Increased root-derived carbon buffers 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 inputs to soil organic matter 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 year^-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 135 g C m^-2 year^-1. In contrast, N enrichment increased root-derived soil organic carbon (SOC) accumulation by 47% and reduced root respiration by 40%, contributing to a near-neutral soil C balance. When combined, warming × N addition increased root-derived SOC fourfold (from 70 to 281 g C m^-2 year^-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 balancing effects on root C allocation and microbial losses, highlighting soil N availability as a critical control determining whether warming accelerates SOC depletion or stabilizes new root-derived C.