The Role of Bacteria and Archaea in Nitrification, Nitrate Leaching and Nitrous Oxide Emissions in Nitrogen-Rich Grassland Soils

Nitrate (NO3-) leaching into fresh water and nitrous oxide (N2O) greenhouse gas emissions are two serious environmental impacts that occur from intensively grazed grassland soils. The oxidation of ammonia (NH3) to NO3- is a key process in the nitrogen (N) cycle which has implications both in influencing nitrous oxide emissions and NO3- leaching. We investigated the relationships between nitrification rate, NO3- leaching and N2O emissions with ammonia oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA) in nitrogen rich grassland soils. Both AOA and AOB were detected in large numbers in these grassland soils. The AOB abundance grew by 3.2-10.4 fold and activity increased by 177 fold in response to the addition of urine-N, and the AOB growthwas significantly inhibited by a nitrification inhibitor, dicyandiamide (DCD). However, neither the AOA abundance, nor activity, increased with the application of urine-N substrate. AOB prefer to grow under high nitrogen environments whereas AOA prefer to grow under low nitrogen environments. DCD decreased NO3- leaching by 59% and decreased N2O emissions by 64% from animal urine patches. Significant quantitative relationships were found between the AOB abundance and the nitrification rate, NO3- -N leaching losses, and N2O emissions, whereas no such relationships were found with AOA. These findings suggest that nitrification, NO3- leaching and N2O emissions are driven by bacteria rather than archaea in these nitrogen rich grassland soils.