Imagining 42 tons of yard waste piling up outside every year, seems a little off the mark. 500 sqm X 85 g/yr is 42.5 kg/yr Also, 190 gpd / 1000 gal = 0.19 kga

Imagining 42 tons of yard waste piling up outside every year, seems a little off the mark. 500 sqm X 85 g/yr is 42.5 kg/yr Also, 190 gpd / 1000 gal = 0.19 kgal per day 0.004 tons per kgal per day x 0.19 x 365 days = .277 tons per year Therefore 0.0425 tons sequestered, 0.277 tons produced by irrigation. The ratio of carbon sequestered to carbon produced on irrigation is the same as your original calculations. But I think the major source of carbon won't be in irrigation, but in the maintenance, mowing the lawn, transportation for the landscaper, transporting pesticides and fertilizers, etc.