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 (Q ₁₀) of R _h and R _a also increased with thinning intensity, with lower Q ₁₀ 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 Q ₁₀ 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.

Keywords

Heterotrophic respiration / Autotrophic respiration / Long-term thinning impacts / Cold seasons / Subalpine plantations / Temperature sensitivity