The TREK Long Bow has 4 arrow types: Piercing, Explosive, EMP, and Smoke. The last three have effects similar to their corresponding grenade types. You can change the arrow type with the reload key (default 'r').
The bow comes with infinite piercing arrows and two arrows for each of the remaining three types. They regenerate over time and can't be refilled with the utility gun. Also, the effect of the infinite ammo can be transferred to another primary/adrenaline weapon when you picked up bow.