Root biomass distribution in response to increasing active layer thickness across plant communities in a permafrost peatland

In Arctic tundra, the majority of plant biomass is belowground. As temperatures increase and permafrost thaws, new soil volume containing nitrogen becomes available for plant roots, potentially leading to new plant–soil interactions. Whether or not different plant communities adapt their rooting depth distribution in response to increasing active layer thickness, whether roots at depth are similar to shallow ones, and if deep‐rooting species potentially redistribute thawed‐out nitrogen into more shallow layers remains poorly understood. We studied root biomass, rooting‐depth distribution, root dry matter content (RDMC), and 15 N natural abundances in a low Arctic peatland. We did this in two plant communities, dominated either by the graminoid Eriophorum vaginatum or by various dwarf shrubs, each with either a shallow (51.3 ± 0.9 cm) or a deep (69.4 ± 1.9 cm) active layer. Root biomass declined sharply with depth but did not differ between plant communities or active layer thicknesses, although roots reached the thawfront. RDMC was unaffected by active layer thickness, but was lower in E. vaginatum and declined with depth for roots of both functional types. This suggests relatively more acquisitive roots with faster growth and shorter lifespan in deeper soils. Presence of the deep‐rooting E. vaginatum increased foliar δ 15 N ratios of other species, indicating a potential redistribution of nitrogen in the soil column. Our results suggest that, despite the presence of deep‐rooting plants and roots down to the thawfront in shallow and deep active layers, rooting depth distribution remains relatively constant. This indicates that an increase in deep roots in response to thawing permafrost is relatively species‐specific and might not result in measurable differences in root biomass or its distribution, irrespective of plant community. The decreasing RDMC with depth illustrates that resource strategies vary not only among species but also with soil depth, with implications for carbon and nutrient cycling.