Seasonal Dynamics of Soil Ammonia Oxidizing Bacteria (AOB) and Ammonia Oxidizing Archaea (AOA) in Response to Nitrogen Fertilization.

The ability of soil ammonia oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA) to convert ammonia-based fertilizer into nitrite, known as nitrification, is an essential ecosystem process that makes nitrogen available for plant uptake and as well for loss to the environment. However, the relative contribution of AOA to nitrification is largely unknown in soil. And, since both AOA and AOB use ammonia oxidation as an energy source, it is not clear if they compete for resources (ammonia) or occupy separate ecological niches. Here we examine nitrification by soil AOA and AOB and their interactions following nitrogen fertilizer application in spring, summer, and winter to annual (corn) and perennial (switchgrass) cropping systems. We took soil samples before and periodically after fertilization to study the responses of AOA and AOB to fertilizer. Short-term laboratory incubations to assess nitrification revealed that 1) winter fertilization had no effect on potential nitrification rates. In the corn system, AOB were responsible for 60-70% of nitrification and in the switchgrass system AOB and AOA contributed equally. 2) In spring, prior to fertilization, AOB dominated nitrification potentials in both systems (70-75% of ammonia oxidation). Fertilization in switchgrass did not increase nitrification potentials but the relative contribution by AOB increased to 85% five days after fertilization and eight days later dropped back to 75%. In corn, fertilization resulted in an immediate 45% increase in nitrification potential one day after fertilization but the relative contribution of AOB did not change until four days later, when its contribution increased slightly to 80% before decreasing to 50% eight days later. These findings suggest that AOA and AOB may have very different seasonal and cropping system responses to nitrogen fertilizer pulses.