Soil Carbon is Decreasing under “Undisturbed” Amazonian Forest

Warming climate can cause release of carbon stocks in soils, but direct observations in tropical soils have been lacking. A unique data set from a study near Manaus, Brazil, allows comparison of samples taken before and after a ∼28‐yr period in 176 plots in undisturbed forest (i.e., intact forest with no visible sign of modern human action and >100 m from a forest edge, with 98% of the plots being >300 m from an edge). The data indicate a significant loss of carbon in the top 20 cm of soil (2.98 MgC ha ‐1 over 28 yr, an average of 0.11 Mg ha ‐1 yr ‐1 , or 0.3828% yr ‐1 of the carbon stock). Carbon emissions would be substantial if the pattern for the top 20 cm at this location holds throughout Amazonia, and the implications are huge if the same pattern holds for the deeper soil layers. Release of soil carbon can contribute to a positive feedback, where emissions cause greater warming that further augments the emissions.