Humic substances in plots under different land uses in Pernambuco

Soil organic matter (SOM) is one of the main indicators of the quality and stability of terrestrial ecosystems. Its recalcitrant fraction plays an essential role in improving soil structure and maintaining long-term carbon stability under different land uses and management systems. This study evaluated the stable fraction of SOM in areas belonging to the first network of permanent plots established for ecological studies in livestock systems within the Caatinga biome, Pernambuco, Brazil. The evaluated areas included native vegetation and pastures, with and without animal grazing. Soil samples were collected up to 20 cm depth, and the carbon content associated with humic substances was quantified. The results showed that, even in closed Caatinga areas with animal presence, carbon levels in the humin fraction remained high. This indicates that the dense vegetation canopy and litter layer promoted structural resilience, mitigating the negative effects of moderate grazing on SOM stability. These findings align with observations in other semi-arid agroecosystems, where controlled grazing does not necessarily lead to significant SOM degradation. In contrast, open Caatinga areas exhibited greater sensitivity to grazing. Sites without animals presented higher carbon contents than grazed areas, reflecting the importance of vegetation cover in protecting SOM. Reduced plant cover increased soil exposure and vulnerability to SOM losses, a pattern commonly reported in semi-arid regions. Overall, the study highlights that conservation practices, such as maintaining vegetation cover and excluding animals when necessary, are effective strategies for preserving SOM stability, enhancing soil resilience, and sustaining ecosystem functionality in the Caatinga biome.