trapped organic particles, nitrates, phosphates, sulfer derivates (reaaaally bad) and other stuff released from the rotting particles being transformed into smaller components.
I would just do rubble and cheato, that is plenty for refugium. Also 4 inches isnt deep enough to create a DSB, you need 6 inches. 4 inches only collect crap without having an oxygen free zone where the nitrates can be transformed into nitrogen gas, which is what DSB is suppose to do. Personally, i think all those deeper sand beds are ticking time bomb because if disturbed you can easily crash your whole tank.