Responses of Nitrification and Ammonia-Oxidizing Bacteria to Reciprocal Transfers of Soil between Adjacent Coniferous Forest and Meadow Vegetation in the Cascade Mountains of Oregon

Despite the critical position of nitrification in N cycling in coniferous forest soils of western North America, little information exists on the composition of ammonia-oxidizing bacteria (AOB) in these soils, or their response to treatments that promote or reduce nitrification. To this end, an experiment was conducted in which a set of soil cores was reciprocally transplanted between adjacent forest (low nitrification potential) and meadow (high nitrification potential) environments, at two high-elevation (~1500 m) sites in the H.J. Andrews Experimental Forest located in the Cascade Mountains of Oregon. Half of the cores were placed in screened PVC pipe (closed) to prevent new root colonization, large litter debris inputs, and animal disturbance; the other cores were placed in open mesh bags. A duplicate set of open and closed soil cores was not transferred between sites and was incubated in place. Over the 2-year experiment, net nitrification increased in both open and closed cores transferred from forest to meadow, and to a lesser extent in cores remaining in the forest. In three of four forest soil treatments, net nitrification increases were accompanied by increases in nitrification potential rates (NPR) and 10- to 100-fold increases in AOB populations. In open cores remaining in the forests, however, increases in net nitrification were not accompanied by significant increases in either NPR or AOB populations. Although some meadow soil treatments reduced both net nitrification and nitrification potential rates, significant changes were not detected in most probable number (MPN)-based estimates of AOB population densities. Terminal restriction fragment profiles (T-RFs) of a PCR-amplified 491-bp fragment of the ammonia monooxygenase subunit A gene (amoA) changed significantly in response to some soil treatments, and treatment effects differed among locations and between years. A T-RF previously shown to be a specific biomarker of Nitrosospira cluster 4 (Alu390) was widespread and dominant in the majority of soil samples. Despite some treatments causing substantial increases in AOB population densities and nitrification potential rates, nitrosomonads remained undetectable, and the nitrosospirad AOB community composition did not change radically following treatment.