Soil carbon stocks vary predictably with altitude in tropical forests: Implications for soil carbon storage

Tropical forests are intimately linked to atmospheric CO2 levels through their significant capacity for uptake and storage of carbon (C) in biomass and soils. Increasing pressure of deforestation and forest degradation is begging the question as to what extent land use changes will affect C storage and release in tropical areas. Hitherto, many research efforts focused on aboveground C stocks in lowland tropical forests, but a considerable amount of C is stored in tropical soils as well. Some previous studies suggested that soil C storage increases with increasing altitude, while others found no relation with altitude. In this study, we addressed this controversy by quantifying soil organic C (SOC) stocks along an altitudinal gradient spanning a 3000 m altitude difference. In addition, we sampled soils in anthropogenic grasslands in proximity to forests at different altitudes to provide information on effects of land use change. Soil was sampled on 92 forest locations down to 100 cm depth in forest plots, and down to 30 cm in 13 grassland plots. We found that forest SOC stocks varied predictably with altitude in our study area, ranging between 4.8 and 19.4 kgC m(-2) and increasing by 5.1 kgC m(-2) per 1000 m increase in altitude. Soil properties (pH, bulk density, depth) and soil forming processes played an important role in this relationship with altitude. SOC stocks were not significantly different between forests and grasslands along the gradient in our study, due to a higher soil density in grasslands. When grassland SOC stocks were corrected for this difference in soil density, forest soils contained a significantly greater amount of C. In addition, while this difference was negligible at low altitudes, it tended to increase with increasing altitude. This study suggests that montane tropical forest soils consistently contain larger amounts of C compared to lowland tropical forests, and that conversion of forest to grasslands at higher altitudes might lead to larger soil C losses than previously expected. (C) 2013 Elsevier B.V. All rights reserved.