Grazing lands occupy a substantial area around the world and could serve as carbon sink if soil organic carbon sequestration potential is increased. In subtropical climates, grazing lands are more commonly situated on low productive sandy soil profiles with poor soil attributes for carbon sequestration. Soil acidity limits root and beneficial microbial interactions, both critical for increasing soil organic carbon (SOC) sequestration. Adapted grazing and forage (vegetation) management could help to improve root-microbe interactions and SOC stocks in the soil profile. The objective of this study was to quantify SOC density in the soil profile along with aboveground and belowground biomass pools, as influenced by long-term cattle grazing of different forage systems managed with or without inorganic nitrogen (iN) fertilization. The following treatments were implemented continuously for the last 30 years. Warm season perennial grass (C4, Cynodon dactylon) pastures were either over-sown with C3 ryegrass (Lolium multiforum; C4-C3G) or clover (Trifolium sp.; C4-C3L), in order to extend the grazing season into cool season months. Grass-based pastures (C4-C3G) were fertilized, called inorganic treatment (iN), while clover systems (C4-C3L called organic N) did not receive iN fertilization (oN). The two forage systems were further split to compare high and low grazing pressure (HG and LG). Forage, root and microbial biomass and SOC density were estimated from soil cores sampled separately during C4 and C3 grazing seasons. Results indicated that SOC density (Mg ha−1) at soil profile level (0–60 cm) was higher under low grazing pressure compared to high, and in iN fertilized compared to oN treatments. However, subsoil (20–60 cm) SOC stocks were significantly higher under oN compared to iN treatments. Forage biomass and root biomass were higher under iN and HG treatments, but arbuscular mycorrhizae (AM) biomass was decreased. There were no consistent trends for other microbial biomass pools in response to GP or N treatments. It can be concluded that higher SOC density in the surface and subsoil in response to GP and N management were largely due to differences in microbial biomass, but not plant biomass productivity. Among the belowground biomass pools, AM was most responsive to experimental factors and their interaction effects, and thus could be a reliable metric to optimize grazing and achieve stewardship goals.