Your argument is simplistic and ignores several important factors. Grass merely covers soil, protecting it against the kind of disturbance which releases sequestered CO2. If, as part of a "...well managed lawn..." you core aerate, well, then you release a significant amount of CO2 from the soil. Then, if you mow your grass, as I'm certain you do, this usually involves a CO2, CO and ozone  belching mower, employed many times during the growing season.Even more CO2 is produced when a lawn service delivers its services by truck and also employs CO2 producing blowers and line trimmers.Then, if the cut grass is hauled to the land-fill, it produces the potent greenhouse gas, methane, in the landfill. Nitrogen gas based lawn fertilizers are often employed resulting in still more emissions in the extraction, transportation and application process.Trees are the most stable form of sequestration short of formerly millions of years buried petroleum products.Lawns in urban areas, not so much!