Long-term thinning decreases the contribution of heterotrophic respiration to soil respiration in subalpine plantations

Abstract

Interest in the dynamics of soil respiration (R_s) in subalpine forest ecosystems is increasing due to their high soil carbon density and potential sensitivity to environmental changes. However, as a principal silvicultural practice, the long-term impacts of thinning on R_s and its heterotrophic and autotrophic respiration components (R_h and R_a, respectively) in subalpine plantations are poorly understood, especially in winter. A 3-year field observation was carried out with consideration of winter CO₂ efflux in middle-aged subalpine spruce plantations in northwestern China. A trenching method was used to explore the long-term impacts of thinning on R_s, R_h and R_a. Seventeen years after thinning, mean annual R_s, R_h and R_a increased, while the contribution of R_h to R_s decreased with thinning intensity. Thinning significantly decreased winter R_s because of the reduction in R_h but had no significant effect on R_a. The temperature sensitivity (Q₁₀) of R_h and R_a also increased with thinning intensity, with lower Q₁₀ values for R_h (2.1–2.6) than for R_a (2.4–2.8). The results revealed the explanatory variables and pathways related to R_h and R_a dynamics. Thinning increased soil moisture and nitrate nitrogen (\({\text{NO}}_{3}^{ - }\)-N), and the enhanced nitrogen and water availability promoted R_h and R_a by improving fine root biomass and microbial activity. Our results highlight the positive roles of \({\text{NO}}_{3}^{ - }\)-N in stimulating R_s components following long-term thinning. Therefore, applications of nitrogen fertilizer are not recommended while thinning subalpine spruce plantations from the perspective of reducing soil CO₂ emissions. The increased Q₁₀ values of R_s components indicate that a large increase in soil CO₂ emissions would be expected following thinning because of more pronounced climate warming in alpine regions.