Whether that synthetic fertilizer will contribute to the death of the soil microorganisms or not will depend on the amount of organic matter in the soil. A goodly level of OM in the soil will help mitigate damage done while a soil with low levels of OM will see more damage done. If you add enough worm castings and other organic matter to that soil you can almost totally offest what harm that synthetic fertilizer might do.

If you send those worthless worm castings to me then I'll gladly dispose of them for you. I've placed them in my containers and spread them around various areas in my raised beds. Fortunately the plants grew mightily on their own and ignored the castings. In spite of those nasty castings I harvested two pound tomatoes and massive amounts of edible greens thus proving the castings were of no benefit. You'll do yourself a big favor by sending them this way immediatly. Us gardeners gotta stick together.

kimmsr wrote: "A goodly level of OM in the soil will help mitigate damage done while a soil with low levels of OM will see more damage done. If you add enough worm castings and other organic matter to that soil you can almost totally offest what harm that synthetic fertilizer might do."
So what you're saying, if I've got it right, is that with a high enough level of organic matter, the proportion of die-off of worm-casting micro-organisms either would be lower, or the recovery and proliferation of the micro-organisms would be greater. The influence of the synthetic fertilizer - in terms of harm to the micro-organism population - would be would be significantly reduced.

J.