Integrated crop-livestock-forestry (ICLF) systems explore synergistic interactions between soil, plant, and animals, maximizing land-use efficiency and sustainability. However, belowground dynamics under ICLF have not been investigated deeply, particularly the role of incorporating dead root material, a forefront strategy for releasing nutrients and storing carbon. To better understand belowground interactions, we conducted a 21-month assessment of fine-root growth and decomposition in an ICLF system, starting when Eucalyptus urograndis trees were three years old. Eucalyptus rows were spaced 15 m apart and integrated with annual crops and pasture. Distances of 1.9, 4.3, and 7.5 m from the trees were evaluated under two successional periods: (i) annual crop, when corn was interspaced with palisade grass (Urochloa brizantha); and (ii) pasture, when palisade grass was grazed. We used the minirhizotron technique to track fine-root production and decomposition down to a depth of 70 cm, capturing 2400 images. Root longevity was estimated per root diameter class (0-0.5-, 0.5–1.0-, and 1.0–2.0-mm) and phenotypical groups (e.g., grasses [corn + palisade grass] and Eucalyptus). Our data showed that root decomposition rate and necromass inputs into the soil were reduced at the closest distance from the Eucalyptus rows (i.e., 1.9 m). The incorporation of decomposed roots was higher in the topsoil (0–28 cm) and declined with increasing soil depths. The total decomposed root incorporation was 101 m m⁻² of soil image for 7.5 and 4.3 m inter-row positions, almost twice as high as the recorded at 1.9 m (54 m m⁻²) from the trees. Daily root decomposition rates increased during the last rainy season, benefited from numerous dead corn roots, and facilitated by higher soil moisture and temperature. Grasses and Eucalyptus roots at 7.5 m from the tree rows had shorter longevity than those at 1.9 m, remaining 88 and 152 days less, respectively. Root diameter influenced the decomposition rate as thicker roots (diameter between 1.0 and 2.0 mm) of grasses and Eucalyptus stood in the soil for 243 and 261 days longer than the finest roots (diameter <0.5 mm). Our results highlight that root necromass accretion and decomposition are heterogeneous in ICLF systems. Furthermore, 3-to-5-year-old Eucalyptus trees drive the interactions, creating microclimate conditions that impair corn and palisade grass root production and reduce root turnover close to the trees. These findings provide a scientific base for managing the ICLF system (spatial and temporal arrangements) and developing models of soil carbon addition via roots in such complex and heterogeneous systems.