Nitrogen Fertilization Changes Abundance and Community Composition of Ammonia-Oxidizing Bacteria

The effects of N fertilization rates on the abundance and community composition of NH3-oxidizing bacteria (AOB) are poorly understood, particularly in greenhouse-based high-N-input vegetable soils. The abundance and community composition of AOB from these vegetable soils under five N (urea) rates in eastern China were evaluated by polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE). These rates were: the conventional high N rate (N4) as 270 or 300 kg N ha(-1) season(-1) for cucumber (Cucumis sativus L.) or tomato (Solanum lycopersicum L.), respectively, and 80% (N3), 60% (N2), 40% (N1), and 0% (N0) of the N4 rate. Aft er 2 yr of a cucumber-tomato rotation, N fertilization had significant effects on the abundance and community composition of AOB. The abundance of AOB amoA gene was significantly higher under the N2 (2.08 x 10(10) kg(-1) dry weight soil) than under the N0 or N4 (8.74 or 8.75 x 10(9) kg(-1) soil). The DGGE patterns were more diverse under N1 and N2 than under N3 and N4, with the greatest DGGE band numbers (15) under N2. Phylogenetic analyses of amoA gene fragments showed that AOB sequences from different N fertilization rates were affiliated with Nitrosospira species and grouped into Clusters 2, 3, 8, and 12. Our results indicate that a reduction to 40% of the conventional high N rate (N4) is practical to reduce N inputs while maintaining the abundance and community composition of AOB in such greenhouse-based vegetable soils.