Understanding the Distribution of Soil Carbon in Gabon, Central Africa

Soils are the largest reservoir of carbon in terrestrial ecosystems. The top meter is estimated to contain 1,500 gigatonnes of carbon, which is three times the amount held in the world’s vegetation. Despite the importance of these pools to the carbon cycle, research on soil carbon stocks is lacking in many regions worldwide. This is especially true in parts of Africa, such as the Congo Basin. This study centers on Gabon, one of six countries in the Congo Basin. Because of expected land-use changes in the country, it is important to know the contemporary amount of carbon held in the soils of Gabon. The objectives of this study were to 1) quantify Gabon’s soil carbon stocks down to two meters in depth and 2) examine how environmental controls influence soil carbon. On average, there were 162 megagrams (Mg) of carbon per hectare in the top 2 meters of soil in Gabon. The top meter had 110 Mg and the second meter had 52 Mg. The sizable subsoil C pool found in this study (over 30% of total soil carbon was beneath the 1st meter) supports the notion that soil carbon sampling needs to go deeper in order to avoid underestimating soil carbon stocks. A geospatial analysis of sub-Saharan Africa revealed that climate, topography, and soil texture influence soil carbon stocks. A similar analysis using soil carbon data specifically for Gabon yielded different results: only soil texture had a notable effect on belowground carbon storage. These regression analyses showed that estimating soil carbon stocks with environmental factors becomes more difficult on finer scales. Lastly, this study found that exponential decay curves have the potential to approximate subsoil carbon pools when surface soil carbon values are known. To conclude, this study provides the first national estimate of soil carbon stocks in Central Africa to a 2-meter depth.