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Nitrification plays a central role in the global nitrogen cycle and is responsible for significant losses of nitrogen fertilizer, atmospheric pollution by the greenhouse gas nitrous oxide, and nitrate pollution of groundwaters. Ammonia oxidation, the first step in nitrification, was thought to be performed by autotrophic bacteria until the recent discovery of archaeal ammonia oxidizers. Autotrophic archaeal ammonia oxidizers have been cultivated from marine and thermal spring environments, but the relative importance of bacteria and archaea in soil nitrification is unclear and it is believed that soil archaeal ammonia oxidizers may use organic carbon, rather than growing autotrophically. In this soil microcosm study, stable isotope probing was used to demonstrate incorporation of ¹³C-enriched carbon dioxide into the genomes of thaumarchaea possessing two functional genes: amoA, encoding a subunit of ammonia monooxygenase that catalyses the first step in ammonia oxidation; and hcd, a key gene in the autotrophic 3-hydroxypropionate/4-hydroxybutyrate cycle, which has been found so far only in archaea. Nitrification was accompanied by increases in archaeal amoA gene abundance and changes in amoA gene diversity, but no change was observed in bacterial amoA genes. Archaeal, but not bacterial, amoA genes were also detected in ¹³C-labeled DNA, demonstrating inorganic CO₂ fixation by archaeal, but not bacterial, ammonia oxidizers. Autotrophic archaeal ammonia oxidation was further supported by coordinate increases in amoA and hcd gene abundance in ¹³C-labeled DNA. The results therefore provide direct evidence for a role for archaea in soil ammonia oxidation and demonstrate autotrophic growth of ammonia oxidizing archaea in soil.

The impact of DMPP (3,4-dimethylpyrazole phosphate), applied at two doses (low: recommended for agronomic use; high: > 100 × the recommended), on the function, diversity, and dynamics of target microorganisms (ammonia-oxidizing microorganisms, AOM), functionally associated microorganisms (nitrite-oxidizing bacteria (NOB) and denitrifiers), and total prokaryotic and fungal microbial communities was assessed in two loamy soils, mainly differing in pH (acidic vs. alkaline), in a 35-day microcosm study. This was achieved via monitoring inorganic N-pools, potential nitrification (PN) rates, amoA gene and transcripts abundance, the abundance of other phylogenetic marker genes (nxrB, narG, nirS, nirK, nosZ, 16S rRNA, 18S rRNA), and amplicon sequencing of amoA, 16S rRNA, and ITS. Overall, DMPP was more persistent in the acidic soil. Its low dose successfully inhibited nitrification in the alkaline but not in the acidic soil, where effective inhibition was observed only at the high dose. This was mainly attributed to the consistently higher activity of DMPP towards ammonia-oxidizing bacteria (AOB) prevailing in the alkaline soil, unlike ammonia-oxidizing archaea (AOA) whose abundance and transcriptional activity was reduced only by the high dose. DMPP, at the high dose, reduced the abundance of Nitrobacter but not Nitrospira NOB, while its low dose increased the abundance of denitrifying bacteria, prokaryotic, and fungal populations in the alkaline soil. Amplicon sequencing revealed that DMPP imposed significant changes in the composition of the prokaryotic, fungal, and AOB communities in both soils, unlike AOA which were less responsive. These were associated with dose-dependent changes in the abundance of bacteria and fungi known to control key soil functions implying possible effects for the soil ecosystem homeostasis. Our study paves the way for a more comprehensive analysis of the effects of NIs on the soil microbial community, beyond the current focus on target AOM.

Background and aims Global warming is predicted to alter the timing and magnitude of biogeochemical nitrogen cycling in paddy soils. However, little is known about its effect on active nitrifying populations. Here we investigated the responses of nitrification activity and active nitrifiers to elevated temperature in an acidic paddy soil. Methods 13 CO 2 -DNA-stable isotope probing (SIP), qPCR and high-throughput sequencing were used to determine active nitrifying phylotypes as well as difference in their abundance and community composition incubated at field temperature (15 °C) and elevated temperature (20 °C). Results Urea application led to significant production of nitrate and growth of ammonia-oxidizing bacteria (AOB) at both temperatures. Nitrification activity at elevated temperature was 148.3% and 18.5% higher than that of low temperature at day 28 and 56, respectively, accompanied by an increase in the extent of 13 C-label incorporation by AOB. 13 CO 2 -based SIP experiment indicated that both AOB and ammonia-oxidizing archaea (AOA) were involved in the nitrification activity and the active ammonia oxidizers changed from AOA to AOB with elevated temperature. Significant variation of AOA communities was observed under different temperatures. Dominant 13 C-labeled nitrite-oxidizing bacteria (NOB) shifted from Nitrospira moscoviensis to Nitrospira japonica with higher temperature. Conclusions Our findings emphasized that elevated temperature had pronounced effects on autotrophic nitrification which was mediated by altering relative abundance of active AOB and AOA, as well as the community composition of AOA and NOB. AOB were more adaptable than AOA with increasing abundance but no alteration of composition at elevated temperature.

Nitrification is the microbial conversion of reduced forms of nitrogen (N) to nitrate (NO3−), and in fertilized soils it can lead to substantial N losses via NO3− leaching or nitrous oxide (N2O) production. To limit such problems, synthetic nitrification inhibitors have been applied but their performance differs between soils. In recent years, there has been an increasing interest in the occurrence of biological nitrification inhibition (BNI), a natural phenomenon according to which certain plants can inhibit nitrification through the release of active compounds in root exudates. Here, we synthesize the current state of research but also unravel knowledge gaps in the field. The nitrification process is discussed considering recent discoveries in genomics, biochemistry and ecology of nitrifiers. Secondly, we focus on the ‘where’ and ‘how’ of BNI. The N transformations and their interconnections as they occur in, and are affected by, the rhizosphere, are also discussed. The NH4+ and NO3− retention pathways alternative to BNI are reviewed as well. We also provide hypotheses on how plant compounds with putative BNI ability can reach their targets inside the cell and inhibit ammonia oxidation. Finally, we discuss a set of techniques that can be successfully applied to solve unresearched questions in BNI studies.

ABSTRACT The elevational distribution patterns of microbial functional groups have long been attracting scientific interest. Ammonia-oxidizers (ammonia-oxidizing archaea [AOA] and bacteria [AOB]), complete ammonia oxidation (comammox) Nitrospira and nitrite-oxidizers (e.g. Nitrobacter and Nitrospira) play crucial roles in the nitrogen cycle, yet their activities and abundances in response to elevational gradients in a subtropical forest ecosystem remain unclear. Here, we investigated the distribution of potential functions and abundances of these nitrifiers in forest soils along elevational gradients on Mount Lu, China. Our results showed that AOA and Nitrospira abundance was higher than that of their counterparts. Only AOA, Nitrobacter and comammox Nitrospira abundances followed a hump-backed-model with altitude. Soil potential ammonia-oxidation activity (PAO) and nitrite-oxidation activity (PNO) ranged from 0.003 to 0.084 and 0.34 to 0.53 μg NO2−-N g−1 dry soil h−1, respectively. The biotic (AOA, Nitrobacter, Nitrospira and comammox Nitrospira abundances) and abiotic factors (soil variables) jointly affected PAO, whereas the abiotic factors were mainly responsible for PNO. Variance partitioning analysis showed that contemporary environmental disturbance is the most important driver for the biogeography of nitrifier assemblages. Overall, our findings indicate that forest soil nitrifier assemblages exhibit a biogeographic pattern largely shaped by soil chemistry along an elevational gradient. Soil nitrifier assemblages exhibit biogeographic patterns shaped more by soil variables than by elevational gradients.

Many studies have assessed the responses of soil microbial functional groups to increases in atmospheric CO2 or N deposition alone and more rarely in combination. However, the effects of elevated CO2 and N on the (de)coupling between different microbial functional groups (e.g., different groups of nitrifiers) have been barely studied, despite potential consequences for ecosystem functioning. Here, we investigated the short-term combined effects of elevated CO2 and N supply on the abundances of the four main microbial groups involved in soil nitrification: ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), and nitrite-oxidizing bacteria (belonging to the genera Nitrobacter and Nitrospira) in grassland mesocosms. AOB and AOA abundances responded differently to the treatments: N addition increased AOB abundance, but did not alter AOA abundance. Nitrobacter and Nitrospira abundances also showed contrasted responses to the treatments: N addition increased Nitrobacter abundance, but decreased Nitrospira abundance. Our results support the idea of a niche differentiation between AOB and AOA, and between Nitrobacter and Nitrospira. AOB and Nitrobacter were both promoted at high N and C conditions (and low soil water content for Nitrobacter), while AOA and Nitrospira were favored at low N and C conditions (and high soil water content for Nitrospira). In addition, Nitrobacter abundance was positively correlated to AOB abundance and Nitrospira abundance to AOA abundance. Our results suggest that the couplings between ammonia and nitrite oxidizers are influenced by soil N availability. Multiple environmental changes may thus elicit rapid and contrasted responses between and among the soil ammonia and nitrite oxidizers due to their different ecological requirements.

For decades, the enzymatic conversion of cellulose was thought to rely on the synergistic action of hydrolytic enzymes, but recent work has shown that lytic polysaccharide monooxygenases (LPMOs) are important contributors to this process. We describe the structural and functional characterization of two functionally coupled cellulose-active LPMOs belonging to auxiliary activity family 10 (AA10) that commonly occur in cellulolytic bacteria. One of these LPMOs cleaves glycosidic bonds by oxidation of the C1 carbon, whereas the other can oxidize both C1 and C4. We thus demonstrate that C4 oxidation is not confined to fungal AA9-type LPMOs. X-ray crystallographic structures were obtained for the enzyme pair from Streptomyces coelicolor , solved at 1.3 Å (Sc LPMO10B) and 1.5 Å (CelS2 or Sc LPMO10C) resolution. Structural comparisons revealed differences in active site architecture that could relate to the ability to oxidize C4 (and that also seem to apply to AA9-type LPMOs). Despite variation in active site architecture, the two enzymes exhibited similar affinities for Cu ²⁺ (12–31 nM), redox potentials (242 and 251 mV), and electron paramagnetic resonance spectra, with only the latter clearly different from those of chitin-active AA10-type LPMOs. We conclude that substrate specificity depends not on copper site architecture, but rather on variation in substrate binding and orientation. During cellulose degradation, the members of this LPMO pair act in synergy, indicating different functional roles and providing a rationale for the abundance of these enzymes in biomass-degrading organisms.

Ammonia oxidation carried out by ammonia-oxidizing microorganisms (AOMs) is a central step in the global nitrogen cycle. Aerobic AOMs comprise conventional ammonia-oxidizing bacteria (AOB), novel ammonia-oxidizing archaea (AOA), which could exist in complex and extreme conditions, and complete ammonia oxidizers (comammox), which directly oxidize ammonia to nitrate within a single cell. Anaerobic AOMs mainly comprise anaerobic ammonia-oxidizing bacteria (AnAOB), which can transform NH4+-N and NO2−-N into N2 under anaerobic conditions. In this review, the unique metabolic characteristics, microbial community of AOMs and the influencing factors are discussed. Process applications of nitrification/denitrification, nitritation/denitrification, nitritation/anammox and partial denitrification/anammox in wastewater treatment systems are emphasized. The future development of nitrogen removal processes using AOMs is expected, enrichment of comammox facilitates the complete nitrification performance, inhibiting the activity of comammox and NOB could achieve stable nitritation, and additionally, AnAOB conducting the anammox process in municipal wastewater is a promising development direction.

Soil pH is a major determinant of microbial ecosystem processes and potentially a major driver of evolution, adaptation, and diversity of ammonia oxidizers, which control soil nitrification. Archaea are major components of soil microbial communities and contribute significantly to ammonia oxidation in some soils. To determine whether pH drives evolutionary adaptation and community structure of soil archaeal ammonia oxidizers, sequences of amoA, a key functional gene of ammonia oxidation, were examined in soils at global, regional, and local scales. Globally distributed database sequences clustered into 18 well-supported phylogenetic lineages that dominated specific soil pH ranges classified as acidic (pH <5), acido-neutral (5≤ pH <7), or alkalinophilic (pH ≥7). To determine whether patterns were reproduced at regional and local scales, amoA gene fragments were amplified from DNA extracted from 47 soils in the United Kingdom (pH 3.5—8.7), including a pH-gradient formed by seven soils at a single site (pH 4.5—7.5). High-throughput sequencing and analysis of amoA gene fragments identified an additional, previously undiscovered phylogenetic lineage and revealed similar pH-associated distribution patterns at global, regional, and local scales, which were most evident for the five most abundant clusters. Archaeal amoA abundance and diversity increased with soil pH, which was the only physicochemical characteristic measured that significantly influenced community structure. These results suggest evolution based on specific adaptations to soil pH and niche specialization, resulting in a global distribution of archaeal lineages that have important consequences for soil ecosystem function and nitrogen cycling.

Nitrification is a fundamental component of the global nitrogen cycle and leads to significant fertilizer loss and atmospheric and groundwater pollution. Nitrification rates in acidic soils (pH < 5.5), which comprise 30% of the world's soils, equal or exceed those of neutral soils. Paradoxically, autotrophic ammonia oxidizing bacteria and archaea, which perform the first stage in nitrification, demonstrate little or no growth in suspended liquid culture below pH 6.5, at which ammonia availability is reduced by ionization. Here we report the discovery and cultivation of a chemolithotrophic, obligately acidophilic thaumarchaeal ammonia oxidizer, \"Candidatus Nitrosotalea devanaterra,\" from an acidic agricultural soil. Phylogenetic analysis places the organism within a previously uncultivated thaumarchaeal lineage that has been observed in acidic soils. Growth of the organism is optimal in the pH range 4 to 5 and is restricted to the pH range 4 to 5.5, unlike all previously cultivated ammonia oxidizers. Growth of this organism and associated ammonia oxidation and autotrophy also occur during nitrification in soil at pH 4.5. The discovery of Nitrosotalea devanaterra provides a previously unsuspected explanation for high rates of nitrification in acidic soils, and confirms the vital role that thaumarchaea play in terrestrial nitrogen cycling. Growth at extremely low ammonia concentration (0.18 nM) also challenges accepted views on ammonia uptake and metabolism and indicates novel mechanisms for ammonia oxidation at low pH.