Abstract
Transformations of natural ecosystems in tropical regions, which are usually covered by high-biomass forests, contribute to increased atmospheric CO2. Much of the carbon in forest ecosystems is stored in the soil. This study estimates soil carbon stock in a dense forest in central Amazonia from sets of soil samples collected in three topographic positions (plateau, slope and valley bottom). Soil organic matter (SOM) was fractionated by density and particle size, thus obtaining the free light fraction (FLF), intra-aggregated light fraction (IALF), sand fraction (F-sand), clay fraction (F-clay) and silt fraction (F-silt). Soil organic carbon (SOC) stocks on the plateaus (Oxisol), slopes (Ultisol) and valley bottoms (Spodosol) were 98.4 ± 7.8 Mg·ha-1, 72.6 ± 5.4 Mg·ha-1 and 81.4 ± 8.9 Mg·ha-1, respectively. Distribution of carbon in soil fractions was: 112.6 ± 15 Mg·ha-1 (FLF), 2.5 ± 0 Mg·ha-1 (ILAF), 40.5 ± 1.5 Mg·ha-1 (F-silt), 68.5 ± 4.2 Mg·ha-1 (F-clay) and 28.3 ± 1.4 Mg·ha-1 (F-sand), totaling 252.4 ± 22.1 Mg·ha-1 of carbon. Carbon is largely in labile form and near the soil surface, making it liable to release from deforestation or from climate change. Spodosols are more susceptible to soil carbon losses, demonstrating the need to preserve forested areas close to Amazonian rivers and streams.
Highlights
Amazon forest stocks large quantities of carbon both in plant biomass (Nogueira et al, 2015) and in soil (Fearnside, 2016; Marques et al, 2016)
Carbon is largely in labile form and near the soil surface, making it liable to release from deforestation or from climate change
Most soil carbon present in surface layers is associated with the free light fraction (FLF), ranging from 26% to 57% on the plateau (Figure 2(a)), from 20% to 80%, on the slope (Figure 2(b)) and 49% to 66% in the valley bottom (Figure 2(c))
Summary
Amazon forest stocks large quantities of carbon both in plant biomass (Nogueira et al, 2015) and in soil (Fearnside, 2016; Marques et al, 2016). Soil carbon under Amazonian forests has important roles in global change, making information on the forms and depths of these stocks of considerable interest in efforts to quantify emissions when deforestation converts these forests into pasture or other land uses. Information on these stocks is important in evaluating the risk of this carbon being released to the atmosphere under projected climate scenarios. Maintaining carbon stocks in tropical forests in Amazonia and elsewhere has increasingly become a justification for governments to take measures to keep these forests standing rather than allowing them to be deforested. Forests cleared for cattle pasture, which is the predominant land use in deforested areas in Brazilian Amazonia, lose substantial amounts of soil carbon under typical management (Fearnside and Barbosa, 1998)
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