Long-term crop diversification increases soil carbon stock and soil health under cotton production in sandy soil of Brazilian savannah

Sandy soils are inherently limited in fertility, water retention and aggregation, and are characterized by a low potential for carbon (C) accumulation. Consequently, promoting management practices that enhance soil health in these soils remains a major challenge. Due to C is a central indicator of soil health, identifying management strategies that increase soil C stocks is essential for sustainable agricultural systems. Therefore, this study aimed to evaluate the relationship between soil C stocks and soil health in cotton production systems on sandy soils in the Brazilian savannah. A 12-year field experiment was conducted under five contrasting cotton-based systems: (i) monoculture and (ii) millet–cotton under conventional tillage, (iii) cereal, (iv) legume, and (v) grass-based rotations under no-tillage. Soil C stocks and soil health were assessed using 14 indicators representing biological, physical, and chemical soil properties. Relationships between soil C stocks and overall soil health, as well as individual component indices, were evaluated using linear regression analyses. Grass- and cereal-based rotations under no-tillage increased soil C stocks, which were positively associated with overall soil health and the biological index (R² = 0.33 and 0.72, respectively; p < 0.05). In contrast, physical and chemical indices showed no significant relationships with soil C stocks (R² = 0.00 and 0.02, respectively; p > 0.05). Overall, long-term no-tillage combined with diversified grass-based rotations was associated with higher soil C stocks and an improved soil health, particularly through biological processes, in sandy soils under cotton production in the Brazilian savannah.