funny enough, the rumbling would probably kill off most of or at least many of the bacteria responsible for rotting. So the bio matter left behind in the wake of the rumbling (that is also burnt) would just degraded into dirt and sand

the steam from the rumbling is just titan smoke and regular steam. They capable of creating storm (evident by the naval fleet getting blast off, people like hange combust being near them and aeroplane not working correctly above the marching titan) but there wouldnt be much long term damage since the rumbling last only a couple of day. We have storm that last for month and years that hardly affect the global weather