Deep soil carbon stock, origin, and root interaction in a tropical integrated crop–livestock system

Integrated crop–livestock systems (ICLS) with fast-growing tree plantations are an environmentally sound approach for agricultural production. Soil carbon stocks can be increased under these systems, but there is no information on the relative contributions of the pasture and the trees in building soil C in tropical regions. This study aimed to investigate C storage, origin, and roots sequestration in the soil profile (1.0 m) in a pasture-forest system. The study was conducted in a 6-year-old ICLS in which Eucalyptus (C3) were introduced into a palisade grass pasture [Urochloa brizantha (Hochst. Ex A. Rich.) R. Webster ‘Marandu’, C4]. Samples were collected at four locations perpendicular to the Eucalyptus planting line: 0 (planting line), 2.0, 4.0, and 6.0 m (middle of the plot). A monoculture palisade grass pasture was included for comparison. The insertion of Eucalyptus in the pasture modified the quantity and distribution of grass above and belowground. Forage root and shoot biomass were lower in the integrated system compared to the monoculture pasture, mainly near Eucalyptus. However, plant C sequestration in the ICLS was 68% higher than in the monoculture pasture, due to the C accumulated by aboveground trees biomass. Overall, monoculture pasture had higher soil C4–C stocks at the 0–0.20 m depth, while integrated system with Eucalyptus stored more C3–C in deeper soil layers (0.60–1.0 m) near Eucalyptus. Soil C stock increased in both systems, but the contribution of Eucalyptus was deeper in the soil profile, while the grass contribution was greater in the upper 0.20 m soil layers.