Increased root-derived carbon offsets soil carbon loss under simultaneous warming and nitrogen addition

Plant roots are primary drivers of soil organic matter dynamics, mediating belowground carbon (C) inputs, stabilization, and losses. Yet, how global changes such as rising temperatures and altered nitrogen (N) availability interact to affect these dynamics has rarely been tested empirically in the field. Here, we quantify how root-derived C inputs from fine root production, root exudates, and root-associated fungi respond to long-term (16 years) soil warming (+5°C), nitrogen (N) enrichment (+5 g N m⁻² yr⁻¹), and their combination in a temperate hardwood forest. Warming alone reduced root-derived C inputs by 21% and increased microbial respiration by 46%, resulting in a net soil C loss of 200 g C m⁻² yr⁻¹. In contrast, N enrichment increased root-derived C accumulation by 113% and reduced root respiration by 40%, contributing to net soil C gains of ~110 g C m⁻² yr⁻¹. When combined, warming × N addition increased root-derived inputs fourfold (from 70 to 281 g C m⁻² yr⁻¹), fully offsetting warming-induced C losses and maintaining soil C stocks at control levels. Root-derived SOC accumulation was positively related to fine root production (r² = 0.42) and to maple:oak exudate ratios (r² = 0.31), highlighting species-specific control over C stabilization. These findings demonstrate that interacting global change factors can have offsetting effects on root C allocation and microbial losses, emphasizing the need to incorporate both processes when predicting soil C feedbacks.