Effect of nitrification inhibitors on the Baltic Sea ammonia-oxidizing community and precision of the denitrifier method

Nitrification is a key process in nitrogen cycling. Several nitrification rate measurement techniques rely on the use of chemical inhibitors that effectively and specifically inhibit nitrifying bacteria. The discovery of ammonia-oxidizing archaea has raised the question of whether these inhibitors are also effective for archaea. The efficiency of 2 commonly used chemicals--allylthiourea (ATU) and nitrapyrin (commercial name N-serve)--in inhibiting nitrification was tested in natural seawater samples containing archaea. In addition, the effect of these inhibitors was tested in a pure culture of Pseudomonas chlororaphis, a bacterium that is used in the denitrifier method to reduce nitrate to nitrous oxide, whose isotopic composition can be analyzed with isotopic ratio mass spectrometers. This method is commonly used in stable isotope based nitrification rate measurements. ATU inhibited nitrification by 80% at a 100 mu M concentration, whereas N-serve inhibited nitrification completely at a 150 mu M concentration. Although the inhibition of nitrification by ATU was not complete for the natural seawater ammonia-oxidizing community,