You need leaf mould to produce a leaf mulch, not bark. Leaf mould will add to the nutrients whereas bark will only help to keep moisture in and make it easier to keep weeds down.
What sort of conifers are suffering from this browning - aphids cause browning on cypresses, frequently starting on the lower parts of the trees. This is an extract from the RHS website "Cypress aphid
Cypress aphid (Cinara cupressivora) is a relatively common cause of brown patches. A recent Link to information on the cypress aphid research project found cypress aphid to be associated with half of the cases of brown patches investigated.
Damage caused by cypress aphid develops in late spring and summer. It is found most often at the base of the hedge, but can develop at any height.Large greyish greenfly are sometimes found, but the browning often develops long after the aphids have left the foliage. Clues are left behind, including cast aphid skins and a black fungal growth (sooty mould) that grows on the sugary honeydew excreted by the pest."