The aim of this study was to compare the relative contribution of ammonia-oxidizing archaea (AOA) and bacteria (AOB) to nitrification during agricultural waste composting. The AOA and AOB amoA gene abundance and composition were determined by quantitative PCR and denaturing gradient gel electrophoresis (DGGE), respectively. The results showed that the archaeal amoA gene was abundant throughout the composting process, while the bacterial amoA gene abundance decreased to undetectable level during the thermophilic and cooling stages. DGGE showed more diverse archaeal amoA gene composition when the potential ammonia oxidation (PAO) rate reached peak values. A significant positive relationship was observed between the PAO rate and the archaeal amoA gene abundance (R(2) = 0.554; P < 0.001), indicating that archaea dominated ammonia oxidation during the thermophilic and cooling stages. Bacteria were also related to ammonia oxidation activity (R(2) = 0.503; P = 0.03) especially during the mesophilic and maturation stages. (C) 2011 Elsevier Ltd. All rights reserved.