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Forests in seasonally dry areas of eastern Amazonia near Paragominas, Pará, Brazil, maintain an evergreen forest canopy through an extended dry season by taking up soil water through deep (>1 m) roots. Belowground allocation of C in these deep‐rooting forests is very large (1900 g C m⁻² yr⁻¹) relative to litterfall (460 g C m⁻² yr⁻¹). The presence of live roots drives an active carbon cycle deeper than l m in the soil. Although bulk C concentrations and ¹⁴C contents of soil organic matter at >l‐m depths are low, estimates of turnover from fine‐root inputs, CO₂ production, and the ¹⁴C content of CO₂ produced at depth show that up to 15% of the carbon inventory in the deep soil has turnover times of decades or less. Thus the amount of fast‐cycling soil carbon between 1 and 8‐m depths (2–3 kg C m⁻², out of 17–18 kg C m⁻²) is significant compared to the amount present in the upper meter of soil (3–4 kg C m⁻² …