Contributions of ammonia-oxidizing archaea and bacteria to nitrification in Oregon forest soils

Ammonia oxidation, the first step of nitrification, is mediated by both ammonia-oxidizing archaea (AOA) and bacteria (AOB); however, the relative contributions of AOA and AOB to soil nitrification are not well understood. In this study we used 1-octyne to discriminate between AOA- and AOB-supported nitrification determined both in soil-water slurries and in unsaturated whole soil at field moisture. Soils were collected from stands of red alder (Alnus rubra Bong.) and Douglas-fir (Pseudotsuga menziesii Mirb. Franco) at three sites (Cascade Head, the H.J. Andrews, and McDonald Forest) on acidic soils (pH 3.9-5.7) in Oregon, USA. The abundances of AOA and AOB were measured using quantitative PCR by targeting the amoA gene, which encodes subunit A of ammonia monooxygenase. Total and AOA-specific (octyneresistant) nitrification activities in soil slurries were significantly higher at Cascade Head (the most acidic soils, pH < 5) than at either the H.J. Andrews or McDonald Forest, and greater in red alder compared with Douglas-fir soils. The fraction of octyne-resistant nitrification varied among sites (21-74%) and was highest at Cascade Head than at the other two locations. Net nitrification rates of whole soil without NH4 amendment ranged from 0.4 to 3.3 mg N kg-1 soil d-1. Overall, net nitrification rates of whole soil were stimulated 2- to 8-fold by addition of 140 mg NH4-N kg-1 soil; this was significant for red alder at Cascade Head and the H.J. Andrews. Red alder at Cascade Head was unique in that the majority of NH4-stimulated nitrifying activity was octyne-resistant (73%). At all other sites, NH4-stimulated nitrification was octyne-sensitive (68-90%). The octyne-sensitive activity- presumably AOB - was affected more by soil pH whereas the octyne-resistant (AOA) activity was more strongly related to N availability.
Keywords: Nitrification, Ammonia oxidation,Archaea, Bacteria, Forest soil