Background: The spruce forests are dominant communities in northwest China,

and play a key role in national carbon budgets. However, the patterns of carbon

stock distribution and accumulation potential across stand ages are poorly

documented.

Methods: We investigated the carbon stocks in biomass and soil in the natural

spruce forests in the region by surveys on 39 plots. Biomass of tree components were

estimated using allometric equations previously established based on tree height

and diameter at breast height, while biomass in understory (shrub and herb) and

forest floor were determined by total harvesting method. Fine root biomass was

estimated by soil coring technique. Carbon stocks in various biomass components

and soil (0–100 cm) were estimated by analyzing the carbon content of each

component.

Results: The results showed that carbon stock in these forest ecosystems can be as

high as 510.1 t ha-1, with an average of 449.4 t ha-1. Carbon stock ranged from

28.1 to 93.9 t ha-1 and from 0.6 to 8.7 t ha-1 with stand ages in trees and deadwoods,

respectively. The proportion of shrubs, herbs, fine roots, litter and deadwoods

ranged from 0.1% to 1% of the total ecosystem carbon, and was age-independent.

Fine roots and deadwood which contribute to about 2% of the biomass carbon

should be attached considerable weight in the investigation of natural forests.

Soil carbon stock did not show a changing trend with stand age, ranging from

254.2 to 420.0 t ha-1 with an average of 358.7 t ha-1. The average value of carbon

sequestration potential for these forests was estimated as 29.4 t ha-1, with the lower

aged ones being the dominant contributor. The maximum carbon sequestration rate

was 2.47 t ha-1 year-1 appearing in the growth stage of 37–56 years.

Conclusion: The carbon stock in biomass was the major contributor to the

increment of carbon stock in ecosystems. Stand age is not a good predictor of soil carbon stocks and accurate evaluation of the soil carbon dynamics thus requireslong-term monitoring in situ. The results not only revealed carbon stock status and dynamics in these natural forests but were helpful to understand the role of Natural Forest Protection project in forest carbon sequestration as well.