Do Archaea dominate nitrification in acid soils?

Which organisms are responsible for nitrogen cycling in acid soils? Nitrification is essential to the cycling of nitrogen throughout the environment. It involves the conversion of ammonia to nitrate by two groups of microorganisms. Ammonia oxidisers convert ammonia to nitrite, which is subsequently converted to nitrite by nitrite oxidisers. Although an essential process, nitrification can also have deleterious consequences. Ammonium can be retained in soil, but after conversion to nitrate, it can be readily leached resulting in the pollution of groundwaters that may be used to supply drinking water. In addition, ammonia oxidation is accompanied by the production of nitrous oxide, a potent greenhouse gas which is also involved in destroying stratospheric ozone in the atmosphere. Approximately 30% of the world's soil can be considered acid (with a pH less than 5.5) and represent a wide range of natural and managed habitats including forestry, agriculture and grasslands. They are therefore of great environmental and economic importance. In contrast to other soils (such as managed arable land), the processes and organisms responsible for nitrification in acid soils are not fully understood, as typical ammonia oxidisers that were thought to be primarily responsible for ammonia oxidation are unable to grow in laboratory media at acidic pH. Until recently it was believed that the most important soil ammonia oxidisers were bacteria. However, this view has changed with the recent discovery of another group of abundant group of organisms that possess the ability to oxidise ammonia, the Crenarchaea. These organisms belong to another domain of microbial life, the archaea. Although they resemble bacteria in many ways, they are evolutionary distinct. These organisms were once thought to be restricted to hyperthermophilic environments such as deep-sea vents or terrestrial thermal springs. However, with the use of molecular techniques, these organisms have been demonstrated to be ubiquitous in the environment. We have recently obtained evidence that in one experimental field site, ammonia oxidising Crenarchaea show greatest activity in acidic soils, in direct contrast to ammonia oxidising bacteria which show greatest activity in neutral soils. This therefore indicates that there are fundamental ecological and physiological differences between these two groups. This research programme aims to determine whether soil crenarchaea, rather than bacteria, are primarily responsible for ammonia oxidation in acid soils from a range of different habitats. Their abundance will be measured in acidic soils and neighbouring neutral pH soils from four sites from within the UK, and their relative abundance will also be determined in forest and pasture soils from three other countries (Finland, Canada and Australia). The findings will benefit those in agriculture, forestry and industry by increasing our knowledge of the organisms responsible for nitrogen cycling in natural and managed acidic soil systems. The findings will also be important to researchers trying to understand the links between biodiversity and ecosystem function, including the mechanisms and organisms potentially responsible for the emission of nitrous oxide from soil.