Air pollution in Germany: Spatio-temporal variations and their driving factors based on continuous data from 2008 to 2018.

This study analyzed long-term observational data of particulate matter (PM2.5, PM10) variability, gaseous pollutants (CO, NO2, NOX, SO2, and O3), and meteorological factors in 412 fixed monitoring stations from January 2008 to December 2018 in Germany. Based on Hurst index analysis, the trend of atmospheric pollutants in Germany was stable during the research period. The relative correlations of gaseous pollutants and meteorological factors on PM2.5 and PM10 concentrations were analyzed by Back Propagation Neural Network model, showing that CO and temperature had the greater correlations with PM2.5 and PM10. Following that, PM2.5 and PM10 show a strong positive correlation (R2 = 0.96, p < 0.01), suggesting that the reduction of PM2.5 is essential for reducing PM pollution and enhancing air quality in Germany. Based on typical PM10/CO ratios obtained under ideal weather conditions, it is conducive to roughly estimate the contribution of natural sources. In winter, the earth's crust contributed about 20.1% to PM10. Taken together, exploring the prediction methods and analyzing the characteristic variation of pollutants will contribute an essential implication for air quality control in Germany.

Spatiotemporal Characteristics and Driving Factors of Black Carbon in Augsburg, Germany: Combination of Mobile Monitoring and Street View Images.

The study investigates the spatial pattern of black carbon (BC) at a high spatial resolution in Augsburg, Germany. Sixty two walks were performed to assess the concentrations of equivalent black carbon (eBC), ultraviolet particulate matter (UVPM), and equivalent brown carbon (eBrC) in different seasons and at different times of the day with a mobile platform (i.e., trolley). Along with BC measurements, images of street microenvironments were recorded. Meteorological parameters, including temperature, relative humidity, and wind speed, were monitored. The BC concentrations showed significant spatial heterogeneity and diurnal variations peaking in the morning and at night. The highest BC concentrations were observed near dense traffic. The correlations between BC and street views (buildings, roads, cars, and vegetation) were weak but highly significant. Moreover, meteorological factors also influenced the BC concentration. A model based on street view images and meteorological data was developed to examine the driving factors of the spatial variability of BC concentrations at a higher spatial resolution as different microenvironments based on traffic density. The best results were obtained for UVPM and eBC (71 and 70% explained variability). eBrC (53%), to which other sources besides road traffic can also make significant contributions, is modeled less well.

Integration of air pollution data collected by mobile measurement to derive a preliminary spatiotemporal air pollution profile from two neighboring German-Czech border villages.

Generally, there are only a few fixed air quality monitoring stations installed in villages or rural areas and only a few studies on small-scale variations in air pollution have been described in detail, which make it difficult to estimate human exposure in such environments and related adverse health effects. Moreover, biomass combustion can be an important source of air pollution in rural areas, comparable to vehicle and industrial emissions in urban planning. And their air pollutants are mainly affected by local sources. For this reason, a survey on rural air pollution was carried out in this study. Therefore, portable, battery-powered monitoring devices were used to measure particulate matter (PM10, PM2.5, PM1, particle number concentration, and black carbon) in order to study air quality in rural communities. The focus of the investigations was to explore the application of mobile monitoring equipment in small-scale environments, compare the differences in rural air pollutants between two neighboring villages in two countries, and the identification of pollution hotspots. The measurements were carried out in November 2018 in two villages on the German-Czech border. Over a period of four days, 21 mobile measurements along fixed routes were carried out simultaneously at both locations. The analysis of the data revealed significant differences in PN and PM concentrations in rural air pollutants between the two countries. The spatial and temporal distribution of air pollution hotspots in the Czech village was higher than that in the German village. The relationships between the measurement parameters were weak but highly significant and the meteorological parameters can effect air pollution. Overall, the results of this study show that mobile measurements are suitable for effectively recording and distinguishing spatial and temporal characteristics of air quality.

Biological inhibition of soil nitrification by forest tree species affects Nitrobacter populations.

Some temperate tree species are associated with very low soil nitrification rates, with important implications for forest N dynamics, presumably due to their potential for biological nitrification inhibition (BNI). However, evidence for BNI in forest ecosystems is scarce so far and the nitrifier groups controlled by BNI-tree species have not been identified. Here, we evaluated how some tree species can control soil nitrification by providing direct evidence of BNI and identifying the nitrifier group(s) affected. First, by comparing 28 year-old monocultures of several tree species, we showed that nitrification rates correlated strongly with the abundance of the nitrite oxidizers Nitrobacter (50- to 1000-fold changes between tree monocultures) and only weakly with the abundance of ammonia oxidizing archaea (AOA). Second, using reciprocal transplantation of soil cores between low and high nitrification stands, we demonstrated that nitrification changed 16 months after transplantation and was correlated with changes in the abundance of Nitrobacter, not AOA. Third, extracts of litter or soil collected from the low nitrification stands of Picea abies and Abies nordmanniana inhibited the growth of Nitrobacter hamburgensis X14. Our results provide for the first time direct evidence of BNI by tree species directly affecting the abundance of Nitrobacter.

Environmental factors determining distribution and activity of anammox bacteria in minerotrophic fen soils.

In contrast to the pervasive occurrence of denitrification in soils, anammox (anaerobic ammonium oxidation) is a spatially restricted process that depends on specific ecological conditions. To identify the factors that constrain the distribution and activity of anammox bacteria in terrestrial environments, we investigated four different soil types along a catena with opposing ecological gradients of nitrogen and water content, from an amended pasture to an ombrotrophic bog. Anammox was detected by polymerase chain reaction (PCR) and quantitative PCR (qPCR) only in the nitrophilic wet meadow and the minerotrophic fen, in soil sections remaining water-saturated for most of the year and whose interstitial water contained inorganic nitrogen. Contrastingly, aerobic ammonia oxidizing microorganisms were present in all examined samples and outnumbered anammox bacteria usually by at least one order of magnitude. 16S rRNA gene sequencing revealed a relatively high diversity of anammox bacteria with one Ca. Brocadia cluster. Three additional clusters could not be affiliated to known anammox genera, but have been previously detected in other soil systems. Soil incubations using 15N-labeled substrates revealed that anammox processes contributed about <2% to total N2 formation, leaving nitrification and denitrification as the dominant N-removal mechanism in these soils that represent important buffer zones between agricultural land and ombrotrophic peat bogs.

Influence of lung CT changes in chronic obstructive pulmonary disease (COPD) on the human lung microbiome.

BACKGROUND: Changes in microbial community composition in the lung of patients suffering from moderate to severe COPD have been well documented. However, knowledge about specific microbiome structures in the human lung associated with CT defined abnormalities is limited. METHODS: Bacterial community composition derived from brush samples from lungs of 16 patients suffering from different CT defined subtypes of COPD and 9 healthy subjects was analyzed using a cultivation independent barcoding approach applying 454-pyrosequencing of 16S rRNA gene fragment amplicons. RESULTS: We could show that bacterial community composition in patients with changes in CT (either airway or emphysema type changes, designated as severe subtypes) was different from community composition in lungs of patients without visible changes in CT as well as from healthy subjects (designated as mild COPD subtype and control group) (PC1, Padj = 0.002). Higher abundance of Prevotella in samples from patients with mild COPD subtype and from controls and of Streptococcus in the severe subtype cases mainly contributed to the separation of bacterial communities of subjects. No significant effects of treatment with inhaled glucocorticoids on bacterial community composition were detected within COPD cases with and without abnormalities in CT in PCoA. Co-occurrence analysis suggests the presence of networks of co-occurring bacteria. Four communities of positively correlated bacteria were revealed. The microbial communities can clearly be distinguished by their associations with the CT defined disease phenotype. CONCLUSION: Our findings indicate that CT detectable structural changes in the lung of COPD patients, which we termed severe subtypes, are associated with alterations in bacterial communities, which may induce further changes in the interaction between microbes and host cells. This might result in a changed interplay with the host immune system.

Rhizosphere microbiomes of potato cultivated in the High Andes show stable and dynamic core microbiomes with different responses to plant development.

The rhizosphere hosts a rich microflora supporting plant nutrition and health. We examined bacterial rhizosphere microbiota of Solanum tuberosum grown in its center of origin, the Central Andean Highlands, at different vegetation stages and sites at altitudes ranging from 3245 to 4070 m.a.s.l., differing in soil characteristics, climate and the agricultural practices by 454 sequence analysis of 16S rRNA genes. We observed that the taxonomic composition of bacteria repeatedly occurring at particular stages of plant development was almost unaffected by highly diverse environmental conditions. A detailed statistical analysis on the operational taxonomic unit (OTU) level, representing bacterial species, revealed a complex community structure of the rhizosphere. We identified an opportunistic microbiome which comprises OTUs that occur randomly or under specific environmental conditions. In contrast, core microbiome members were found at all sites. The 'stable' component of the core microbiome consisted of few ubiquitous OTUs that were continuously abundant in all samples and vegetation stages, whereas the 'dynamic' component comprised OTUs that were enriched at specific vegetation stages.

Response of Archaeal and Bacterial Soil Communities to Changes Associated with Outdoor Cattle Overwintering.

Archaea and bacteria are important drivers for nutrient transformations in soils and catalyse the production and consumption of important greenhouse gases. In this study, we investigate changes in archaeal and bacterial communities of four Czech grassland soils affected by outdoor cattle husbandry. Two show short-term (3 years; STI) and long-term impact (17 years; LTI), one is regenerating from cattle impact (REG) and a control is unaffected by cattle (CON). Cattle manure (CMN), the source of allochthonous microbes, was collected from the same area. We used pyrosequencing of 16S rRNA genes to assess the composition of archaeal and bacterial communities in each soil type and CMN. Both short- and long- term cattle impact negatively altered archaeal and bacterial diversity, leading to increase of homogenization of microbial communities in overwintering soils over time. Moreover, strong shifts in the prokaryotic communities were observed in response to cattle overwintering, with the greatest impact on archaea. Oligotrophic and acidophilic microorganisms (e.g. Thaumarchaeota, Acidobacteria, and α-Proteobacteria) dominated in CON and expressed strong negative response to increased pH, total C and N. Whereas copiotrophic and alkalophilic microbes (e.g. methanogenic Euryarchaeota, Firmicutes, Chloroflexi, Actinobacteria, and Bacteroidetes) were common in LTI showing opposite trends. Crenarchaeota were also found in LTI, though their trophic interactions remain cryptic. Firmicutes, Bacteroidetes, Methanobacteriaceae, and Methanomicrobiaceae indicated the introduction and establishment of faecal microbes into the impacted soils, while Chloroflexi and Methanosarcinaceae suggested increased abundance of soil-borne microbes under altered environmental conditions. The observed changes in prokaryotic community composition may have driven corresponding changes in soil functioning.

pH as a Driver for Ammonia-Oxidizing Archaea in Forest Soils.

In this study, we investigated the impact of soil pH on the diversity and abundance of archaeal ammonia oxidizers in 27 different forest soils across Germany. DNA was extracted from topsoil samples, the amoA gene, encoding ammonia monooxygenase, was amplified; and the amplicons were sequenced using a 454-based pyrosequencing approach. As expected, the ratio of archaeal (AOA) to bacterial (AOB) ammonia oxidizers' amoA genes increased sharply with decreasing soil pH. The diversity of AOA differed significantly between sites with ultra-acidic soil pH (<3.5) and sites with higher pH values. The major OTUs from soil samples with low pH could be detected at each site with a soil pH <3.5 but not at sites with pH >4.5, regardless of geographic position and vegetation. These OTUs could be related to the Nitrosotalea group 1.1 and the Nitrososphaera subcluster 7.2, respectively, and showed significant similarities to OTUs described from other acidic environments. Conversely, none of the major OTUs typical of sites with a soil pH >4.6 could be found in the ultra- and extreme acidic soils. Based on a comparison with the amoA gene sequence data from a previous study performed on agricultural soils, we could clearly show that the development of AOA communities in soils with ultra-acidic pH (<3.5) is mainly triggered by soil pH and is not influenced significantly by the type of land use, the soil type, or the geographic position of the site, which was observed for sites with acido-neutral soil pH.

Dynamics of soil bacterial communities in response to repeated application of manure containing sulfadiazine.

Large amounts of manure have been applied to arable soils as fertilizer worldwide. Manure is often contaminated with veterinary antibiotics which enter the soil together with antibiotic resistant bacteria. However, little information is available regarding the main responders of bacterial communities in soil affected by repeated inputs of antibiotics via manure. In this study, a microcosm experiment was performed with two concentrations of the antibiotic sulfadiazine (SDZ) which were applied together with manure at three different time points over a period of 133 days. Samples were taken 3 and 60 days after each manure application. The effects of SDZ on soil bacterial communities were explored by barcoded pyrosequencing of 16S rRNA gene fragments amplified from total community DNA. Samples with high concentration of SDZ were analyzed on day 193 only. Repeated inputs of SDZ, especially at a high concentration, caused pronounced changes in bacterial community compositions. By comparison with the initial soil, we could observe an increase of the disturbance and a decrease of the stability of soil bacterial communities as a result of SDZ manure application compared to the manure treatment without SDZ. The number of taxa significantly affected by the presence of SDZ increased with the times of manure application and was highest during the treatment with high SDZ-concentration. Numerous taxa, known to harbor also human pathogens, such as Devosia, Shinella, Stenotrophomonas, Clostridium, Peptostreptococcus, Leifsonia, Gemmatimonas, were enriched in the soil when SDZ was present while the abundance of bacteria which typically contribute to high soil quality belonging to the genera Pseudomonas and Lysobacter, Hydrogenophaga, and Adhaeribacter decreased in response to the repeated application of manure and SDZ.

Temporal dynamics of abundance and composition of nitrogen-fixing communities across agricultural soils.

BACKGROUND: Despite the fact that the fixation of nitrogen is one of the most significant nutrient processes in the terrestrial ecosystem, a thorough study of the spatial and temporal patterns in the abundance and distribution of N-fixing communities has been missing so far. METHODOLOGY/PRINCIPAL FINDINGS: In order to understand the dynamics of diazotrophic communities and their resilience to external changes, we quantified the abundance and characterized the bacterial community structures based on the nifH gene, using real-time PCR, PCR-DGGE and 454-pyrosequencing, across four representative Dutch soils during one growing season. In general, higher nifH gene copy numbers were observed in soils with higher pH than in those with lower pH, but lower numbers were related to increased nitrate and ammonium levels. Results from nifH gene pyrosequencing confirmed the observed PCR-DGGE patterns, which indicated that the N fixers are highly dynamic across time, shifting around 60%. Forward selection on CCA analysis identified N availability as the main driver of these variations, as well as of the evenness of the communities, leading to very unequal communities. Moreover, deep sequencing of the nifH gene revealed that sandy soils (B and D) had the lowest percentage of shared OTUs across time, compared with clayey soils (G and K), indicating the presence of a community under constant change. Cosmopolitan nifH species (present throughout the season) were affiliated with Bradyrhizobium, Azospirillum and Methylocistis, whereas other species increased their abundances progressively over time, when appropriate conditions were met, as was notably the case for Paenibacilus and Burkholderia. CONCLUSIONS: Our study provides the first in-depth pyrosequencing analysis of the N-fixing community at both spatial and temporal scales, providing insights into the cosmopolitan and specific portions of the nitrogen fixing bacterial communities in soil.

Effects of season and experimental warming on the bacterial community in a temperate mountain forest soil assessed by 16S rRNA gene pyrosequencing.

Climate warming may induce shifts in soil microbial communities possibly altering the long-term carbon mineralization potential of soils. We assessed the response of the bacterial community in a forest soil to experimental soil warming (+4 °C) in the context of seasonal fluctuations. Three experimental plots were sampled in the fourth year of warming in summer and winter and compared to control plots by 16S rRNA gene pyrosequencing. We sequenced 17,308 amplicons per sample and analysed operational taxonomic units at genetic distances of 0.03, 0.10 and 0.25, with respective Good's coverages of 0.900, 0.977 and 0.998. Diversity indices did not differ between summer, winter, control or warmed samples. Summer and winter samples differed in community structure at a genetic distance of 0.25, corresponding approximately to phylum level. This was mainly because of an increase of Actinobacteria in winter. Abundance patterns of dominant taxa (> 0.06% of all reads) were analysed individually and revealed, that seasonal shifts were coherent among related phylogenetic groups. Seasonal community dynamics were subtle compared to the dynamics of soil respiration. Despite a pronounced respiration response to soil warming, we did not detect warming effects on community structure or composition. Fine-scale shifts may have been concealed by the considerable spatial variation.

Phylotype-level 16S rRNA analysis reveals new bacterial indicators of health state in acute murine colitis.

Human inflammatory bowel disease and experimental colitis models in mice are associated with shifts in intestinal microbiota composition, but it is unclear at what taxonomic/phylogenetic level such microbiota dynamics can be indicative for health or disease. Here, we report that dextran sodium sulfate (DSS)-induced colitis is accompanied by major shifts in the composition and function of the intestinal microbiota of STAT1(-/-) and wild-type mice, as determined by 454 pyrosequencing of bacterial 16S rRNA (gene) amplicons, metatranscriptomics and quantitative fluorescence in situ hybridization of selected phylotypes. The bacterial families Ruminococcaceae, Bacteroidaceae, Enterobacteriaceae, Deferribacteraceae and Verrucomicrobiaceae increased in relative abundance in DSS-treated mice. Comparative 16S rRNA sequence analysis at maximum possible phylogenetic resolution identified several indicator phylotypes for DSS treatment, including the putative mucin degraders Akkermansia and Mucispirillum. The analysis additionally revealed strongly contrasting abundance changes among phylotypes of the same family, particularly within the Lachnospiraceae. These extensive phylotype-level dynamics were hidden when reads were grouped at higher taxonomic levels. Metatranscriptomic analysis provided insights into functional shifts in the murine intestinal microbiota, with increased transcription of genes associated with regulation and cell signaling, carbohydrate metabolism and respiration and decreased transcription of flagellin genes during inflammation. These findings (i) establish the first in-depth inventory of the mouse gut microbiota and its metatranscriptome in the DSS colitis model, (ii) reveal that family-level microbial community analyses are insufficient to reveal important colitis-associated microbiota shifts and (iii) support a scenario of shifting intra-family structure and function in the phylotype-rich and phylogenetically diverse Lachnospiraceae in DSS-treated mice.

Niche specialization of terrestrial archaeal ammonia oxidizers.

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.

Nitrogen turnover in soil and global change.

Nitrogen management in soils has been considered as key to the sustainable use of terrestrial ecosystems and a protection of major ecosystem services. However, the microorganisms driving processes like nitrification, denitrification, N-fixation and mineralization are highly influenced by changing climatic conditions, intensification of agriculture and the application of new chemicals to a so far unknown extent. In this review, the current knowledge concerning the influence of selected scenarios of global change on the abundance, diversity and activity of microorganisms involved in nitrogen turnover, notably in agricultural and grassland soils, is summarized and linked to the corresponding processes. In this context, data are presented on nitrogen-cycling processes and the corresponding microbial key players during ecosystem development and changes in functional diversity patterns during shifts in land use. Furthermore, the impact of increased temperature, carbon dioxide and changes in precipitation regimes on microbial nitrogen turnover is discussed. Finally, some examples of the effects of pesticides and antibiotics after application to soil for selected processes of nitrogen transformation are also shown.

Improved protocol for the simultaneous extraction and column-based separation of DNA and RNA from different soils.

We developed an improved protocol, allowing the simultaneous extraction of DNA and RNA from soil using phenol-chloroform with subsequent column-based separation of DNA and RNA (PCS). We compared this new approach with the well established protocol published by Griffiths et al. (2000), where DNA and RNA are separated by selective enzymatic digestions and two commercial kits used for DNA or RNA extraction, respectively, using four different agricultural soils. We compared yield and purity of the nucleic acids as well as abundance and diversity profiles of the soil bacterial communities targeting the nosZ gene via quantitative real-time PCR and terminal restriction fragment length polymorphism on DNA and RNA level. The newly developed protocol provided purer nucleic acid extracts compared to the used kit-based protocols. All protocols were suitable for DNA- and RNA-based gene quantification, however high variations between replicates were obtained for RNA samples using the original Griffiths protocol. Diversity patterns of nosZ were highly influenced by the extraction protocol used both on the DNA and RNA level. Finally, our data showed that the new protocol allows a simultaneous and reproducible extraction and separation of DNA and RNA, which were suitable for reliable analyses of gene and transcript copy numbers and diversity pattern.

Quantification of key genes steering the microbial nitrogen cycle in the rhizosphere of sorghum cultivars in tropical agroecosystems.

The effect of agricultural management practices on geochemical cycles in moderate ecosystems is by far better understood than in semiarid regions, where fertilizer availability and climatic conditions are less favorable. We studied the impact of different fertilizer regimens in an agricultural long-term observatory in Burkina Faso at three different plant development stages (early leaf development, flowering, and senescence) of sorghum cultivars. Using real-time PCR, we investigated functional microbial communities involved in key processes of the nitrogen cycle (nitrogen fixation, ammonia oxidation, and denitrification) in the rhizosphere. The results indicate that fertilizer treatments and plant development stages combined with environmental factors affected the abundance of the targeted functional genes in the rhizosphere. While nitrogen-fixing populations dominated the investigated communities when organic fertilizers (manure and straw) were applied, their numbers were comparatively reduced in urea-treated plots. In contrast, ammonia-oxidizing bacteria (AOB) increased not only in absolute numbers but also in relation to the other bacterial groups investigated in the urea-amended plots. Ammonia-oxidizing archaea exhibited higher numbers compared to AOB independent of fertilizer application. Similarly, denitrifiers were also more abundant in the urea-treated plots. Our data imply as well that, more than in moderate regions, water availability might shape microbial communities in the rhizosphere, since low gene abundance data were obtained for all tested genes at the flowering stage, when water availability was very limited.

Influence of different Sinorhizobium meliloti inocula on abundance of genes involved in nitrogen transformations in the rhizosphere of alfalfa (Medicago sativa L.).

Inoculation of leguminous seeds with selected rhizobial strains is practised in agriculture to ameliorate the plant yield by enhanced root nodulation and nitrogen uptake of the plant. However, effective symbiosis between legumes and rhizobia does not only depend on the capacity of nitrogen fixation but also on the entire nitrogen turnover in the rhizosphere. We investigated the influence of seed inoculation with two indigenous Sinorhizobium meliloti strains exhibiting different efficiency concerning plant growth promotion on nitrogen turnover processes in the rhizosphere during the growth of alfalfa. Quantification of six target genes (bacterial amoA, nirK, nirS, nosZ, nifH and archaeal amoA) within the nitrogen cycle was performed in rhizosphere samples before nodule formation, at bud development and at the late flowering stage. The results clearly demonstrated that effectiveness of rhizobial inocula is related to abundance of nifH genes in the late flowering phase of alfalfa. Moreover, other genes involved in nitrogen turnover had been affected by the inocula, e.g. higher numbers of amoA copies were observed during flowering when the more effective strain had been inoculated. However, the respective gene abundances differed overall to a greater extent between the three plant development stages than between the inoculation variants.

Effects of aboveground grazing on coupling among nitrifier activity, abundance and community structure.

The influence of switches in grassland management to or from grazing on the dynamics of nitrifier activity, as well as the abundance of ammonia-oxidizing bacteria, AOB and ammonia-oxidizing archeae, AOA, was analyzed for two years after changing management. Additionally community structure of AOB was surveyed. Four treatments were compared in mesocosms: grazing on previously grazed grassland (G-G); no grazing on ungrazed grassland (U-U); grazing on ungrazed grassland (U-G) and cessation of grazing on grazed grassland (G-U). Nitrifier activity and abundance were always higher for G-G than U-U treatments and AOB community structure differed between these treatments. AOA abundance was in the same range as AOB abundance and followed the same trend. Grazing led to a change in AOB community structure within <5 months and a subsequent (5-12 months) increase in nitrifier activity and abundance. In contrast, cessation of grazing led to a decrease in nitrifier activity and abundance within <5 months and to a later (5-12 months) change in AOB community structure. Activity in G-U and U-G was similar to that in U-U and G-G, respectively, after 12 months. Sequence analysis of 16S rRNA gene clones showed that AOB retrieved from soils fell within the Nitrosospira lineage and percentages of AOB related to known Nitrosospira groups were affected by grazing. These results demonstrate that AOB and AOA respond quickly to changes in management. The selection of nitrifiers adapted to novel environmental conditions was a prerequisite for nitrification enhancement in U-G, whereas nitrification decrease in G-U was likely due to a partial starvation and decrease in the abundance of nitrifiers initially present. The results also suggest that taxonomic affiliation does not fully infer functional traits of AOB.