| Ammonia-oxidizing bacteria (AOB) mediate the first rate-liming step of autotrophic nitrification, which is considered as the key control point in N cycling due to the mobility of nitrate and loss of oxidized N forms. Understanding the genetics and metabolism of AOB in general and the effect of animal waste application on their community structure and associated N transformations is desirable for proper and efficient management of animal waste N in soils.;Molecular techniques were employed to investigate quantitative and compositional changes in AOB community after application of dairy waste compost (DC), liquid dairy waste (LW), and ammonium sulfate fertilizer (AS) at 100 and 200 kg available N ha-1 in an agricultural soil for several years. Field isotope dilution techniques and laboratory incubation experiments were done to determine treatment-associated changes in N transformation rates.;AOB population size ranged from 105 to 106 cells g-1 soil in the variously treated soils. Clone libraries of partial amoA sequences indicated that cluster 3 sequences were the common AOB with Nitrosospira multiformis strains being the most dominant. AS-treated soils seemed to have the least diverse AOB with all the clones grouping under cluster 3 while DC- and LW-treated soils had clones from more than one cluster.;The DC treatment significantly increased soil organic C, N, and labile organic N pools as well as urease activity as compared to the other treatments. The mean gross N mineralization rates for 1999 to 2002 for the high-level DC, LW, and AS treatments were 5.72, 2.89, and 1.27 mg N kg-1 d-1, respectively, whereas gross nitrification rates were 10.24, 1.57, and 0.74 mg N kg-1 d-1, respectively. Net mineralization rates were less than 35% of gross rates while nitrate consumption was not significant under any treatment. Soils receiving high-level DC and LW applications showed a three-fold increase in nitrifier activity compared to the control.;AOB responded differentially to dairy waste and ammonium fertilizer treatments. Congruent with the change in the AOB community structure, the compost treatments resulted in significantly different nitrification rates than the ammonium sulfate treatment, suggesting that changes in AOB community structure and function were related. |
