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Organic-rich riparian soils in northern boreal landscapes are often the primary source of organic and inorganic carbon (C) to headwater streams. During extreme hydro-climatic events, such as droughts, the production and mobilization of C in these soils may be sensitive to changes in groundwater levels. Yet, the biogeochemical effects of drying and rewetting have been under-investigated in boreal riparian zones, particularly when compared to peat soils in discrete landscape components (i.e., mires). Here, we experimentally assess the response of riparian soil cores to simulated drought and rewetting and test whether mobilization of dissolved organic matter (DOM), carbon dioxide (CO₂), and methane (CH₄) are altered by geochemical and biological drivers over a two-month rewetting period. Drought oxidized the soil profile, upregulated activities of oxidative enzymes, and replenished terminal electron acceptors (TEAs), most notably sulfate (SO₄²⁻), which likely suppressed DOM concentrations over the short term. However, over the longer term, soil DOM mobilization increased in response to rewetting, unrelated to the intensity of experimental drought. Enzyme activity during the rewetting phase indicates that the persistent increases in DOM may be linked to microbially-mediated decomposition of organic matter following drought. By contrast, CO₂ production was sensitive to drought intensity, with concentrations suppressed in soils subjected to the most extreme drying treatment. Elevated SO₄²⁻ concentrations also delayed the recovery of CH₄ production in soils by creating a pool of more favorable TEAs. Our results collectively show that mobilization of different C forms in riparian soils is influenced by drying-rewetting events through multiple biogeochemical mechanisms operating at different time scales. These findings have broader implications for the lateral transfer of organic and inorganic C from riparian zones to streams in response to predicted increases in climate variability.

Riparian zones are important ecological interfaces, acting as control points for biogeochemical cycling in landscapes. Yet we know relatively little about how the local hydrogeomorphic structure of riparian zones shapes the belowground microbial processes that underpin C and nutrient cycles. Here we assessed how topographically driven variation in riparian hydrogeomorphology along a boreal stream influences resource accumulation, microbial biomass and community composition, and extracellular enzyme activity in soils at the land-water interface. We found that riparian interfaces with lower average groundwater levels supported soils with greater organic matter content and capacity to generate solutes at the land-water interface. By contrast, microbial biomass in soils was elevated at interface sites with the greatest variability in groundwater level, whereas the fungal:bacterial ratio was lowest at sites with persistently high groundwater levels. Extracellular enzyme activities also varied with local hydrogeomorphology, but these responses were distinct among targeted enzymes. Specifically, patterns for some enzymes (beta-glucosidase and protease) were linked to soil properties (e.g., soil % loss on ignition, C:N), whereas others (cellulase and peroxidase) were more influenced by local hydrological variability. Collectively, our study shows how variation in the hydrogeomorphic template can drive heterogeneity in the capacity of riparian soils to store resources and support microbial activity along small streams. In combination with variation in local hydrologic connectivity, this heterogeneity adds complexity to the mechanisms regulating solute production, transformation, and exchange at the land-water interface.

The transport of biodegradable dissolved organic carbon (bDOC) across land-water boundaries is central to supporting the ecological and biogeochemical functioning of freshwater ecosystems. Yet, we know little about how the generation and supply of terrestrial bDOC to streams and lakes is regulated by the physical, biological, and hydrological properties of the riparian interface. Here, we assessed how terrestrial, groundwater, and aquatic bDOC differ along flowpaths connecting riparian soils to a headwater boreal stream. We further tested how bDOC generation and supply differs among interfaces with distinct hydrogeomorphologies, as reflected by differences in soil properties, groundwater dynamics, and hydrological connectivity to the stream. We found that bDOC quantity declined sharply from terrestrial sources, to groundwater, to aquatic systems, and that these differences were associated with changes in the optical and chemical properties of the dissolved organic matter pool. However, bDOC generation and potential transport in groundwater varied across site types and reflected local differences in soil organic matter storage, depth to groundwater, and soil microbial community activity. Interface zones with organic-rich soils but weak hydrological connections had a large capacity to produce bDOC, but likely only laterally contributed organic resources during floods. By contrast, sites with stronger lateral hydrological connectivity served as persistent conduits for organic resources generated further upslope, even if the capacity to generate bDOC locally was weak. Overall, our results illustrate how hydrogeomorphic heterogeneity at the land-water interface can add spatial and temporal complexity to the generation and transfer of bDOC from soils to the inland water continuum.

Riparian zones are hydrogeomorphically heterogeneous, which may influence how, when, and where groundwater forms lateral connections between riparian soils and streams. These lateral connections represent important linkages between organic riparian soils and the expansive network of smaller headwater streams that are ubiquitous features of northern boreal landscapes. By way of lateral connections, riparian zones function as a source of organic and inorganic solutes to aquatic ecosystems. Yet, heterogeneity within the riparian interface complicates how we can mechanistically explain the role of the riparian zone as a source. 

Here, I explore the influence of hydrogeomorphic heterogeneity on the biogeochemical role of boreal riparian zones. By assessing soil properties, extracellular enzyme activity and microbial community composition I show that the capacity of a riparian site to do important biogeochemical work is directly influenced by the hydrogeomorphic setting. This creates substantial spatial and temporal complexity in the generation and transfer of degradable organic matter from riparian soils to inland water continua. Additionally, we experimentally test how drying and rewetting changes the mobilization of dissolved organic carbon in riparian soils, which may hint to how riparian zones will respond to drought as part of a changing climate. Further, I use relationships between solute concentration and groundwater discharge to show that a range of solute mobilization patterns from riparian sites are linked to the hydrogeomorphic heterogeneity of the riparian interface. Finally, I show that contrasting modes of lateral connectivity can simultaneously influence the stream, but do so via different mechanisms, across different response variables and at different spatial scales. 

This thesis shows that hydrogeomorphic heterogeneity influences a boreal riparian zone by determining the capacity of riparian soils to do important biogeochemical work, and the level of hydrological opportunity that riparian zones have to influence the adjacent stream via lateral connectivity. Ultimately, the riparian zone offers us the chance to use interactions between capacity and opportunity to more effectively manage and protect streams and aquatic networks in headwater catchments.