Responses of Archaeal and Bacterial Ammonia Oxidizers in Mulched and Vegetated Soils at Different Temperatures.

Increases in the extent of urban areas are expected to be accompanied by an increase in soils used for landscaping purposes.  Bark and gravel mulch are among the most commonly used landscaping practices.  The use of organic vs. inorganic mulches create soil conditions that can affect N cycling, providing a practical system for the study of ammonia oxidizing prokaryotes (AOP).  The effects of bark and gravel mulch, lawn, and fallow on the abundance of AOP were monitored every 34 days for a period of 102 days.  Pots with each of these land covers were incubated at 18 and 28 ºC.  Soil variables including moisture, in situ temperature, pH, ammonium and nitrate content were also monitored.  Both ammonia oxidizing bacteria (AOB) and archaea (AOA) responded differently to the combined effects of mulching and temperature.  After 34 days, AOA abundance decreased significantly at 28 ºC.  However, the drop was delayed under bark mulch.  Soils incubated at 18 ºC under bark and gravel mulch favored an increase in AOB over time.  At the same temperature, AOA decreased from 34 to 68 days.  Fluctuations in potential nitrification correlated significantly with fluctuations in AOA abundance over time, while no correlation was found with AOB.  After 102 days, while no transcriptional activity of AOB was detected, AOA amoA transcripts were significantly higher at 18 ºC. Soil cover did not affect transcriptional activity.  Our data suggest that AOA were the main drivers of potential nitrification in these non-fertilized soils.