Carbon accumulation in soil layers under degraded, intact and planted forest cover types in tropical semi-deciduous and moist evergreen forests

Tropical forest soils can contribute significantly to mitigating climate change by sequestering and storing carbon in their layers. However, in West Africa, knowledge of how much carbon is stored in deeper soil layers of various forest cover types and protected from further release into the atmosphere remains scanty. We quantified the carbon (C) and nitrogen (N) contents of tropical soils in Ghana at 0-20 cm and 20-50 cm, by comparing a degraded forest and an intact forest in the semi-deciduous forest zone, and intact forest, degraded forest, and agroforestry plantations in the moist evergreen forest zone. In semi-deciduous forests, C concentration was significantly higher for intact forest than degraded forest, but total C content of soils from the intact forest was not greater than the degraded forest due to compensating differences in bulk density. C content differed by depth for the two forests, with values at 0-20 cm of 48.1 vs 38.4 Mg ha(-1) and at 20-50 cm of 20.6 vs 26.5 Mg ha(-1), for degraded and intact forests respectively. In moist evergreen forests, soil C concentrations were similar between intact forest, degraded forest and plantations, yet, differed between the depths. Among the three, soils under plantations had the highest C content due to higher bulk density. For N, differences among cover types and soil depths followed similar patterns as those for C. Our results suggest the potential for formerly disturbed or degraded forests to gain more C lies primarily in live forest biomass, not in soil, especially if forests have maintained vegetative cover of some type since disturbance. The potentially large capacity for deeper soil layers to store C, and their reduced susceptibility to forest disturbance makes them an important soil carbon pool to further quantify and preserve.