Temporal changes in soil organic carbon stratification across contrasting pedoclimatic environments in Brazil

Soil organic carbon (SOC) is typically stratified, with higher concentrations in the topsoil than in subsoil layers due to environmental gradients, land cover, and soil properties. Thus, our study aimed to assess to what extent soil type may drive SOC stratification. Our experimental sites are located across three different regions of Brazil, spanning from tropical to subtropical climates (Bahia - R1, Espírito Santo - R2, and São Paulo - R3). Soil samples were collected between 1984–1993 and resampled in 2001 and 2010 at two depths (0–15 cm and 15–30 cm). Carbon stocks (Mg ha⁻¹) were calculated by multiplying carbon concentration by bulk density. Three soil types (Oxisols, Ultisols, and Entisols) were identified in each region. We calculated the carbon stratification ratio (CSR) as the ratio of SOC stocks in the superficial and the adjacent soil layer in each site. Our findings showed that, in most cases, CSR values were greater than 1, indicating higher SOC stocks in the topsoil. However, in the earliest sampling period (1984–1993), Entisol and Ultisol in R3 presented CSR values as low as 0.49 and 0.36, respectively. Over time, CSR increased in several sites, suggesting recovery of surface SOC stocks, whereas declines were also observed in specific soil types and regions, reflecting redistribution or loss of carbon from the surface layer. These contrasting temporal trends highlight the combined influence of soil type, site conditions, and management history on SOC vertical distribution.