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 hydrolytic enzyme activities, and microbial carbon metabolism remain unclear. We evaluated the influences of five planting densities (D1: 1667 stems·ha, D2: 3333 stems·ha, D3: 4444 stems·ha, D4: 5000 stems·ha, and D5: 6667 stems·ha) on the concentrations of SOC, microbial biomass carbon (MBC), easily oxidizable 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 plantations were 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-density plantations was lower than that in high-density plantations in both 5- and 35-year-old plantations. The high planting density was associated with higher MBC, activities of invertase and β-glucosidase, and microbial carbon metabolism activity in 5-year-old plantations, but the opposite was found in 35-year-old plantations. The high-density plantations exhibited a significant decrease in cellulase activity in 35-year-old plantations. These results highlight 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.