Soil organic carbon variation is associated with soil chemical conditions under long-term nitrogen enrichment in a temperate forest

Soil organic carbon (SOC) accumulation is influenced by multiple interacting processes, yet how long-term nitrogen (N) enrichment alters their relative importance and whether species-level trait divergence leads to differentiated SOC-regulatory pathways remain unclear. A long-term N-addition experiment was conducted in a temperate mixed forest. We quantified soil chemical and structural properties together with species-specific fine-root and mycorrhizal morphological traits. N enrichment significantly altered soil chemical conditions, with soil pH decreasing (6.18-5.00) and NO₃⁻–N increasing (12.11-36.89 μg g⁻¹), while SOC also increased across treatments (22.88–27.46 g kg⁻¹). Although soil aggregate stability increased under N addition, multivariate and random forest analyses identified soil pH and NO₃⁻–N as the dominant predictors of SOC, whereas aggregate indices and biotic traits contributed secondarily. Mycorrhizal morphological traits declined with increasing N inputs, and fine-root traits varied among species, root orders, and treatments; however, these biotic differences did not improve SOC prediction beyond plot-scale soil chemical variables, indicating that SOC variation under long-term N enrichment was associated mainly with soil chemical conditions.