Soil Warming Shifts Seasonal and Soil Depth-Related Root Growth Dynamics in Subarctic Grasslands

Climate warming in northern ecosystems is accelerating, yet the responses of belowground growth dynamics under warming remain poorly understood. We assessed how soil warming affects seasonal root growth dynamics in subarctic grasslands, aiming to predict belowground responses under future climate scenarios. We investigated root growth across soil depths and seasons in geothermally warmed subarctic grasslands, comparing two warming durations: medium-term (MTW; 11-14 years) and long-term (LTW; > 60 years) and three warming magnitudes (ambient: +0°C, < +2°C, and > +2°C). Root length and projection area were quantified from minirhizotron images using the automated segmentation model RS_SAG. The automated segmentation model trained and used in this study performed robustly, with accuracy comparable to manual labels. Warming duration and magnitude effects on root length and projection area were season- and soil-depth-dependent. Root growth responses were more pronounced under LTW than MTW, particularly in the subsoil under warming > +2°C. Significant differences from ambient conditions occurred during summer, where root length and projection area were larger than in ambient conditions. Warming shifted root growth phenology by accelerating peak-season growth and intensifying winter decline. Enhanced subsoil root growth was related to a higher topsoil rhizome biomass, which likely provided additional energy to support deeper root growth and a higher abundance of Kobresia myosuroides-a deep-rooted rhizomatous sedge species with extensive belowground biomass. Higher subsoil root length and projection area under LTW indicate a shift towards deeper rooting, potentially associated with changes in species composition or nutrient availability. Enhanced root growth, proliferation, temporal shifts and seasonal amplitude may increase litter inputs, influencing microbial activity and organic matter dynamics in subsoils. These deep-root responses may partially offset warming-induced organic matter decomposition but could also affect the stability of soil carbon pool, emphasising the role of deep-soil roots in carbon cycling in subarctic soils.