Soil organic carbon and pH dynamics across land use types and altitudinal gradients in semi-arid Northern Tanzania

Despite growing evidence that land-use change and altitudinal variation influence soil organic carbon (SOC) and soil pH, integrated assessments examining their combined effects across multiple land-use systems and environmental gradients remain limited in semi-arid landscapes of northern Tanzania. This study assessed SOC (%) and pH across four land use types i.e., conserved land, cropping land, grazing land, and abandoned land in Arusha and Monduli Districts, northern Tanzania, encompassing lowland (< 1400 m), midland (1400–1600 m), and highland (> 1600 m) zones altitude categories. A total of 586 soil samples were collected across 2 Soil depth classes and analyzed for SOC (%) and pH. Results demonstrated that land use type was the dominant driver of both SOC and pH variability (one-way ANOVA: FSOC = 107.19, P = 2.95 × 10− 55; FpH = 48.68, P = 4.43 × 10− 28). Conserved land recorded the highest mean SOC (2.15 ± 0.35%) and lowest mean pH (6.48 ± 0.38), while abandoned land showed the lowest SOC (0.84 ± 0.20%) and highest pH (7.27 ± 0.42). Monduli District consistently outperformed Arusha District across all land use types, with overall mean SOC of 1.87% versus 1.18%. A strong negative correlation was observed between SOC and pH (Pearson r = -0.637; Spearman ρ = -0.653; P < 0.001), and altitude positively predicted SOC (r = + 0.59). Bonferroni-corrected post-hoc comparisons confirmed statistically significant differences in SOC between all six pairwise land use combinations. These findings show the critical role of land conservation, highland ecosystem protection, and active restoration of degraded soils in sustaining carbon stocks and soil health across semi-arid landscapes of northern Tanzania.