Response of Ammonia-Oxidizing Bacteria and Archaea to Ammonium Availability in an Agricultural Soil Under Contrasting N Fertilization.

The first step of nitrification in soils, ammonia oxidation to nitrite, is mediated by ammonia oxidizing bacteria and archaea (AOB and AOA). However, our understanding of the relative contribution of AOA and AOB to overall nitrification in the soil environment is limited. Our goal was to determine the dynamics of AOA and AOB abundance and contributions to nitrification activity under mineral and organic N fertilization. In 2011-2015, corn field plots were treated annually with contrasting N fertilizers: control (no additional N), ammonium sulfate (AS100 & 200 kg N ha^-1), and steer compost (200 kg N ha^-1). In 2014, kinetics of nitrate & nitrite production were determined in shaken soil slurries with nine concentrations of ammonium (0-20 mM). The differential inhibitor,1-octyne, was used to distinguish AOA and AOB contributions to nitrification. We found that the fraction of nitrification due to AOA decreased (from 100% to 8%) with increasing ammonium in these assays. In 2015, soil was sampled pre-fertilization, and 3d (day), 7d, 14d, 21d, 35d, 80d after fertilization. Ammonia oxidizers were examined using DNA and RNA extracted from the soil, and quantification and expression of amoA gene were measured using quantitative PCR. N treatments had no effect on AOA abundance, but significantly affected AOB populations. Both AOA and AOB abundances and transcripts were stimulated at 7d or 14d after application of ammonium fertilizers. We assessed AOA and AOB contributions to soil nitrification in soil slurries with 1 mM supplemental NH₄⁺ and in whole soil without addition of ammonium substrate. Total and AOB-specific (octyne-sensitive) nitrification potentials were increased in AS treated soils by 90-150% at 7d or 14d after fertilization, compared to pre-fertilization. AOB dominantly contributed to nitrification activity over the short-term after fertilization (within 2 weeks) while AOA dominated nitrification activity in whole soils once fertilizer ammonium was depleted. Understanding the response of AOB and AOA to ammonium may inform our ability to manage N more efficiently in agricultural soils.