By definition, removing any product from the land it was grown on leads to nutrient degradation. Unless you have a closed loop (you eat it, poop it, and return that nightsoil to the garden) you will need fertilizer. That's why most early agriculture grew on floodplains, using nutrients washed down from upriver.
There are carbon-free sources of fertilizer. Specifically, scientists brought to scale ammonia through electrolysis. There is no cure for phosphorus deplation, though. We can't sythesize it.
The reality is, without fossil fuels, feeding a world over 2 billion people while maintaining natural ecosystems remains impossible. That is the problem we ned to solve.