Land use and landscape effects on aggregate stability and total carbon of Andisols from the Colombian Andes

The Dosquebradas Basin, in the central coffee-growing region of the Colombian Andes, covers 58 km2 between 1350 and 2150 m of elevation. Soils in this basin are mostly Melanudands. Soil sampling units were defined on the basis of land use (sun coffee, shade coffee and pasture) and slope. Samples from 76 sites were collected during June-July of 2002 to measure water stable aggregates, aggregate carbon concentration, total carbon content and C/N ratios, and analyze the effect of aggregate size, land use, landscape position (summit, shoulder, backslope) and depth on these properties. Analyses of variance indicated an interaction effect of land use and landscape position on water stable aggregates and aggregate carbon concentration, while a main effect of land use on total carbon content and C/N ratios. Higher percentages of water stable aggregates were found under sun coffee (95%) and pasture (93%) compared to shade coffee (83%) at the backslope position. Aggregate carbon concentration was highest under pasture (95 g kg−1) at the summit position and under sun coffee (76.7 g kg−1) at the shoulder position. Aggregate size (>2 mm, 1 to 2 mm and <1 mm) did not have an effect on aggregate carbon concentration, indicating that these Andisols do not conform to the aggregate hierarchy model. Total carbon content of surface samples (0 to 5 cm) was higher under pasture (26 tC ha−1) than under shade coffee (19 tC ha−1). Carbon content under sun coffee (23 tC ha−1) was not significantly different from either one. The most notable findings were (a) the generalized high percentages of water stable aggregates (75% of sites with water stable aggregates above 90%) which point to the relatively high stability of Andisols, and (b) the significant differences in aggregate carbon concentration, particularly at the summit where pasture incorporated almost 30% more carbon into aggregates compared to the coffee systems.