Download Advances in microbial processes of ammonium and nitrate

Advances in microbial processes of ammonium and nitrate transformations in
soils and sediments
Jizheng (Jim) He
Department of Agriculture and Food Systems, Melbourne School of Land and Environment, The University of Melbourne, Parkville,
Victoria 3051, Australia (E-mail: [email protected])
Nitrogen (N) cycle is one of the central processes of terrestrial ecosystems, and contains four basic steps,
i.e. biological N fixation, ammonification, nitrification and denitrification, all of which are mainly driven
by microorganisms. Nitrification and denitrification include the transformations of ammonium (NH4+)
and nitrate (NO3-) through ammonia oxidation, nitrite (NO2-) oxidation, anaerobic ammonium oxidation
(anammox), dissimilatory nitrate reduction to ammonium (DNRA), denitrification of nitrate to nitrite, nitric
oxide (NO), nitrous oxide (N2O) and nitrogen gas (N2). In the last decade, with the rapid development
of culture-independent molecular techniques and high-throughput sequencing technologies, breakthrough
advances has been made on the aerobic and anaerobic ammonia oxidation processes and the involving
microorganisms. For over 100 years, ammonia oxidizing bacteria (AOB) were considered as the only group
mediating the first and rate-limiting step of nitrification. Recent progress in ammonia-oxidizing archaea
(AOA) studies suggested their putative contributions to the global nitrogen cycle by possessing the potential
capacity to oxidize ammonia, and phylogenetic analysis categorized AOA into the Thaumarchaeota lineage,
a novel archaeal phylum. Our research findings provided strong evidence that AOA play a more important
role than AOB in autotrophic ammonia oxidation in acid soils. The discovery of anammox process and
related bacteria in the early 1990s offered new insight into the mechanism of anaerobic ammonia oxidation,
which challenged the traditional view that denitrification was the only pathway leading to N loss. It has
been documented that anammox may contribute up to 50% of N loss in marine systems and some wetland
soils. The novel findings of AOA and anammox bacteria have shed new light on ammonium and nitrate
transformations, especially under oxygen-limited conditions, such as at the oxic/anoxic interface of soils and
sediments. However, there are still many unresolved questions which need to be answered, e.g., the relative
contributions of AOB and AOA to soil nitrification, of anammox and DNRA to soil nitrogen transformations,
and of nitrification and denitrification to nitrous oxide emissions. This paper will present a snapshot of the
achievements of microbial mediated ammonium and nitrate transformations during last decade and propose
future perspectives in the field.