Agroforestry systems affect soil organic carbon stocks and fractions in deforested landscapes of Amazonia

Soil organic carbon (SOC) is the main component of carbon in terrestrial ecosystems and an indicator of soil quality. The study aimed to investigate the stock and vertical distribution of SOC fractions and the SOC sequestration of different agroforestry systems (AFS) and other land uses in the Colombian Amazon. In each land use (Secondary forest-SF, Natural regeneration-NR, Alley cropping 1-AC1, Alley cropping 2-AC2, Forest plantations 1-FP1, Forest plantations 2-FP2, Crops in forest plantation-CFP, Shade trees for crops-STC, Homegarden-HG, Silvopastoral system-SPS and Pasture-P), four soil pits were made (1 x 1 x 1 m) to collect soil samples to determine SOC concentration by carbon lability fractions (C-VL: very labile, C-L: labile, C-LL: less labile, C-NL: non-labile) and bulk density at four depths (0-10, 10-20, 20-40 and 40-100 cm). The mean SOC concentration was 15.3 +/- 1.2 g kg(-1), with a higher concentration in 0-10 cm (26.1 +/- 2.4 g kg(-1)). The concentration of the different fractions presented the following order C-VL > C-NL > C-L > C-LL (7.4 +/- 0.3, 4.8 +/- 0.2, 2.7 +/- 0.1 and 2.1 +/- 0.1 g kg(-1), respectively). SOC stock was 13.3 to 220.0 Mg C ha(-1) in the 0-100 cm layers, where FP1, FP2, AC1 and AC2 showed the highest values (174.0 to 199.0 Mg ha(-1)). In contrast, HG, P and STC showed the lowest values with 134.0, 116.0 and 96.2 Mg ha(-1), respectively. Therefore, and due to the contribution of the most stable carbon fractions in the soil (C-LL and C-NL), land uses such as AC1 and AC2 presented the highest levels of carbon stability measured by the carbon management index. Therefore, the different land uses with agroforestry systems increased the amount and stability of carbon accumulated in the soil compared to pasture.