Soil organic carbon (SOC) storage is a key ecosystem service in semi-arid regions that watershed management may modify. We compared ecosystem organic carbon stocks and apparent annual accumulation rates among nine watershed-management practices and two reference conditions in the Taham watershed, Iran. These comprised four biological, four mechanical and one bio-mechanical practice, plus a natural rangeland and an eroded gully. The experimental unit was the independent site: five sites at least 100 m apart per replicated practice, with within-site subsamples aggregated before analysis. Because each reference condition and the earth dam was a single site, inference was restricted to the eight replicated practices (one-way ANOVA, df 7 and 32); these three were reported descriptively. Soil was sampled at 0–25 and 25–50 cm; SOC stocks were calculated at fixed depth and on an equivalent-soil-mass (ESM) basis, with biomass and litter carbon measured separately. All values refer to organic carbon only: soil inorganic carbon, a potentially substantial pool in these calcareous soils, was not measured. Soil held 94.4–99.2% of ecosystem organic carbon. Among the eight replicated practices, 0–50 cm SOC stock ranged from 23.21 to 29.65 t ha⁻¹ and did not differ significantly (F(7,32) = 1.24; P = 0.31; ω² = 0.04), whereas biomass carbon (F = 30.55; P < 0.001) and litter carbon (F = 3.39; P = 0.008) did differ. Descriptively, the natural rangeland stored 59.94 and the eroded gully 19.26 t ha⁻¹; ESM correction raised their ratio from 3.11 to 3.93. Apparent annual SOC accumulation ranged from 0.198 to 0.415 t ha⁻¹ yr⁻¹. Watershed management was associated with greater SOC stocks than the degraded reference, but no practice was statistically superior in the soil pool and none reached the natural-rangeland stock after two to two and a half decades. Conserving remaining natural rangelands should therefore be a priority.