Soil organic carbon fractions, C-cycling hydrolytic enzymes, and microbial carbon metabolism in Chinese fir plantations

The mechanisms by which planting density affects soil organic carbon (SOC) fractions, C-cycling associated hydrolyticenzyme activities, and microbial carbon metabolism remain unclear. We evaluated the influences offive planting densities (D1 1667 stems·ha−1, D2 3333 stems·ha−1, D3 4444 stems·ha−1, D4 5000stems·ha−1, and D5 6667 stems·ha−1) on the concentrations of SOC, microbial biomass carbon (MBC), easilyoxidizable carbon (EOC), and dissolved organic carbon (DOC), the activities of invertase, cellulase, and β-glucosidase, and microbial carbon metabolism activities in 5- and 35-year-old Chinese fir plantations. Generally,no significant differences in the SOC and DOC concentrations among five planting densities in 5-year-old plantationswere found, but the SOC and DOC were significantly higher in high-density plantations (D3, D4, and D5)than in low-density plantations (D1 and D2) in 35-year-old plantations. The EOC concentration in low-densityplantations was lower than that in high-density plantations in both 5- and 35-year-old plantations. The highplanting density was associatedwith higherMBC, activities of invertase and β-glucosidase, and microbial carbonmetabolism activity in 5-year-old plantations, but the opposite was found in 35-year-old plantations. The highdensityplantations exhibited a significant decrease in cellulase activity in 35-year-old plantations.These resultshighlight that although increased planting density would enrich SOC storage after a long-term rotation of plantations,it also reduces microbial and enzymatic activities. This has important implications in the formulation of planting density management strategies to increase SOC stocks while maintaining soil fertility.