A fire turns dead trees/decaying trees into ash which nutrients return to the soil.

After a wild fire when trees burn out and all there is left of them are the ashes. This ash has a high alkaline level i.e. high pH level whereas the soil of the forest is acidic. The alkaline ashes basically neutralize the soil of the forest.

The chemical change that occurs due to the fire and the risen temperature of soil due to fire, gives a push to the increased microbial activity. This leads to the increased rate of decomposition which makes the soil all the more nutritious for new plants to grow.