Abstract:

Objective: Compare the differences in soil aggregation and carbon sequestration between two poplar plantation forests to reveal the basic process of carbon sequestration in plantation forests, thereby providing a theoretical basis for sustainable forest management. Method: We collected the surface soil of high density (400 plants·hm^?2) and low density (278 plants·hm^?2) poplar plantation (16 a), and classified soil aggregates into <0.25 mm, 0.25–2 mm, and >2 mm particle levels by the optimal moisture-dry sieving method, studying the content of active, slow and passive organic carbon, and the rate of soil carbon mineralization in each particle level. Result: Compared with the low-density poplar plantation forests, the composition of soil aggregates in the high-density poplar plantation forests had a higher proportion of <0.25 mm and 0.25–2 mm particle sizes and a lower proportion of >2 mm particle sizes, respectively 1.06%, 21.92% and 77.02%. There were significant differences in the organic carbon fractions in soil aggregates at all particle levels among poplar plantations of different densities. Among them, active and slow organic carbon were higher in all particle levels of high-density plantation forest soil, while passive organic carbon was lower in <0.25 mm and 0.25–2 mm particle levels. The indoor incubation experiments revealed that soil carbon mineralization rates at all particle levels were generally lower in high-density plantation forests compared to low-density plantation forests throughout the incubation stage, suggesting that conservative soil microbial carbon metabolism strategies are the key to soil organic carbon accumulation in high-density plantation forests. Further analysis revealed that stand density had a significant effect on soil aggregate particle size distribution and active and slow organic carbon, but not on soil passive organic carbon; in which the reduction of soil density under high-density stands led to the accumulation of active and slow organic carbon content. Conclusion: In summary, reasonable regulation of stand density is an important silvicultural measure that affects soil aggregate formation and organic carbon fractions in plantation forests. Reduced soil bulk density and conserved soil carbon metabolism capacity in high-density poplar plantation forests would be favorable for soil carbon sequestration.

Key words: plantation, forest managements, soil aggregates, soil carbon sinks