Deciphering the impact of active aluminum and iron on soil organic carbon stabilization in volcanic and non-volcanic paddy soils of the central highlands of Madagascar

Understanding the controlling factors of soil organic carbon (SOC) content in tropical rice paddy fields is crucial for improving soil fertility and carbon sequestration. In tropical soil, SOC is primarily stabilized by interactions with active aluminum (Al) and iron (Fe), yet the relative importance of these elements remains unclear. This study aimed to (1) determine key factors of SOC content in paddy fields, and (2) examine the relationship between SOC and Al and Fe, specifically oxalate-extractable Al (Al_ox) and Fe (Fe_ox). We analyzed 306 rice paddy soils (0–15 cm) in six communes in the central highlands of Madagascar, three influenced by volcanic activity. Structural equation modeling revealed that SOC content was significantly and positively correlated with Al_ox + Fe_ox content across volcanic and non-volcanic soils. Al_ox was the primary SOC-controlling factor across all soils, while Fe_ox played a significant role when its relative abundance to Al_ox increased, particularly in non-volcanic soils. This suggests that SOC stabilization depends on the relative proportions of Al_ox and Fe_ox. Additionally, SOC content decreased with increasing soil pH. Overall, these findings underscore the importance of incorporating site-specific geochemical characteristics—particularly the balance between active Al and Fe—into the development of accurate SOC sequestration models for tropical paddy soils.