Restoring soil climate-regulation capacity in the Amazon biome through biochar addition: A 19-year assessment

Human-induced changes in the Amazon biome have impaired several soil ecosystem services, including climate regulation. A promising strategy to restore this function is the use of biochar, a charred material rich in recalcitrant carbon (C) compounds that can enhance long-term C sequestration. However, long-term assessments of soil C dynamics in the Amazon biome remain scarce, particularly under tropical conditions where organic matter turnover is accelerated, and C accumulation is more challenging. This study evaluated the long-term effects of biochar on soil C pools 19 years after its application. Soil samples were collected from a long-term field experiment located within the Amazon biome, comprising four biochar treatments: 0 (CTR), 40 (T40), 80 (T80), and 120 (T120) Mg ha^-1. Additionally, soils from an adjacent native forest (NF) were sampled as a reference system. Our results showed that biochar application at rates of 80 and 120 Mg ha^-1 effectively restored total soil C stocks to levels comparable to those observed in the native forest. However, only the T120 treatment significantly increased soil C stocks compared to CTR (n = 4, p ≤ 0.05), resulting in a 45% increase. The main gains associated with biochar addition were observed in the particulate organic C (POC) fraction, with T120 showing the largest increases in both POC and mineral-associated organic C. These findings indicate that, even under tropical conditions, biochar application at higher rates is an effective long-term strategy to restore soil C stocks and recover its capacity to regulate climate in the Amazon biome.