Decomposing animal remains and animal wastes also return organic nitrogen to the soil as ammonia. Many different kinds of decay microorganisms participate in ammonification. The nitrifying bacteria of the genus Nitrosomonas oxidize the ammonia to nitrites, and Nitrobacter oxidize the nitrites to nitrates. The nitrates can then be taken up again by the green plant. The cycle of fixation-decay-nitrification-fixation can proceed indefinitely without any nitrogen being returned to a gaseous state. But still another group of microorganisms, the denitrifying bacteria, can reduce nitrates all the way to molecular nitrogen. Denitrification occurs only in the absence of oxygen and is not common in well-cultivated soils.