a DSB works for Nitrates, as it offers an anerobic enviroment(at the correct depth and grain size; usually 3-6"+ in depth) for the Denitrifying bacteria to survive.
DSB if done incorrectly can be a huge headache though, and can cause more harm than good, never disturb a settled DSB, and always choose the correct grain size and depth. Other wise it will become a nitrate/detritus facatory...