Tree plantations in the high elevations of the tropics constitute a growing land use, but their effect on ecosystem processes and services is not well known. We examined changes in soil organic carbon (C) and water retention in a chronosequence of Pinus radiata stands planted in páramo grasslands in Cotopaxi province, Ecuador. Water retention at 10, 33, and 1,500 kPa declined with stand age, with soils in the oldest pine stands retaining 39%, 55%, and 63% less water than grassland soils at the three pressures tested. Soil organic C in the 0-10-cm depth also declined with stand age, from 5.0 kg m-2 in grasslands to 3.5 kg m-2 in 20-25-year-old pine stands (P < 0.001); at greater depth in the A horizon, C contents decreased from 2.8 to 1.2 kg m-2 (P = 0.047). There were no significant differences among age classes in the AC and C horizons (P = 0.15 and P = 0.34, respectively), where little or no weathering of the primary material has occurred. Inputs of C may be affected by the significantly higher carbon-nitrogen (C:N) ratio of the litter under older pine stands (P = 0.005), whereas outputs are influenced by substrate quality as well as soil environmental factors. Soil ratios at the 0-10 cm depth were significantly higher in grasslands and young pine stands (P < 0.001), whereas carbon-phosphorous (C:P) ratios at 0-10-cm depth followed a similar but not significant trend. However, there was no significant difference in short-term decomposition rates (P = 0.60) when the soils were incubated under uniform temperature and moisture conditions. In páramo ecosystems, where high soil moisture plays an important role in retarding decomposition and driving high C storage, the loss of water retention after afforestation may be the dominant factor in C loss. These results suggest that soil C buildup and water retention respond rapidly to changes in biota and need to be assessed with regard to implications for C sequestration and watershed management.