When you say carbon, you mean activated carbon right? In this case, activated carbon provides no bio-accessible carbon.
Activated carbon will absorb organic products in your water. Your fertilizers are inorganic, so they won't be affected.
That being said, there is a debate on adding carbon to planted tanks as some think that it takes things out of the water that plants would be able to use. Others thing that's a load of hogwash. Read up on the debate and decide what's right for you.