Duality of root‐dominated net soil organic carbon gain and mycelium‐mediated carbon loss in hyper‐arid afforestation

Summary Afforestation regulates soil organic carbon (SOC) sequestration via plant‐ and microbial‐derived carbon (C) inputs, but root‐ and mycelium‐derived C inputs differ in quantity, turnover, and stabilization mechanisms, and these differences may shift as stands develop. However, how root vs mycelium pathways drive SOC dynamics across a dryland afforestation chronosequence remains unclear. Using the ingrowth‐core method along a dryland afforestation chronosequence (young forests, middle‐aged forests, and mature forests), we quantified the contributions of root and mycelium pathways to SOC. The root pathway promoted SOC accumulation via particulate organic C and mineral‐associated organic C inputs, contributing to 77.19–85.05% of the SOC changes. By contrast, the mycelium pathway induced net SOC loss via particulate organic C depletion, contributing to 14.95–22.81% of the SOC changes. SOC concentrations peaked in middle‐aged forests, whereas relative SOC stability peaked in young forests. This study provides the first quantitative assessment of the distinct contributions of root and mycelium pathways to SOC accumulation and stabilization during dryland afforestation, advancing understanding of their roles in regulating SOC dynamics. As climate change intensifies land‐use pressure, management strategies should integrate stage‐specific root and mycelial traits to enhance SOC sequestration in arid forests.