Potential of different Urochloa grass hybrids to enhance soil organic carbon stocks in a Mollisol of Valle del Cauca, Colombia

Increasing soil organic carbon (SOC) stocks in grasslands is a promising strategy for offsetting greenhouse gas (GHG) emissions from cattle ranching. Previous studies have shown that certain Urochloa grasses develop an extensive deep rooting system that contributes to higher SOC accumulation in acidic soils with low fertility and aluminum toxicity. At the Alliance Bioversity and CIAT, genetically diverse a Urochloa grasses are utilized in breeding programs aimed at developing hybrids with improved adaptation to biotic (e.g., spittlebug) and abiotic (acidic soils, aluminum toxicity, drought, and waterlogging) stresses, enhanced nutritional quality, and reduced nitrous oxide from soil and methane from cattle. However, the capacity for SOC accumulation from their deep rooting ability has not yet been considered as a target trait within these breeding programs. Since these improved grasses are pivotal for sustainable livestock intensification, this study aims to evaluate the differences in the contribution of nine Urochloa genotypes to increase SOC stocks in a Mollisol of Valle del Cauca, Colombia. The field trial was established in 2016 with the planting of nine Urochloa genotypes: four cultivars/accessions (U. brizantha  cv. Marandu,  U. humidicola  cv. Tully,  U. humidicola  CIAT 26146, and U. humidicola  CIAT 16888), and five hybrids (cv. Mulato II, Uh08 675, Uh08 1149, Uh 72, and Uh 91) as well as a control treatment with bare soil. Each genotype was planted in 20 x 20 m plots with three replicates, and three control plots were maintained as bare soil. Six years after establishment (early 2023), soil samples were collected from each plot at four depths (0-10, 10-20, 20-60, 60-100 cm) to estimate SOC stocks using the values of SOC concentration and bulk density. We observed that SOC concentrations decreased with depth. In grass-covered plots, the average SOC concentration was 2.17% in the surface layer (0–10 cm) and 0.25% in the deeper layer (60–100 cm). In contrast, in bare soil plots, the average SOC concentration was 1.7% in the surface layer and 0.11% in the deeper layer. Results on the stocks of SOC (0-100 cm soil depth), based on equivalent soil mass, indicated that after 6 years of establishment, plots planted with Urochloa grasses showed an average value of 127.4 Mg C ha⁻¹, which is 47% more compared to 86.4 Mg C ha⁻¹ found in bare soil. We observed a clear trend with Urochloa hybrids showing a greater ability to increase SOC stocks, in the following order: Uh 91 > Uh 72 > Uh08 675 > cv. Marandu > Uh08 1149 > cv. Mulato > CIAT 26146 > CIAT 16888 > cv. Tully. For example, the Urochloa hybrid of Uh 91 had a SOC stock of 155.66 Mg C ha⁻¹, while cv. Tully had 97.17 Mg C ha⁻¹. These results highlight the potential of Urochloa grass hybrids to contribute towards mitigation of climate change. On-going Urochloa grass breeding programs should consider incorporating the trait of SOC accumulation ability together with other desirable traits of stress resistance, improved nutritional quality, and reduced GHG emissions.