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

Bowering, Keri L. (2021) Mobilization of soil dissolved organic matter in mesic boreal forests of Newfoundland and Labrador, Canada. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

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.

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