Biogeochemical patterns vary with hydrogeomorphology in riparian soils along a boreal headwater stream

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 (β-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.