Soil carbon stocks under improved tropical pasture and silvopastoral systems in Colombian Amazonia.

Soil carbon stocks of primary forest, degraded pasture, and four improved pasture systems in Colombian Amazonia were compared in a flat and a sloping landscape. The improved grasslands were Brachiaria humidicola , and B. decumbens , both in monoculture and in combination with a legume ( Arachis pintoi ). The age of the treatments was 30 years for degraded pasture and 10 or 15 years for each of the improved pastures. Carbon fractions were Total C, Oxidizable C, and Stable C. Stocks were compared using a fixed soil mass base. Although forest soils reputedly have the highest C stocks, this is not necessarily true if the forest is on poorly drained soil. The degraded pasture in the flat landscape was abandoned and dominated by weeds, while that in the sloping area was overgrazed. The latter has much lower C stocks than the former. B. humidicola monoculture had the highest stocks in both areas, while the effect of the other three treatments varied. The effect of improved pastures, although they tend to increase soil C stocks, depends strongly on both soil and management practices.