The soil would definitely help to keep the flames contained. Sure, it might take Gaja longer to claw his way out of it once he's reborn, but that's something that will only affect him. It this lush riverbank were to catch fire... So many living things would be harmed, directly or indirectly. It's not a risk Gaja wants to take.

Though his crater is probably big enough to keep the fire contained, sparks still fly.

No, this creature will be well within his rights to do whatever he feels is necessary to protect his lands. Gaja has no argument. In fact, he simply nods his head. "Thank you."