Soil Carbon Dynamics Following Reforestation of Tropical Pastures

Abstract

Soil carbon changes following reforestation of tropical pasture soils have been variable and the mechanisms poorly understood. This limits predictive capabilities and therefore management decisions. Photosynthetic pathways of C4 grasses and C3 trees create unique stable carbon isotopic signatures that can be used to determine soil carbon dynamics. In this study, mineral soils were sampled to 45 cm from paired land cover types consisting of an unmanaged forest and pasture, pastures paired with broadleaf plantations, and broadleaf paired with conifer plantations. Soil δ13C values were determined from pastures (SOC4) and forests (SOC3), allowing the calculation of turnover time of SOC4 following reforestation. Reforested pasture soils became dominated by SOC3 in 10 years at 0 to 15 cm and in 8 yr at 15 to 30 cm, but remained dominated by SOC3 from original forest cover from 30 to 45 cm. From 0 to 15 cm, the rate of SOC3 accumulation (0.21 Mg C ha-1 yr-1) matched the rate of SOC4 disappearance (-0.21 Mg C ha-1 yr-1). Soil carbon decreased slightly in the 15- to 30-cm (-0.17 Mg C ha-1 yr-1) and 30- to 45-cm (-0.08 Mg C ha-1 yr-1) soil layers from a loss of SOC4 and no gain of SOC3. Overall, reforestation led to relatively small net losses of mineral soil C (-0.20 Mg C ha-1 yr-1, 0–45 cm). Turnover time for SOC4 increased with depth from 30 to 73 yr. The results from this analysis contribute to a better understanding of soil carbon dynamics following reforestation of tropical pastures.