Figure 4

Variation in dissolved organic carbon (DOC) content (a) and DOC/SOC (b) in different N-deposition treatments in 0–20 cm in spring, summer, and autumn. CK: Control (0 kg N·ha⁻¹ yr⁻¹); LN: low-N deposition (50 kg N·ha⁻¹ yr⁻¹); MN: medium-N deposition (100 kg N·ha⁻¹ yr⁻¹); HN: high-N deposition (150 kg N·ha⁻¹ yr⁻¹). Each value in the plot represents the average value of three plots replicates. The error bars represent the standard error and different letters indicate significant differences among treatments (p < 0.05).