The roots from dead plants just enrich the soil of an outdoor vegetable garden, it's valuable organic matter added throughout the soil, not sure why it would be any different in soil used indoors. I would break it up so your next plant can send it's roots down easy, and like another person said, an enzyme product might help, though honestly I'm on the fence about them. Any organic matter in the soil will "take away" some nitrogen, it's just part of the composting process, so I guess it's good news that we feed these plants more than enough nitrogen to compensate anyway! I grow in coco now, and I plan on reusing it for the first time after my next grow to see if I can save some money. Right now it just goes into my outdoor garden and improves the soil.