The adoption of management practices to enhance carbon (C) sequestration in pastureland, such as mixed pastures, is crucial for sustainable agriculture, particularly in sandy soils, which have inherently low C storage capacity. The objective of this study was to evaluate how nitrogen (N) fertilizer application or legume integration into Marandu palisade grass (Urochloa brizantha cv. Marandu) pastures affect soil aggregate stability, organic matter fractions, and microbial activity in the surface layer of a sandy soil after eight years of management. This study assessed three pasture types: (1) mixed Marandu palisade grass and ovalifolium (Grona heterocarpa subsp. ovalifolia) legume (GRASS+LEGUME pasture); (2) Marandu palisade grass fertilized with 150 kg N ha-1 yr-1 (GRASS+N pasture); and (3) Marandu palisade grass without N fertilizer application (GRASS pasture) compared with native vegetation. Soil aggregate classes (4-2 mm, 2-0.250 mm, and 0.250-0.053 mm) were analyzed for total, particulate, and mineral-associated C content, as well as C mineralization rate and enzyme activity. Higher C and N contents were recorded for the 4-2 mm and 0.250-0.053 mm aggregates. All pasture types exhibited increased C, N, and organic matter in microaggregates compared to native vegetation. The GRASS+LEGUME pasture promoted the formation of larger and more stable aggregates and enhanced microbial necromass and enzymatic activity relative to the other treatments. In contrast, native vegetation exhibited a higher C mineralization rate and microbial necromass C. These findings highlight the role of grass/legume mixed pastures in enhancing C storage and microbial activity in sandy soils, where accumulation predominantly occurs within macroaggregates and microaggregates. Thus, mixed pastures constitute a sustainable alternative for preserving soil organic C and mitigating climate change.