Critical limits of soil quality and modeled soil organic carbon stocks in the Brazilian Atlantic Forest

The Brazilian Atlantic Forest (BAF) harbors outstanding biodiversity but is increasingly affected by land degradation driven by human activities. Conserving its soil is a pressing priority for sustaining its biodiversity and providing ecosystem services, which requires a data-driven assessment of its quality. However, critical soil quality limits linked to soil organic carbon (SOC) stocks remain poorly defined at the regional scale in the BAF. This study aimed to identify critical soil quality limits in remaining BAF fragments. We integrated process-based carbon modeling with a soil quality index (SQI) to derive quantitative limits of soil quality across remaining forest fragments. The SOC stock was modeled using the CENTURY model and calibrated and validated with empirical data. The SQIs were integrated using an additive approach, linear scoring, and a minimum data set, in which organic carbon was the dominant indicator strongly associated with SOC stock. Critical limits were established by linear regression between SOC stock and SQI values. SOC stocks provide representative estimates of soil carbon compartments in the biome. SOC stock in BAF remaining forest fragments ranges from 20 to 110 Mg C ha −1 . The lower critical limit for SOC stock is below 40.73 Mg C ha −1 . Approximately 85% of soils in the BAF fragments are classified as having low to moderate soil quality (40.73–83.36 Mg C ha −1 ), occurring in patches of 5–50 ha distributed throughout the biome. These limits provide a practical benchmark for monitoring soil quality and supporting conservation and restoration strategies in this ecologically important and continentally extensive tropical forest. • Modeled soil organic carbon stocks reveal critical limits of soil quality. • 40.73 Mg C ha −1 of soil organic carbon stocks is a lower limit of soil quality. • Most forest fragment soils exhibit moderate to very low levels of inherent soil quality. • Soil organic carbon stocks vary widely across the Brazilian Atlantic Forest. • Process-based modeling enables regional-scale assessment of soil carbon dynamics.