Responses of soil organic carbon, soil respiration, and associated soil properties to long‐term thinning in a semiarid spruce plantation in northwestern China

Silvicultural thinning using whole‐tree harvesting (WTH) has been an important and common forest management practice for Picea crassifolia plantations in the Qilian Mountains of China. However, consequences of this silvicultural practice are still not well known. We examined the influence of three thinning levels on long‐term soil carbon storage, soil respiration (Rs), and soil properties. Our results showed that soil carbon stocks decreased significantly with increasing thinning intensity at a soil depth of 0–70 cm, whereas soil water storage increased, especially in the deep soil layers (30–70 cm). Mean Rs rates during the growing season increased significantly with increasing thinning intensity, and the dynamics of Rs coincided with that of soil temperature. Generally, 65% to 73% of the variation in Rs rates in three thinning levels was explained by the changes in soil temperature. WTH significantly increased soil bulk density at the 0‐ to 30‐cm and soil pH at the 0‐ to 20‐cm depths and significantly decreased soil nitrogen and C:N ratio in the 0‐ to 20‐cm layers. As no significant effect of WTH was detected on fine root biomass, we attributed the elevated soil respiration to accelerated decomposition of organic matter as a result of elevated soil temperature and substrate quality. Our results demonstrate the potential for WTH to relieve water deficits in spruce plantations in semiarid regions but suggest that WTH has a negative impact on carbon sequestration.