The nut is a normal thread, simply stuck on maybe with some rust or was tightened with an impact driver or both. See if you can soak the nut and the washer with WD-40 and if available, try to use an impact driver to unscrew it. By the way, if yours is John Deere D130 22-HP V-Twin Hydrostatic 42" Riding Lawn Mower , here is the direct link to its owners manual:
formatting link
Scroll down to Service Mower -> Servicing Mower Blades (
formatting link
)