Ammonia-oxidizing microorganisms: key players in the promotion of plant growth

Nitrogen (N) is the most important mineral nutrient required by plants. Although some plants can directly utilize N, ammonium (NH4+) and nitrate (NO3-) are the only forms of N that are usable to most plants. Ammoniaoxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) are the key drivers that are responsible for the conversion of N into usable forms. Due to the importance of ammonia oxidizers (AOs), factors affecting their abundance and activity have been a major research focus over the years. This review focuses on the diversity of AOs, the factors that affect their abundance and activity in different soil types and the mechanism of nitrification. An improved understanding of the combinatorial effects of highly active AOs and methods for limiting nitrate loss from soils may enhance the management of nitrate in soils and improve plant yields.