Dictionary.net

• the circulation of nitrogen; nitrates from the soil are absorbed by plants which are eaten by animals that die and decay returning the nitrogen back to the soil

2006 - WordNet 3.0
By Princeton University

• the circulation of nitrogen; nitrates from the soil are absorbed by plants which are eaten by animals that die and decay returning the nitrogen back to the soil

2011 - English Dictionary Database
By DataStellar Co., Ltd