Grassland soils are a reservoir of organic and inorganic carbon. Regionally, grasslands are CO₂ sources or sinks depending on management, soil organic carbon (SOC) concentration and climate. Land management changes (LMC) affect SOC sequestration rate, duration and C-store at steady state. A common hypothesis is that higher grassland productivity increases SOC sequestration to a higher steady state. High SOC-sequestration rates occur for 5 to 10 yr after LMC, but continue slowly up to 50 yr. Permanent grasslands are at steady state for CO₂ exchange. The most significant LMC for SOC accumulation is conversion from cropland to grassland where C-inputs from perennial roots and residues are higher than annual crops. Residue- and root-C inputs for Alberta perennial forage systems were 2.3 to 3 times greater, than barley silage. SOC sequestration rates for improved pastures on degraded soils in South-Eastern US were up to 1.4 Mg SOC ha⁻¹ yr⁻¹. Applications of fertilizer-N, manure and using legumes increased SOC through higher root-C inputs, but not always permanently, due to the increased degradable-C fraction. Manure-C application effectively replaces surface soil-C on Quebec dairy farms. But, micro-meteorological studies on net ecosystem exchange (NEE) on old pastures in Pennsylvania, and Alberta (>6% soil organic matter), indicated that when harvested forage-C was removed from NEE, net Biome-C loss occurred in 90% of the years. Increasing productivity through N-application in Pennsylvania resulted in lost SOC, but a 5-species forage mixture increased productivity and sequestered more SOC than a 2-species mixture. Specific LMC in this grassland was less important to sequestration rate than initial SOC content. Quebec studies show that grass and legume species respond differently to increased atmospheric temperature and CO₂ concentration. Elevated temperature and CO₂ caused a higher root C: N ratio, resulting in a lower in vitro degradation rate, indicating that reduced SOC-degradation rates under future climates might be possible. How a concomitant change in species mixture affects SOC and what effects changing moisture and temperature regimens have on potential SOC-sequestration at a regional level can only be speculated.