Repeated forest fires drive soil carbon losses that far exceed pyrogenic carbon inputs in southern Amazonia

Recurrent forest fires are expanding across the Amazon, producing pyrogenic carbon (PyC) while depleting soil C. To assess how fire frequency shapes this trade-off, we analyzed a 2-m soil profile in 50-ha burn plots within an Amazon–Cerrado transition forest in Mato Grosso, Brazil. In 2019, we sampled an unburned control and plots burned annually (B1yr: 2004–2010, except 2008) or triennially (B3yr: 2004, 2007, 2010). Bulk soil and PyC (via hydrogen pyrolysis) were analyzed for total C and δ¹³C in 5-cm intervals to 30 cm, plus 30–50, 50–100, 100–150, and 150–200 cm; stocks were calculated using bulk density. Total C was lower in burned plots down to 50 cm, whereas PyC was >2 times higher at 0–5 cm, with modest increases at 5–10 cm. PyC comprised >4% of total C in the upper 10 cm of burned plots versus ~2% in the control, and increased with depth across treatments, exceeding 12% below 1 m. Down to 2 m, total C stocks were 127.3, 104.0 and 112.1 Mg ha^-1 (control, B1yr and B3yr), while PyC stocks were 8.7, 9.1 and 9.5 Mg ha⁻¹. Thus, fires added ≤0.8 Mg PyC ha^-1 but caused >15 Mg C ha^-1 loss, mostly in the top 30 cm, greatest under B1yr. PyC/C correlated negatively with total C and positively with δ¹³C (R²>0.7, p<0.01). In conclusion, frequent Amazon forest fires incorporate PyC but remain far from offsetting soil C losses, highlighting priorities for fire prevention and long-term post-fire soil monitoring to determine soil C recovery trajectories.