Total soil carbon changes and stable carbon isotope dynamics following reforestation of pasture with broadleaf and conifer plantations in Brazil

Background/Question/Methods

Reforestation has been proposed as a means to increase soil carbon sequestration for climate change mitigation and soil quality restoration. Changes in soil organic carbon have been shown to vary according to tree species established.  To quantify differences in total soil organic carbon stocks (SOC_T) between conifer and broadleaf tree plantations following establishment on cattle pasture in southeastern Brazil, we sampled the organic horizon and three depths from 0-45 cm of mineral soil in adjacent, paired vegetation plots consisting of ten Pasture-Broadleaf (PB), ten Conifer-Broadleaf (CB) plantations, and one Reference unmanaged forest and pasture pair.  We used a chronosequence approach (ages 6-34 years) to test for soil carbon accumulation over time.  Photosynthetic pathways of C4 pasture grasses and C3 tree plantations created unique isotopic carbon signatures in the mineral soil that we used to determine soil organic carbon dynamics following reforestation.  Based on the stable isotope signatures in the chronosequence, we estimated soil organic carbon accumulation from planted forests (SOC₃), and mean residence times of pasture carbon (SOC₄).  Independent variables included paired-plot blocks, vegetation, and depth as main effects and age as a covariate in a general linear model.  Multiple comparisons were tested with Tukey’s HSD (α = 0.05). 

Results/Conclusions

Our results indicated no significant difference in SOC_T between broadleaf plantations and pastures, and a small increase (6%) in broadleaf plantations compared to conifers.  No significant relationship existed between forest stand age and SOC_T, but stable carbon isotopes showed a significant decrease in SOC₄.  The estimated age when plantations became dominated by SOC₃ was 9.7 years at 0-15 cm and 8 years at 15-30 cm.  At 30-45 cm, soils remained dominated by SOC₃ from the original forest.  The rate of forest SOC₃ accumulation at 0-15 cm (0.21 Mg C ha-1 yr-1) matched the rate of SOC₄ disappearance (-0.21 Mg C ha-1 yr-1).  A slight SOC_T decrease in from 15-30 cm and 30-45 cm was due to a loss of SOC₄ (-0.17 and -0.083 Mg C ha-1 yr-1, respectively).  Mean residence times for SOC₄ increased significantly with depth averaging (± standard error) 30 ± 4.5, 33 ± 4.7, and 73 ± 18 years from 0-45 cm.  These results suggest slower rates of pasture derived soil carbon decomposition with increasing depth and contribute to a better understanding of soil carbon dynamics following land-use change.