关键词: 土壤呼吸, 温度敏感性, 人工林树种, 林分密度

Abstract:

Objective: The objective of this study is to reveal the characteristics of soil respiration rate (R_S) and soil respiration temperature sensitivity index (Q₁₀) under different tree species and stand density conditions in plantations in the Loess Plateau of western Shanxi Province, and identify the key driving factors, so as to provide a scientific basis for plantation management and parameter optimization of carbon cycle models in this region. Method: Five stand density gradients were selected in pure Robinia pseudoacacia and Pinus tabuliformis plantations. The densities were 900, 1 200, 1 500, 1 800, and 2 400 trees·hm^–2 for R. pseudoacacia, and 900, 1 200, 1 800, 2 400, and 2 800 trees·hm^–2 for P. tabuliformis. A LI-8100 soil carbon flux monitoring system was used to continuously measure R_S during the growing season (from May to October, 2024). Soil physicochemical properties, soil microbial characteristics, as well as litter standing crop, litter carbon content, fine root biomass density, and carbon, nitrogen and phosphorus contents of fine roots simultaneously determined. Q₁₀ was estimated using an exponential model, and the main driving mechanisms underlying the variation in R_S and Q₁₀ were further elucidated. Result: 1) After controlling for soil temperature, soil water content, and month, tree species significantly affected R_S. The R_S in the P. tabuliformis plantations [(3.47±0.17) μmol·m^–2s^–1 ] was significantly higher than that in the R. pseudoacacia plantations [(3.02±0.09) μmol·m^–2s^–1]. The Q₁₀ of P. tabuliformis plantations (3.23±0.26) was also numerically higher than that of R. pseudoacacia plantations (2.50±0.13). 2) As the forest density increased, R_S in the two plantations increased, but there were no significant differences in R_S among different density treatments within each species. The response of Q₁₀ to stand density differed between the two species. There was no significant difference in Q₁₀ among different density treatments in R. pseudoacacia plantations, while in P. tabuliformis plantations, Q₁₀ values at the density of 2 800 trees·hm^?2 was significantly higher than that at 1 200 trees·hm^?2 (P<0.05). 3) Correlation analysis indicated that soil clay content and litter standing crop were key drivers for variation in R_S, and plant litter traits, fine root traits, and soil microbial properties played central roles in fluctuations in Q₁₀ values. Conclusion: Soil respiration and soil respiration temperature sensitivity index in plantations are jointly regulated by tree species and stand density. Specifically, the characteristics of plant litter, fine root, and soil microorganisms are the key factors driving the temperature response of soil carbon release. Tree species influence soil respiration primarily through differences in the quantity and quality of litter input. P. tabuliformis plantation has larger litter standing crop and higher carbon content, providing more abundant substrates for microbial respiration, resulting in overall higher R_S and Q₁₀ values than R. pseudoacacia plantations. The effect of stand density on soil respiration ias species-dependent: Q₁₀ values remains stable across density classes in R. pseudoacacia plantation, while the high-density P. tabuliformis stands have sufficient substrates, and the microbial metabolism is more sensitive to temperature, jointly driving a significant increase in Q₁₀ values. This study deepens understanding of changes in soil carbon flux and the mechanisms underlying its temperature response in plantations, and provides a theoretical basis for density regulation and structural optimization of plantations.

Key words: soil respiration, temperature sensitivity, plantation tree species, stand density