Early silvicultural practices increase deep soil carbon and total ecosystem carbon stocks in a mid-rotation Pinus radiata plantation

Soil organic carbon (SOC) is the largest and most persistent carbon pool in forest ecosystems, yet its response to early silvicultural practices remains poorly understood in intensively managed plantations. This study evaluated the effects of soil preparation intensity and weed-control opportunity on SOC stocks and ecosystem carbon pools in a 12-year-old Pinus radiata D. Don plantation established on ancient volcanic ash soil in south-central Chile. Treatments followed a 3 × 3 factorial design with three blocks, combining three soil preparation intensities: shovel planting, shallow disking to 30 cm, and subsoiling to 80 cm; and three weed-control opportunities: no weed control, pre + post planting weed control, and post-planting weed control only. Tree diameter and height were measured annually to estimate stand volume and biomass carbon. SOC was quantified at 0-20, 20-40, and 40-100 cm depths, while forest floor carbon was estimated from litter and woody residues. Pre + post-planting weed control consistently increased stand productivity, SOC, and total ecosystem carbon stocks regardless of soil preparation intensity.

The highest values were observed under subsoiling plus pre + post planting weed control, reaching 241 m³ ha^-1 of stand volume, 207 Mg C ha^-1 of SOC, and 291 Mg C ha^-1 of total ecosystem carbon stock, compared with 111 m³ ha^-1, 107 Mg C ha^-1, and 150 Mg C ha^-1, respectively, under shovel planting without weed control. SOC showed a strong positive relationship with stand volume (R² = 0.71; p < 0.05), while total ecosystem carbon was closely related to stand productivity (R² = 0.87; p < 0.05). Mineral soil remained the dominant carbon pool, representing 65-72% of total site carbon. These preliminary results suggest that early weed control is a key practice for increasing both stand growth and ecosystem carbon sequestration in P. radiata plantations.