Ammonium availability and temperature control contributions of ammonia oxidizing bacteria and archaea to nitrification in an agricultural soil

Soil ammonia-oxidizing bacteria and archaea (AOB and AOA) convert ammonium/ammonia to nitrite in the process of nitrification. However, the potentially differential responses of these AO to substrate and temperature and the effects of conventional and organic nitrogen management on these responses remains poorly understood. We determined the response of nitrification to ammonium substrate concentration and temperature using an AOB specific inhibitor to distinguish the contribution of AOB and AOA to nitrification. Soils were sampled from cornfield plots that had been treated for four years with contrasting nitrogen sources: control (no additional N), ammonium sulfate at two rates and compost. Nitrification potential and net rates were stimulated for one month after fertilization with ammonium sulfate compared to relatively lower and stable rates in control and compost treated soils. For soils that had been fertilized with ammonium sulfate, the proportion of nitrification mediated by AOB in slurry assays was over 90% at 1.0 mM but less than 50% at 0.01 mM. Kinetic analysis showed maximum nitrification activity (V-max) for AOB ranged from 032 to 4.8 mmol N kg(-1)d(-1) with a half saturation constant (Km) of 14-160 mu M ammonium; parameters were higher for soils from ammonium sulfate treated plots. Vmax and Km for AOA averaged 0.24 mmol N kg(-1) d(-1) and 4.28 mu M ammonium with no effect of field treatment. The proportion of nitrification due to AOA was lowest at 5 degrees C, increased with temperature, and was near to 100% at 50 degrees C; optimum temperature was 41 degrees C for AOA versus 31 degrees C for AOB. Understanding the kinetic and temperature response of microbes responsible for nitrification may allow ecosystem models to include these populations as dynamic components driving nitrogen flux. (C) 2017 Elsevier Ltd. All rights reserved.