Mobilization of soil dissolved organic matter in mesic boreal forests of Newfoundland and Labrador, Canada

The mobilization of soil dissolved organic matter (DOM) distributes carbon and nutrients within ecosystems and links terrestrial to aquatic environments. As a hydrologically and biogeochemically mediated flux, DOM mobilization encapsulates a number of interacting ecological processes. This presents a major challenge for identifying the main drivers of DOM mobilization at different spatial and temporal scales. In this thesis, I use two mesic boreal forest research platforms to investigate the drivers of DOM mobilization from the organic horizon at different spatiotemporal scales. Using an experimentally harvested site, I show that total annual DOC flux from O horizons is due to both vertical and lateral flow, and was 30% percent greater in the harvested plots with significantly reduced organic horizons. Additionally, the C:N of DOM and absorbance characteristics of samples in both treatments demonstrated a stronger control of season over harvesting on the composition of DOM mobilized. One of the most significant of these seasonal controls was the snowpack insulation throughout winter. The lower C:N, higher SUVA₂₅₄nm and lower molecular weight of chromophoric DOM mobilized during winter and snowmelt indicates relatively more decomposed DOM, compared to that mobilized in summer and autumn. This shows that the decomposition of soil organic matter underneath a consistently deep snowpack is a key determinant of the composition of DOM mobilized from O horizons during winter and the hydrologically significant snowmelt period. Additionally, I show that air temperature and snowpack duration best explain DOM mobilization dynamics both interannually within boreal sites and among boreal forest sites along a climate transect. This suggests that air temperature indirectly affects DOC mobilization through a direct control on snowpack season length in these forests. Furthermore, climate influenced differences in ecosystem properties such as organic horizon thickness, moss coverage and stand density, may additionally influence DOM mobilization through a direct control on soil hydrology. These results enhance our understanding of the relationship between boreal forest soil organic matter and soil DOM and the potential impacts of climate change on soil organic matter losses as DOM, contributing to a predictive understanding of forest C and nutrient distribution and the potential effect on aquatic environments

Dissolved organic carbon mobilized from organic horizons of mature and harvested black spruce plots in a mesic boreal region

Boreal forests are subject to a wide range of temporally and spatially variable environmental conditions driven by season, climate, and disturbances such as forest harvesting and climate change. We captured dissolved organic carbon (DOC) from surface organic (O) horizons in a boreal forest hillslope using passive pan lysimeters in order to identify controls and hot moments of DOC mobilization from this key C source. We specifically addressed (1) how DOC fluxes from O horizons vary on a weekly to seasonal basis in forest and paired harvested plots and (2) how soil temperature, soil moisture, and water input relate to DOC flux trends in these plots over time. The total annual DOC flux from O horizons contain contributions from both vertical and lateral flow and was 30 % greater in the harvested plots than in the forest plots (54 g C m−2 vs. 38 g C m−2, respectively; p=0.008). This was despite smaller aboveground C inputs and smaller soil organic carbon stocks in the harvested plots but analogous to larger annual O horizon water fluxes measured in the harvested plots. Water input, measured as rain, throughfall, and/or snowmelt depending on season and plot type, was positively correlated to variations in O horizon water fluxes and DOC fluxes within the study year. Soil temperature was positively correlated to temporal variations of DOC concentration ([DOC]) of soil water and negatively correlated with water fluxes, but no relationship existed between soil temperature and DOC fluxes at the weekly to monthly scale. The relationship between water input to soil and DOC fluxes was seasonally dependent in both plot types. In summer, a water limitation on DOC flux existed where weekly periods of no flux alternated with periods of large fluxes at high DOC concentrations. This suggests that DOC fluxes were water-limited and that increased water fluxes over this period result in proportional increases in DOC fluxes. In contrast, a flushing of DOC from O horizons (observed as decreasing DOC concentrations) occurred during increasing water input and decreasing soil temperature in autumn, prior to snowpack development. Soils of both plot types remained snow-covered all winter, which protected soils from frost and limited percolation. The largest water input and soil water fluxes occurred during spring snowmelt but did not result in the largest fluxes of DOC, suggesting a production limitation on DOC fluxes over both the wet autumn and snowmelt periods. While future increases in annual precipitation could lead to increased DOC fluxes, the magnitude of this response will be dependent on the type and intra-annual distribution of this increased precipitation