there's normally an idler pulley at the lower back of the engine that tensions the belt as seen in this illustration from nissan4u.com
note the PAS idler pulley Pushes to tension while non-PAS pulleys Pull to tension
here's a pic of a non-PAS idler pulley. the PAS pulley is slightly different with a vertical adjusting thread.
don't have a pic of mine but will take a pic soon
note the nut in the middle of the pulley axle is to lock it in position while the petruding bolt head shaft nearby is for moving the pulley up/down.