Sources of soil organic carbon under Eucalyptus silvopastures and other land-use systems in Brazilian Oxisols

Silvopastoral systems (SPS) incorporating non-native tree species such as Eucalyptus are increasingly being adopted in the Cerrado region of Brazil. The climate-change mitigation potential of the practice, although hypothesized, has not been assessed vigorously. The objective of this research was to measure the extent of SOC accumulation under SPS involving C3 and C4 plants and other prevalent land-use systems in an Oxisol-dominated location in Minas Gerais, Brazil. We determined the overall delta 13C in three soil fraction-size groups within six land-use systems. These systems consisted of 8-year-old planted forests, native forest, managed pastures, and three variations of 8-year-old SPS with diverse tree-planting layouts. In the SPS and planted forest treatments, the tree species was Eucalyptus hybrid, and Urochloa decumbens was the grass species in the SPS and pasture treatments. For each treatment, soil samples were collected at depths of 0-10, 10-30, 30-60, and 60-100 cm. These samples were fractionated into three size classes (2000-250 mu m, 250-53 mu m, and <53 mu m) and analyzed for delta 13C. The native forest had higher C3 contribution in the topsoil layers. The highest C3 derived SOC stock up to 1 m depth was in the pasture (137 Mg ha(-1)), and the lowest in the forest (74 Mg ha(-1)). The pasture also had the highest C4 derived SOC stock and the forest the lowest, but not significantly. The SPS had intermediate values. We infer that the study site had a mixed vegetation in the past, and after the conversion of the land to planted forest/SPS/pasture, the "native" SOC stock could be maintained better in a well-managed pasture than in a degraded native forest. The higher C3-derived SOC stock in the pasture treatment suggests that managed SPS could help retain more soil C in a stable form than in degraded forests.