There is little information on the effects of land use change on soil Carbon stocks in Colombian Amazonia. Such information would be needed to assess the impact of this area on the global C cycle and the sustainability of agricultural systems that are replacing native forest. The aim of this study was to evaluate soil carbon stocks and changes after the clearing of the native forest, the establishment of pastures and the reclamation of the degraded pasture, in Caquetá, Colombia. We compared the contents of Total C, Oxidizable C and Non-Oxidizable (stable) C in four different land use systems, namely Monoculture (Brachiaria grassland), Association (Brachiaria + Arachis pintoi), Forage Bank (a mixture of forage tree species), and Natural Regeneration of the pasture in both a flat area and a sloping one. The Degraded Pasture was the reference. Results showed that in the sloping area all treatments have higher Total Carbon stocks than the Degraded Pasture, while three of the treatments significantly increased the stocks of Non-Oxidizable C. In the flat landscape, only the Association significantly increased Total C stocks. Plowing and fertilization cause significant increases in Oxidizable carbon and decreases in Non-Oxidizable carbon. This effect needs further research, as C stability will influence equilibrium stocks. In the sloping area, improved pastures and fodder bank rapidly increased Total Carbon contents and stocks, with increases as large as10 ton.ha-1 yr-1. In the Traditional Fodder Bank, which showed the largest increase, this is partially due to the application of organic manure. Surprisingly, also C stocks under Natural Regeneration were significantly higher than under the original Degraded Grassland. This increase was fully due to Non-Oxidizable Carbon, which is difficult to explain. Stable isotope analysis indicated that under improved grassland, especially Brachiaria monoculture, up to 40% of the original C in the upper 10 cm was replaced in 3.3 years.
- soil organic-matter
- humic substances