Abstract:

In this paper, we investigated influences of plantation measures and forest ages on soil organic carbon (SOC) of Larix principis-rupprechtii plantations with different slope aspects in the Southern Liupan Mountains was studied. The results showed that the SOC content on all aspects presented the same trend of firstly decreasing after tree-planting and then recovering. The SOC sensibility to environment variation, such as plantation disturbance, decreased with increasing soil depth. The time required for restoring the SOC density in the soil layers of 0-45 cm to the level of pre-afforestation varied with site conditions. On the sunny and semi-sunny aspects, the SOC density (96.33 t·hm^-2) in 0-45 cm soil layers of a 10 years young plantation was still lower than that of the shrub land (122.12 t·hm^-2); however, the SOC density (189.27 t·hm^-2) of the 20 years middle-aged plantation after afforestation was higher than that of the shrub land, indicating that the SOC was restored before this forest age. On the shady and semi-shady aspects, the SOC density of the young and middle-aged plantations was 192.37 and 222.03 t·hm^-2, respectively, and both were lower than that of the naturally regenerated forests (256.64 t·hm^-2), indicating that the SOC was still not restored after 20 years afforestation. Based on the statistical analysis of the investigated SOC content in the soil layers of 0-45 cm, the SOC content of 0-45 cm soil layers would decrease to the lowest point after 8 years afforestation on the sunny/semi-sunny aspects, with the decrease of 3.72 g·kg^-1 compared to the control of shrub land (32.13 g·kg^-1) on sunny aspect, and the fully recovering of SOC to the pre-plantation level would require 16 years after plantation. On the shady/semi-shady aspects, the SOC content would decrease to the lowest point after 16 years plantation, with the decrease of 22.77 g·kg^-1 compared to the control of secondary forests (66.30 g·kg^-1), and the fully recovering of SOC to the pre-plantation level would require 32 years after plantation. The SOC on shady aspects after plantation had a bigger loss, longer decreasing period and slower recovering rate than those on the sunny aspect. However, the absolute values of the SOC pool on the shady aspects were always higher than that on the shady aspects at any forest age, suggesting that the capacity of carbon sequestration at shady aspects was higher than that of sunny aspects. Our study demonstrated that the planting disturbance was able to affect the change of SOC pool. The mean SOC content in the 0-45 cm soil layers on sunny/semi-sunny aspect was 31.05 g·kg^-1 after 10 years plantation if the trees were planted with sparse spacing, considerably higher than that (23.17 g·kg^-1) of forests planted with normal density, however still lower than that of the control of shrub land (35.55 g·kg^-1).

Key words: Larix principis-rupprechtii, plantation, soil organic carbon, plantation measure, forest age, Liupan Mountains in Ningxia