Abstract: Soil respiration is the key process driving CO₂ exchange between forest soils and the atmosphere and regulated by soil organic carbon (SOC) characteristics and extracellular enzyme activities. However, the direction and magnitude of the effects of stand density on labile SOC fractions, extracellular enzymes, and soil respiration across plantation ages remain unclear. We constructed enhanced soil respiration models using heterogeneous soil data under density regulation to better characterize soil processes. Study plots encompassing stand-density gradients were implemented in Larix principis-rupprechtii plantations spanning three age-class strata. During the growing season, systematic measurements were conducted on soil respiration rates, labile organic carbon fractions, and extracellular enzyme activities. A process-driven soil respiration model was developed by integrating nonlinear mixed-effects modeling frameworks with measured data. The moderate density stands showed increases in soil respiration (R_s), microbial biomass carbon (MBC), light fraction organic carbon (LFOC), β-1,4-glucosidase (BGC), and β-N-acetyl glycosaminidase + leucine aminopeptidase (NAG + LAP). In 36a and 48a stands, the moderate-density stands NAG + LAP had a ~ 35% increase compared to other density levels, while readily oxidized carbon (ROC) concentrations showed a significant ~ 30–50% reduction. All labile organic carbon components were stable with age, so that soil microorganisms were promoted to acquire C, N, and P. Temperature, moisture, MBC, BGC, and NAG + LAP were essential factors that affected soil respiration. Stand density has important impacts on soil respiration as it regulates the soil organic carbon and activities of extracellular enzymes. The roles of temperature, microbial biomass carbon, soil organic carbon and dissolved organic carbon are complex and directly affect autotrophic and heterotrophic respiration and regulate soil respiration by influencing microbial C and N acquisition. A mixed-effects model with nested stand density and age mathematically optimized the soil respiration model, enabling enhanced characterization of covariation patterns of soil respiration with related soil carbon pool variables.

Key words: Stand density, Soil organic carbon, Soil enzyme activities, Soil respiration, Soil respiration model