Forest soils are major carbon reservoirs that may become greater sources of CO₂ to the atmosphere depending on how forests are managed. Characterization of forest soil C, N and their natural stable isotopic abundance within bulk soil and size fractions from sites subjected to various forest disturbance regimes provide insights to the dynamics of soil organic matter within soil profiles. The objective of this study was to examine how forest disturbance affects the depth distribution of soil organic C, N, d¹³C and d¹⁵N within bulk soil samples (<2 mm) and three size fractions (coarse: 2-0.5 mm; medium: 0.5-0.053 mm; and fine: <0.053 mm). Soil samples were collected from each of three sites (LV1: 84 years old intact forest, LV2: selectively cut forest, and LV3: clear cut in 2002) and five depths (0-4, 4-8, 8-20, 20-40 and 40-60 cm).

The fine fraction (silt and clay) only accounted for less than 15% of the bulk soil at all sites. No consistent pattern was found in d¹⁵N, N, and C concentrations among size fractions. The fine fraction was significantly enriched with ¹³C only at LV1. The three sites were significantly different in their SOC concentration with mean values ranging between 1.3 ± 0.2% (mean ± 1SD) at LV1 to 3.2 ± 0.3% at LV3. Mean C and N concentrations did not follow a general decreasing pattern with depth. For LV1 and LV2, the highest C concentration was within the 20-40 cm depth, characteristic of spodic horizons containing illuvial humus. Similarly N concentration, with measured ranges of 0.02 to 0.32% from all three sites, was significantly higher within the 20-40 cm depth at LV1. For LV3, N concentration was the highest within the 0-4 cm depth while C concentration was the highest within the 0-20 cm depth. The d¹³C values were identical (-26.1± 0.8‰) for the three sites and varied with depth by 3.4‰ with the 20-40 cm depth showing the highest average value (-25.1 ± 0.3 ‰). C/N ratios generally decreased with depth and ranged between 16.9 and 43.9 with greater depth variations more apparent at LV3 and LV2 than LV1. The C/N ratio and d¹³C from all three sites were significantly negatively correlated (r = -0.66, P < 0.001) implying the strong bearing of decomposition upon d¹³C, especially for LV3 where the correlation coefficient was the highest (r = -0.91, P < 0.001). The relationship between C/N ratio and d¹⁵N was more complex and suggests that factors other than decomposition may be important in shaping the depth distribution of d¹⁵N.