Soil carbon responses to afforestation in Eucalyptus and pine stands

Afforestation of pasture can have significant but highly variable effects on soil carbon, with important implications for forest carbon budgets. The effects of tree species and management practices such as planting density on soil carbon are not well understood.

A recent paper from a study of eight-year-old stands of Eucalyptus grandis and loblolly pine (Pinus taeda) planted at different densities on native pasture in Uruguay concluded there was no overall effect of the plantings on soil carbon stocks compared to pasture. Soil carbon dynamics with depth were different in forested and pasture sites, however, with lower soil carbon in the upper 15 cm soil layer under forested stands vs. native pasture and greater soil carbon accumulation in deeper layers under forest. Using a carbon isotope technique, investigators estimated that accumulation of “young” carbon from the trees would compensate for reduced carbon near the soil surface by the time stands were 11 to 15 years old. 

The study was supported by a Weyerhaeuser Company Foundation grant to NCASI and led by a team of scientists at the Universidad de la República, based in Montevideo, Uruguay. The paper, which was recently published in Forest Ecology and Management, was written by Jorge Hernández, Amabelia del Pino, Álvaro Califra, Fabián Del Giorgio, Leticia Martínez, and Pablo González-Barrios of the Universidad de la República and Eric D. Vance of NCASI. The abstract follows:

“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.”  

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Hernández, J., A. del Pino, E.D. Vance, Á. Califra, F. Del Giorgio, L. Martínez, P. González-Barrios. 2016. Eucalyptus and Pinus stand density effects on soil carbon sequestration. Forest Ecology and Management 368:28–38.