Eucalyptus and Pinus stand density effects on soil carbon sequestration

• No changes in SOC stocks (0–90 cm) in 8-year-old Eucalyptus or Pinus afforestation.
• Higher SOC stocks in AB and Bt1 horizons under afforestation than pasture soil use.
• New vegetation (Eucalyptus and Pinus) contributed to replenish SOC in the upper 30 cm.
• No significant effects in SOC were found of Eucalyptus and Pinus stands density.

Although afforestation of pasture and agricultural systems can increase C sequestration in soils, studies have shown wide variation in the magnitude, timing and direction of soil organic C (SOC) dynamics, depending on site conditions, management practices, and previous land use. Effects of stocking density on SOC have yet to be elucidated, however. The objectives proposed for this work were to quantify: (i) SOC content and distribution for the top 30 cm of the A horizon under native pasture compared with that following 8 years of afforestation (Eucalyptus grandis and Pinus taeda planted at different densities); (ii) SOC accumulation in the AB and the top of Bt horizons in afforested soils; and (iii) the contribution of the new vegetation (Eucalyptus and Pinus) to SOC. A field trial with three stand-densities of E. grandis and P. taeda and corresponding native pasture was established and soil samples were collected and assayed for five layers of the A horizon (0–5, 5–10, 10–15, 15–20, 20–30 cm), and the AB and Bt1 horizons. Soil organic C concentration and δ13C were determined, and the total SOC stock and C derived from the Eucalyptus and Pinus vegetation (young C) were calculated. Our results suggest that there was likely no significant change in SOC stocks in response to 8 years of afforestation with either Eucalyptus or Pinus. No significant differences in SOC stocks in the upper 30 cm soil layer as a whole were found among pasture, Eucalyptus, and Pinus treatments. By contrast, SOC in the AB and Bt1 horizons was significantly higher under afforested sites than native pasture (P < 0.001 and P < 0.039, respectively). Soil δ13C in the afforested treatments (0–30 cm) reflects the contribution of the new vegetation (Eucalyptus or Pinus) to SOC. Net accumulation of new SOC from the planted trees in the top 15 cm soil layer was equivalent to 0.20 Mg C ha−1 yr−1 for Eucalyptus and 0.30 Mg C ha−1 yr−1 for Pinus. Assuming initial declines in SOC following afforestation have largely ended and young C sequestration rates continue at the same rate, SOC stocks in the top 15 cm layer would reach those found under pasture after 15 and 11 years for Eucalyptus and Pinus plantations, respectively. Measurements of additional sites afforested with Eucalyptus and Pinus of different ages are needed to draw firm conclusions about the net SOC balance in these afforested pasture soils.