Because of the nature of the process, predominately aerobic composting usually leads to the loss of at least some nitrogen. The loss is associated with high temperatures, low moisture content and eventual alkaline conditions that are attained during the process. The presence of excess nitrogen in the form of ammonium carbonate or ammonia can be traced to the microbial metabolism of protein or other sources of nitrogen. If the C/N ratio is too low, the energy source (carbon or carbohydrates) may be less than that required for converting available nitrogen into microbial cells. In such an event the organisms make full use of available carbon and the excess nitrogen is eliminated as ammonia. If excess nitrogen in a decomposing mass is too great, ammonia may be formed in amounts sufficient to be toxic to the microbial population and cause air pollution as well.
Ammonia losses can be minimized when compost ingredients are adequately mixed in proper proportions and all other factors discussed above are taken into account. Wastes which are high in nitrogen such as poultry manure generally cannot be sufficiently and economically amended with carbon (bulking agents) to avoid ammonia losses. The exhaust air from such compost piles must be scrubbed with water to prevent a serious air pollution problem.