Multiscale Temporal Dynamics and Depth–Dependent Controls of Soil Respiration Components Across Subtropical Forest Types

Soil respiration (Rs) is a key component of terrestrial carbon cycling, yet its temporal structure and vertical environmental controls remain insufficiently understood. This study investigated Rs and its components across four subtropical forest types in Hunan, China, using high–frequency 24–h observations combined with multi–month monitoring and measurements at three soil depths. Temporal dynamics were analyzed using harmonic fitting and two–way ANOVA–based variance partitioning, and environmental controls were quantified using linear mixed–effects models and relative importance analysis. Results showed that annual Rs ranged from 473.50 g C m−2 yr−1 in secondary broadleaf forest to 652.50 g C m−2 yr−1 in pure Pinus massoniana Lamb. plantation, with heterotrophic respiration dominating all stands (68.9%–82.1% of Rs). The cosine model best described diel cycles in 64.6% of cases, with amplitude significantly higher in summer than in winter. Variance partitioning revealed that seasonal (month) scale explained 89.2%–97.3% of total variation in Rs and Rh, whereas diel (hour) scale contributed only 0.4%–4.8%; for Ra, seasonal contribution dropped to 36.1%–73.7% and residual variance surged to 21.9%–54.3%, indicating substantially greater stochasticity. Linear mixed–effects models showed that topsoil temperature (0–10 cm) was the dominant driver of Rs and Rh, explaining 48% of their variation, whereas Ra was additionally regulated by subsurface soil moisture and conductivity, with only 19% of variation explained. Overall, soil respiration exhibited a structured, multi–scale, and depth–dependent response system shaped by forest type, temporal variability, and depth–specific soil heterogeneity.