Bacterial decay anaerobic conditions

Anaerobic digestion, like pyrolysis, occurs in the absence of air. But, the decomposition is caused by bacterial action rather than high temperatures. This process takes place in most biological materials, but it is accelerated by warm, wet and airless conditions. It occurs naturally in decaying vegetation in ponds, producing the type of marsh gas that can catch fire. 

To complete the cycle, another bacterial bug that flourishes under anaerobic (i.e., oxygenless) conditions feeds on nitrogen-containing compounds of decay and returns nitrogen gas, N2, to the air. 

---