOC concentrations in two boreal forest streams of northern Sweden. By accounting for the effects of seasonal hydrology, I was able to statistically evaluate the importance of antecedent winter climatic conditions to inter-annual variation in stream DOC concentrations during the spring snow-melt periods. This descriptive work was complemented by a long-term field experiment where snow packs were manipulated to investigate the impacts of soil freezing on the concentration and quality of DOC in soil water. The effect of soil freezing on DOC was further addressed in a multi-factor laboratory experiment on soil samples taken from the riparian zone. The laboratory experiment was designed based on a central composite face-centered (CCF) model which applied three levels of four freezing-related factors: temperature, water content, duration and frequency of freeze-thaw cycles. The responses of soil microbial- activity and composition to the same experimental factors as well as their potential link to frost induced changes in DOC were also tested. Large inter-annual variations were observed in spring snow-melt DOC concentrations in streams. Lower export of DOC during the preceding seasons and longer, and colder, winters resulted in higher spring snow-melt DOC concentrations. Soil water DOC concentrations and lability were significantly enhanced in the upper soil horizons which experienced extensive soil frost and longer frost duration. In the laboratory experiment, similar responses of soil water DOC were observed in that the higher concentrations and greater lability were found in samples incubated at the lowest temperatures (-12°C). The fungal to bacterial growth ratio also increased in the lower temperature treatments. In addition, fungal growth rate and soil basal respiration responded positively to frost-induced increases in DOC concentration. The frequency of freeze-thaw cycle did not appear to be an influential factor in the laboratory experiment. Several significant interactions of the factors were also detected. By conducting and integrating field and laboratory experiments I highlight the importance of soil frost regime and winter climatic conditions for regulating DOC in riparian soils and their adjacent streams in areas with seasonally frozen soils. However, in a changing future winter climate, alterations in soil frost should be assessed as the result of changes in air temperature and snow-pack formation and extent, and implications for streams should be investigated with regard to changes in hydrology and export processes in soil-water interface.