Nitrification is an important process in the nitrogen cycle and has the most obvious environmental implications. The end-product of nitrification is nitrate (NO3-) which can be leached into groundwater (see diagram below). High concentrations of nitrate can cause algal blooms and contamination of lakes and rivers. In drinking water, high concentrations can cause the potentially fatal blue baby syndrome in young children. This occurs when nitrate is converted to nitrite in the baby’s digestive system. This then reacts with the body’s oxygen carrying molecule, haemoglobin, forming a non-oxygen carrying molecule called methaemoglobin which decreases the infant’s ability to utilise oxygen. Nitrate from nitrification can also lead to the production of nitrous oxide (see diagram), a greenhouse gas, which has further implications for the environment through global warming and climate change. It is therefore important to understand the biological process of nitrification for the application of mitigation strategies to reduce these environmental hazards.

It has long been thought that ammonia oxidising archaea (AOA) (single celled micro-organisms which have no nucleus or any other bound organelles) only drive the nitrogen cycle in harsh environments. However, in a paper published in Nature, Linginer and colleagues discovered that AOA may be the most abundant ammonia oxidising organisms in soil ecosystems. This questions the traditional assumption that nitrification is dominantly the role of ammonia oxidizing bacteria (AOB) and begs to question what really is driving the nitrogen cycle in the soil?

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Although it has been shown that AOB dominate the nitrification process in some agricultural soils, it is interesting to see how AOA may react to nutrient changes in the soil and in the presence of an inhibitory substance.
In a recently published study in the Federation of European Microbiological Societies Microbiology from Hong Di and his team at the Centre for Soil and Environmental Research, Lincoln University, they investigated the role of AOA and AOB in differing layers of soil and determined the effects of animal urine and a nitrification inhibitor (DCD) application on the two organisms. Samples from Waikato, Canterbury and Southland were collected to gain a holistic view of New Zealand soils. They found that AOA was more abundant in the soil than AOB in both soil layers with the bacteria decreasing in the lower layer. With the addition of urine AOB greatly increased whereas AOA seemed to be inhibited. This demonstrates that AOA and AOB prefer different soil nutrient conditions with AOA favouring low-nutrient environments.

Adding nitrification inhibitor sufficiently reduced the AOB nitrification rates and hence lowered nitrate leaching and nitrous oxide production. The AOA were inhibited by the urine addition but this could have been due to the urine or DCD which were both present in the treatment. It would be useful to determine the AOA reaction to the DCD alone. However, because nitrification inhibitors work by preventing the enzyme produced from the organism rather than targeting the actual producer, Archaea would most likely react in a similar way to the bacteria and be inhibited.
This study demonstrates that AOB are the hardest workers in the soil nitrogen cycle, although AOA should not be underestimated. AOA’s abundance in the soil suggests their importance, but the processes they undertake are yet to be understood. Continued research from Lincoln University will continue to decipher the roles of AOA and their potential presence in agricultural soils as well as their value in lower nutrient environments.