How Do Soil Properties Affect the Efficacy of Nitrification Inhibitors On Nitrous Oxide Emission and Distribution of Ammonia Oxidizers?.

Nitrification is an important nitrogen (N) transformation process in soils, which converts relatively immobile ammonium (NH₄⁺) and ammonia (NH₃) into more mobile nitrate (NO₃^-). The NO₃^- can then be subjected to losses by leaching and as gaseous emissions (N₂O or N₂) from subsequent denitrification. Nitrous oxide (N₂O) is a potent greenhouse gas with a global warming potential 300 times that of carbon dioxide (CO₂). The application of nitrification inhibitors is one strategy to reduce these N losses as they inhibit the bacterial ammonia monooxygenase (amoA) enzyme that is involved in the oxidation of NH₃ to NH₂OH, the first step of nitrification. Previous studies have indicated that the efficiency of nitrification inhibitors is variable both spatially and temporally, and this depends greatly on the environmental conditions and soil properties.

Some studies have verified that ammonia oxidizers can directly regulate N₂O production. Ammonia-oxidizing bacteria (AOB) are generally believed to be the most important contributors to nitrification in agricultural soils, but ammonia-oxidizing archaea (AOA) can also be present in large numbers. Inhibition of nitrification after application of fertilizers treated with nitrification inhibitors has been observed for soil AOB populations but not for AOA populations. Differences in soil properties seems to be a key parameter responsible for the variation in their distributions, however there is no clearly identified soil chemical or physical parameter that is most important, and limited data exists regarding the importance of the soil microbial community.  Therefore, we undertook a laboratory incubation experiment aimed at evaluating the efficiency of the nitrification inhibitors 3, 4- dimethylpyrazol phosphate (DMPP) and acetylene in reducing N₂O production from three soils of different physical and chemical properties, and their impact on the dynamics of AOB and AOA abundance.

The results showed that the application of DMPP and acetylene can significantly reduce N₂O emission from all three soils. DMPP inhibited N₂O emissions better than acetylene in the neutral clay loam soil, less effectively in the acid loam soil, and equally well in the alkaline clay loam soil. It was concluded that soil pH is a key parameter influencing the effectiveness of the nitrification inhibitors.

Further studies are currently being undertaken to quantify the amoA gene from bacteria and archaea in our soils to test if there is a direct relationship between these nitrifiers and soil properties that can influence N₂O production from nitrification.