Kevin Bishop

Boreal catchments are composed of different land covers, such as forests, peatlands and lakes, which differ in their runoff response to rainfall events. Understanding the individual and combined responses to rainfall events of these different land cover types is crucial for predicting potential impacts of future climate conditions on boreal water cycling. A common assumption is that peatlands attenuate peak flows, which is used as a motivation to restore drained boreal wetlands. However, it remains unclear how and to what extent peatlands can affect peak flow response. Only a few previous studies have looked at the hydrologic dynamics of peatlands in response to specific rainfall events across a wide range of nested sub‐catchments with varying peatland cover. In this study, we use nine years of hourly hydrometric data from 14 catchments within the Krycklan Catchment Study in northern Sweden to examine how peatlands contribute to flood attenuation at both local and stream network scales. Our analysis at the local scale demonstrated that during large events with low antecedent wetness conditions, peatland‐dominated catchment exhibited more muted responses compared to the similar‐sized forest‐dominated catchment. However, during events with high antecedent wetness conditions, the peatland‐dominated catchment exhibited flood magnitudes similar to the forest‐dominated catchment, although the elevated flow condition at the peatland‐dominated catchment persisted for longer periods. Finally, our analysis revealed no significant influence of peatlands on the attenuation or amplification of floods at the stream network scale.

Summary Tree growth in boreal forests is driven by ectomycorrhizal fungal mobilisation of organic nitrogen and mineral nutrients in soils with discrete organic and mineral horizons. However, there are no studies of how ectomycorrhizal mineral weathering and organic nitrogen mobilisation processes are integrated across the soil profile. We studied effects of organic matter (OM) availability on ectomycorrhizal functioning by altering the proportions of natural organic and mineral soil in reconstructed podzol profiles containing Pinus sylvestris plants, using 13 CO 2 pulse labelling, patterns of naturally occurring stable isotopes ( 26 Mg and 15 N) and high‐throughput DNA sequencing of fungal amplicons. Reduction in OM resulted in nitrogen limitation of plant growth and decreased allocation of photosynthetically derived carbon and mycelial growth in mineral horizons. Fractionation patterns of 26 Mg indicated that magnesium mobilisation and uptake occurred primarily in the deeper mineral horizon and was driven by carbon allocation to ectomycorrhizal mycelium. In this horizon, relative abundance of ectomycorrhizal fungi, carbon allocation and base cation mobilisation all increased with increased OM availability. Allocation of carbon through ectomycorrhizal fungi integrates organic nitrogen mobilisation and mineral weathering across soil horizons, improving the efficiency of plant nutrient acquisition. Our findings have fundamental implications for sustainable forest management and belowground carbon sequestration.