Nitrification In Acid Soils Research Articles

Dominance of comammox Nitrospira in soil nitrification.

The first and limiting step of nitrification is catalyzed by ammonia-oxidizing archaea (AOA) and bacteria (AOB). Recently, complete ammonia oxidizers (comammox Nitrospira) have been discovered to perform complete nitrification in one cell, yet their role in soil nitrification is still unclear. This study investigated the abundance and contribution of aerobic ammonia oxidizers in typical soil habitats, and assessed the role of comammox Nitrospira in ammonia-oxidizing communities. The results showed that comammox Nitrospira were dominant in 70% of the samples and their abundance displayed a significant positive correlation with nitrification potential. The median amoA gene transcription level of comammox Nitrospira exceeded that of AOA and AOB in in-situ soils. The abundance of comammox Nitrospira was negatively correlated with soil pH, dominating in 84% of soil samples with pH<6.17. The results challenge the role of AOA and AOB in soils, highlighting the importance of comammox Nitrospira in soil nitrification, especially in acid soils. This work supports better understanding and regulation of the soil nitrogen cycle.

The effect of C:N ratio on heterotrophic nitrification in acidic soils

Generally, a high carbon to nitrogen (C:N) ratio is considered an important factor inducing heterotrophic nitrification in acidic soils; however, few studies have investigated the regulation of C:N ratio on heterotrophic nitrification. In the present study, a C:N ratio gradient (C:N 23, C:N 19, C:N 15, C:N 10) was regulated in acidic forest soil (SF) and acidic agricultural soil (SC) under 24 h and 30 days incubation, and 15N-labeling techniques combined with acetylene inhibition were used to distinguish between heterotrophic and autotrophic nitrification. Gene copy abundance and soil fungi and bacteria composition were also determined under 30 days C:N ratio regulation. The results showed that N input in C-abundant soils (e.g. SF in this investigation) and C input in N-abundant soil (e.g. SC in this investigation), instead of high C:N ratios, may stimulate soil heterotrophic nitrification. The stimulatory effect of acetic acid on heterotrophic nitrification in SC rather than SF further confirmed our conclusion. Soil fungal populations were significantly and positively correlated with gross heterotrophic nitrification rates and the ratio of heterotrophic nitrification rate to total nitrification rate, in addition to the contribution of heterotrophic NO3− production to total NO3− production in both the 15N-NH4+ and 15N-Glycine treatments, indicating soil fungi could play the key role in driving heterotrophic nitrification. In addition, our results suggested that Mortierella and Trichoderma were the potential fungal species driving heterotrophic nitrification in acidic forest soils, while Exophiala and Acidomelania seemed to be the fungal species driving heterotrophic nitrification in acidic agricultural soils according to their high abundance in C-abundant treatments. Consequently, initial soil C and soil N status should be taken into account when predicting the responses of soil heterotrophic nitrification to C or N soil inputs.

Heterotrophic Nitrification - Effects of N-Serve on Ammonia Oxidation and Nitrification in Acid Soils of the Nilgiris

Nitrification process in the acid soils (pH 6.7, 5.6 and 4.3) of the Nilgiris was studied. Accumulation of nitrate was found to be higher in the soils with pH 6.7 and 5.6 than the soil with pH 4.3. Accruement of nitrite was observed along with nitrate only in the soil with pH 4.3 after 42 days of incubation. Addition of N-Serve, an inhibitor of autotrophic ammonium oxidizers and chlorate, (an inhibitor of nitrite oxidizers) completely inhibited the nitrate and nitrite formation in the soils of pH 5.6 and 6.7, but only 50% reduction in the formation of nitrite and nitrate was observed in the soil of pH 4.3. Nine heterotrophic organisms isolated from the soil with pH 4.3 were tested for their heterotrophic nitrifying ability in glucose peptone liquid medium. Among nine, two fungal isolates identified as Fusarium sp. and Penicillium sp. were found to produce significant quantities of nitrite and nitrate. The amount of nitrite and nitrate production by these fungi varied with different cultural conditions viz., nitrogen sources and pH.

Improving the fertility of sandy soils in the temperate region by combined biochar and microbial inoculant treatments

ABSTRACTAchieving a sustainable increase in the fertility of sandy soils is a major problem. The application of biochar (BC) is a relatively new method, but results for a temperate climate are scarce. This work investigates various combined doses of BC and plant growth-promoting rhizobacteria inoculants on the fertility of acidic and calcareous sandy soils, hypothesising that joint application could have a synergistic impact. The effects were followed by measuring chemical (pH, organic matter, nutrient availability) and microbiological properties of the soil and maize biomass. 30 t ha−1 BC increased both pH (by 1 unit) and nitrification in acidic soil (causing a 60% drop in NH4-N concentration), and the phosphorus and potassium availability (by 53 and 80%, respectively) in both soils. Substrate-induced respiration increased by up to 100% and 50% in acidic and calcareous soil, respectively, in treatments involving both BC and inoculant. In acidic soil a BC dose of 3 t ha−1 resulted in a 70% decrease in arbuscular mycorrhizal fungal infection. Changes in maize grain yield were not significant, but the increase in above-ground biomass indicated that the combined application of BC and inoculant is more beneficial than separate application of these yield-increasing agents, primarily on acidic sand.

Nitrogen additions inhibit nitrification in acidic soils in a subtropical pine plantation: effects of soil pH and compositional shifts in microbial groups

Plantation forests play a pivotal role in carbon sequestration in terrestrial ecosystems, but enhanced nitrogen (N) deposition in these forests may affect plantation productivity by altering soil N cycling. Hence, understanding how simulated N deposition affects the rate and direction of soil N transformation is critically important in predicting responses of plantation productivity in the context of N loading. This study reports the effects of N addition rate (0, 40, and 120 kg N ha−1 a−1) and form (NH4Cl vs. NaNO3) on net N mineralization and nitrification estimated by in situ soil core incubation and on-soil microbial biomass determined by the phospholipid fatty acid (PLFA) method in a subtropical pine plantation. N additions had no influences on net N mineralization throughout the year. Net nitrification rate was significantly reduced by additions of both NH4Cl (71.5) and NaNO3 (47.1%) during the active growing season, with the stronger inhibitory effect at high N rates. Soil pH was markedly decreased by 0.16 units by NH4Cl additions. N inputs significantly decreased the ratio of fungal-to-bacterial PLFAs on average by 0.28 (49.1%) in November. Under NH4Cl additions, nitrification was positively related with fungal biomass and soil pH. Under NaNO3 additions, nitrification was positively related with all microbial groups except for bacterial biomass. We conclude that simulated N deposition inhibited net nitrification in the acidic soils of a subtropical plantation forest in China, primarily due to accelerated soil acidification and compositional shifts in microbial functional groups. These findings may facilitate a better mechanistic understanding of soil N cycling in the context of N loading.

Ammonia-oxidising archaea living at low pH: Insights from comparative genomics.

SummaryObligate acidophilic members of the thaumarchaeotal genus Candidatus Nitrosotalea play an important role in nitrification in acidic soils, but their evolutionary and physiological adaptations to acidic environments are still poorly understood, with only a single member of this genus (Ca. N. devanaterra) having its genome sequenced. In this study, we sequenced the genomes of two additional cultured Ca. Nitrosotalea strains, extracted an almost complete Ca. Nitrosotalea metagenome‐assembled genome from an acidic fen, and performed comparative genomics of the four Ca. Nitrosotalea genomes with 19 other archaeal ammonia oxidiser genomes. Average nucleotide and amino acid identities revealed that the four Ca. Nitrosotalea strains represent separate species within the genus. The four Ca. Nitrosotalea genomes contained a core set of 103 orthologous gene families absent from all other ammonia‐oxidizing archaea and, for most of these gene families, expression could be demonstrated in laboratory culture or the environment via proteomic or metatranscriptomic analyses respectively. Phylogenetic analyses indicated that four of these core gene families were acquired by the Ca. Nitrosotalea common ancestor via horizontal gene transfer from acidophilic representatives of Euryarchaeota. We hypothesize that gene exchange with these acidophiles contributed to the competitive success of the Ca. Nitrosotalea lineage in acidic environments.

PH rather than nitrification and urease inhibitors determines the community of ammonia oxidizers in a vegetable soil

Nitrification inhibitors and urease inhibitors, such as nitrapyrin and N-(n-butyl) thiophosphoric triamide (NBPT), can improve the efficiencies of nitrogen fertilizers in cropland. However, their effects on ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) across different soil pH levels are still unclear. In the present work, vegetable soils at four pH levels were tested to determine the impacts of nitrification and urease inhibitors on the nitrification activities, abundances and diversities of ammonia oxidizers at different pHs by real-time PCR, terminal restriction fragment length polymorphism (T-RFLP) and clone sequence analysis. The analyses of the abundance of ammonia oxidizers and net nitrification rate suggested that AOA was the dominate ammonia oxidizer and the key driver of nitrification in acidic soil. The relationships between pH and ammonia oxidizer abundance indicated that soil pH dominantly controlled the abundance of AOA but not that of AOB. The T-RFLP results suggested that soil pH could significantly affect the AOA and AOB community structure. Nitrapyrin decreased the net nitrification rate and inhibited the abundance of bacterial amoA genes in this vegetable soil, but exhibited no effect on that of the archaeal amoA genes. In contrast, NBPT just lagged the hydrolysis of urea and kept low NH4+-N levels in the soil at the early stage. It exhibited no or slight effects on the abundance and community structure of ammonia oxidizers. These results indicated that soil pH, rather than the application of urea, nitrapyrin and NBPT, was a critical factor influencing the abundance and community structure of AOA and AOB.

长期施加氮肥及氧化钙调节对酸性土壤硝化作用及氨氧化微生物的影响

High levels of N fertilization and acid deposition could cause soil acidification directly and indirectly. The nitrogen cycle, especially nitrification, makes a great contribution to the acidification of agricultural soils across China, which further leads to the mobilization of potentially toxic metals such as aluminum (Al) and manganese (Mn) and decerases crop yields. Chemicals (e.g., CaO) are amended as soil conditioners to relieve soil acidification. Ammonia oxidation, the rate-limiting step in the nitrification process, is driven by ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA). Increasing evidence demonstrates that pH is one of the most important factors determining the niche separation of AOA and AOB, and AOA play the more important role in nitrification of acidic soils. However, abundant AOB have been detected in acidic soils but little is known about their ecological function. In this study, the effects of long-term N fertilization practices and liming on nitrification and ammonia oxidizers in acidic soils were investigated using quantitative PCR and DGGE methods combined with soil physiochemical analysis. Compared with a previous study conducted 6 years ago at the same site, N fertilizer application without liming further decreased soil pH (3.35—3.47) and potential nitrification rate (PNR) (0.02—0.14 µg NO-2 -N g-1 soil h-1), while 2 years liming alleviated soil acidification (pH 4.10—4.46) and increased PNR (0.22—0.34 µg NO-2 -N g-1 soil h-1) significantly. There was a significantly positive correlation between soil pH and PNR, indicating the increase in soil pH via liming had positive effects on nitrification in acidic soils. AOA amoA gene copy numbers (7.40 ×107—4.08 ×108 copies/ g) were significantly higher than their counterpart AOB (1.67 ×106—2.57 ×107 copies/ g) in soils that received different chemical N fertilizers. Ratios of AOA and AOB amoA gene abundance ranged between 10.9 and 44.3. After amending lime for two years in the N fertilizers treatment plots, AOA amoA gene copy numbers (8.23 ×106—4.89 ×107 copies/ g) decreased to a certain degree but AOB amoA gene copy numbers (1.99 ×107—5.42 ×107 copies/ g) increased significantly compared to their corresponding fertilization treatments without liming,resulting in lower ratios of AOA to AOB amoA gene abundance (0.83—1.94). Moreover, liming showed no clear influence on AOA community structure, but significantly increased AOB community diversity. Taken together, these results indicate that although the activity and abundance of AOA was generally greater than those of AOB in acidic soils, both AOA and AOB actively responded to disturbance in acidic soils based on their niche differentiation and had different effects on nitrification of acidic soils.

PH regulates key players of nitrification in paddy soils

Increasing lines of evidence have suggested the functional importance of ammonia-oxidizing archaea (AOA) rather than bacteria (AOB) for nitrification in upland soils with low pH. However, it remains unclear whether niche specialization of AOA and AOB occurs in rice paddy wetlands constrained by oxygen availability. Using DNA-based stable isotope probing, we conclude that AOA dominated nitrification activity in acidic paddy soils (pH 5.6) while AOB dominated in alkaline soils (pH 8.2). Nitrification activity was stimulated by urea fertilization and accompanied by a significant increase of AOA in acid soils and AOB in alkaline soils. DNA-based stable isotope probing indicated significant assimilation of 13CO2 for AOA only in acidic paddy soil, while AOB was the solely responsible for ammonia oxidation in the alkaline paddy soil. Phylogenetic analysis further indicated that AOA members within the soil group 1.1b lineage dominated nitrification in acid soils. Ammonia oxidation in the alkaline soil was catalyzed by Nitrosospira cluster 3-like AOB, suggesting that the physiological diversity of AOA is more complicated than previously thought, and soil pH plays important roles in shaping the community structures of ammonia oxidizers in paddy field.

Ammonia oxidation-dependent growth of group I.1b Thaumarchaeota in acidic red soil microcosms.

Accumulating evidence suggests that Thaumarchaeota may control nitrification in acidic soils. However, the composition of the thaumarchaeotal communities and their functioning is not well known. Therefore, we studied nitrification activity in relation to abundance and composition of Thaumarchaeota in an acidic red soil from China, using microcosms incubated with and without cellulose amendment. Cellulose was selected to simulate the input of crop residues used to increase soil fertility by local farming. Accumulation of NO3-(-N) was correlated with the growth of Thaumarchaeota as determined by qPCR of 16S rRNA and ammonia monooxygenase (amoA) genes. Both nitrification activity and thaumarchaeotal growth were inhibited by acetylene. They were also inhibited by cellulose amendment, possibly due to the depletion of ammonium by enhanced heterotrophic assimilation. These results indicated that growth of Thaumarchaeota was dependent on ammonia oxidation. The thaumarchaeotal 16S rRNA gene sequences in the red soil were dominated by a clade related to soil fosmid clone 29i4 within the group I.1b, which is widely distributed but so far uncultured. The archaeal amoA sequences were mainly related to the Nitrososphaera sister cluster. These observations suggest that fosmid clone 29i4 and Nitrososphaera sister cluster represent the same group of Thaumarchaeota and dominate ammonia oxidation in acidic red soil.

Cultivation of an obligate acidophilic ammonia oxidizer from a nitrifying acid soil

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.

The influence of soil pH on the diversity, abundance and transcriptional activity of ammonia oxidizing archaea and bacteria

Autotrophic ammonia oxidation occurs in acid soils, even though laboratory cultures of isolated ammonia oxidizing bacteria fail to grow below neutral pH. To investigate whether archaea possessing ammonia monooxygenase genes were responsible for autotrophic nitrification in acid soils, the community structure and phylogeny of ammonia oxidizing bacteria and archaea were determined across a soil pH gradient (4.9-7.5) by amplifying 16S rRNA and amoA genes followed by denaturing gradient gel electrophoresis (DGGE) and sequence analysis. The structure of both communities changed with soil pH, with distinct populations in acid and neutral soils. Phylogenetic reconstructions of crenarchaeal 16S rRNA and amoA genes confirmed selection of distinct lineages within the pH gradient and high similarity in phylogenies indicated a high level of congruence between 16S rRNA and amoA genes. The abundance of archaeal and bacterial amoA gene copies and mRNA transcripts contrasted across the pH gradient. Archaeal amoA gene and transcript abundance decreased with increasing soil pH, while bacterial amoA gene abundance was generally lower and transcripts increased with increasing pH. Short-term activity was investigated by DGGE analysis of gene transcripts in microcosms containing acidic or neutral soil or mixed soil with pH readjusted to that of native soils. Although mixed soil microcosms contained identical archaeal ammonia oxidizer communities, those adapted to acidic or neutral pH ranges showed greater relative activity at their native soil pH. Findings indicate that different bacterial and archaeal ammonia oxidizer phylotypes are selected in soils of different pH and that these differences in community structure and abundances are reflected in different contributions to ammonia oxidizer activity. They also suggest that both groups of ammonia oxidizers have distinct physiological characteristics and ecological niches, with consequences for nitrification in acid soils.

The N 2O product ratio of nitrification and its dependence on long-term changes in soil pH

The contribution of nitrification to the emission of nitrous oxide (N 2O) from soils may be large, but its regulation is not well understood. The soil pH appears to play a central role for controlling N 2O emissions from soil, partly by affecting the N 2O product ratios of both denitrification (N 2O/(N 2+N 2O)) and nitrification (N 2O/(NO 2 −+NO 3 −). Mechanisms responsible for apparently high N 2O product ratios of nitrification in acid soils are uncertain. We have investigated the pH regulation of the N 2O product ratio of nitrification in a series of experiments with slurries of soils from long-term liming experiments, spanning a pH range from 4.1 to 7.8. 15N labelled nitrate (NO 3 −) was added to assess nitrification rates by pool dilution and to distinguish between N 2O from NO 3 − reduction and NH 3 oxidation. Sterilized soil slurries were used to determine the rates of chemodenitrification (i.e. the production of nitric oxide (NO) and N 2O from the chemical decomposition of nitrite (NO 2 −)) as a function of NO 2 − concentrations. Additions of NO 2 − to aerobic soil slurries (with 15N labelled NO 3 − added) were used to assess its potential for inducing denitrification at aerobic conditions. For soils with pH⩾5, we found that the N 2O product ratios for nitrification were low (0.2–0.9‰) and comparable to values found in pure cultures of ammonia-oxidizing bacteria. In mineral soils we found only a minor increase in the N 2O product ratio with increasing soil pH, but the effect was so weak that it justifies a constant N 2O product ratio of nitrification for N 2O emission models. For the soils with pH 4.1 and 4.2, the apparent N 2O product ratio of nitrification was 2 orders of magnitude higher than above pH 5 (76‰ and 14‰). This could partly be accounted for by the rates of chemodenitrification of NO 2 −. We further found convincing evidence for NO 2 −-induction of aerobic denitrification in acid soils. The study underlines the role of NO 2 −, both for regulating denitrification and for the apparent nitrifier-derived N 2O emission.

A hot-spot approach applied to nitrification in tropical acid soils

Several authors have reported that nitrification in acid soils may be restricted to microsites having a more favorable pH. The aim of this study was to propose a conceptual model of the functioning of nitrification in hot-spots, and to test it with the experimental data obtained in laboratory conditions using twelve tropical unamended and NH 4 + -amended soils with a wide range of pH (from 4.2 to 6.9). Nitrification was also measured in two selected soils where the pH was adjusted from 3.5 to 6.2. The model characterizes the relationship between the nitrification rates in unamended and NH 4 + -amended soils as a function of pH. It is based upon the assumption that nitrification of the NH 4 + coming from N mineralization occurs in the hot-spot (RN h), and the nitrification of the added NH 4 + occurs in the hot-spot and also in its adjacent surrounding region (RN s). The experimental design was chosen to be able to estimate both nitrification rates. Soil acidity limited nitrification more in NH 4 + -amended soils than in unamended ones. From our approach, this is due to less favorable conditions for nitrification in the region surrounding the hot-spot. The effect of self-induced acidity on nitrification was not noticeable neither in unamended nor in NH 4 + -amended treatments. The model described well three observations made in the experiments: (i) the minimum pH for nitrification to occur was lower for RN h (pH<4.2) than for RN s (pH<4.7), (ii) the RN h/RN s ratio increased with the decrease of pH (from 1.5 at pH 6 to 8.5 at pH 4), and (iii) for a given pH, the RN h/RN s ratio increased with the decrease of the initial pH of the soil. Among the soil parameters determined in this study (i.e. exchangeable Al, EDTA-extractable Cu and Zn, total C and N), only pH was related to nitrification. However, for a given pH, nitrification varied 3-fold among soils, depending upon their initial pH. This suggests that soil pH as determined on bulk soil is not suitable to predict nitrification in each individual soil, because it is not representative of the acidity level within the hot-spot.

Autotrophic ammonia oxidation at low pH through urea hydrolysis.

Ammonia oxidation in laboratory liquid batch cultures of autotrophic ammonia oxidizers rarely occurs at pH values less than 7, due to ionization of ammonia and the requirement for ammonium transport rather than diffusion of ammonia. Nevertheless, there is strong evidence for autotrophic nitrification in acid soils, which may be carried out by ammonia oxidizers capable of using urea as a source of ammonia. To determine the mechanism of urea-linked ammonia oxidation, a ureolytic autotrophic ammonia oxidizer, Nitrosospira sp. strain NPAV, was grown in liquid batch culture at a range of pH values with either ammonium or urea as the sole nitrogen source. Growth and nitrite production from ammonium did not occur at pH values below 7. Growth on urea occurred at pH values in the range 4 to 7.5 but ceased when urea hydrolysis was complete, even though ammonia, released during urea hydrolysis, remained in the medium. The results support a mechanism whereby urea enters the cells by diffusion and intracellular urea hydrolysis and ammonia oxidation occur independently of extracellular pH in the range 4 to 7.5. A proportion of the ammonia produced during this process diffuses from the cell and is not subsequently available for growth if the extracellular pH is less than 7. Ureolysis therefore provides a mechanism for nitrification in acid soils, but a proportion of the ammonium produced is likely to be released from the cell and may be used by other soil organisms.

Nitrification in acid soils: micro-organisms and mechanisms

Nitrification in acid soils was first reported in the beginning of the 20th century. Although this finding has been well substantiated by countless studies since then, it has until recently remained unclear which micro-organisms were responsible for nitrate production at low pH. Substantial evidence now supports the role of chemolitho–autotrophic bacteria as the main nitrifying agents in most acid soils. Heterotrophs may make some contribution to nitrification in acid soils, but this is difficult to demonstrate conclusively. Current insights in the phylogenetic position of autotrophic nitrifying bacteria in acid soils and the mechanisms that may enable them to be active at low pH are presented. In addition, the spatial variability of their activity and their contribution to the flux of the greenhouse gas N2O is discussed.

Ammonia oxidation at low pH by attached populations of nitrifying bacteria

Biofilm populations of Nitrosomonas europaea were formed in continuous flow sand columns supplied with defined inorganic medium containing 50 μg NH 4 +-N ml −1. Steady-state nitrite concentrations in effluent from the columns decreased as the pH of the inflowing medium was reduced but ammonia oxidation occurred at a pH value of 6, even though the pH minimum for growth of the organism in liquid batch culture was 7. No evidence was obtained for NH 3 limitation at low pH values, resulting from ionization to NH 4 +. The pH minimum for ammonia-oxidizing activity was also reduced in continuous flow vermiculite columns supplied with medium of pH 5.4, with an effluent pH of 6.3, but detailed assessment of the effects of low pH on activity was prevented by the strong buffering capacity of vermiculite. A multispecies, nitrifying biofilm, formed as wall growth in an ammonia-limited chemostat, was capable of ammonia-oxidizing activity, but not growth, at pH 5 while growth was possible at pH 5.5. The data provide evidence of the potential for autotrophic nitrification in acid soils.

Autotrophic nitrification in acid tea soils

Abstract Investigations to analyze the nature of nitrification in acid soils were carried out in the laboratory using 2 soil types from a tea field. The optimum temperature for the nitrification activity in these soils was 25°C and the activity was inhibited above 35°C, suggesting that the nitrifying bacteria became adapted to the soil conditions. The optimum NH4 +-N concentrations for nitrification ranged between 20 and 200 mg N/100 g of soil and the activity decreased at a concentration above 300 mg N. The effect of acidity on nitrification was studied by using soil samples amended with various amounts of CaCO3. Although the pattern of NO2¯ production changed, the rate of ammonia oxidation was not influenced by the addition of CaCO3. The nitrification activity was completely inhibited by the addition of nitrapyrin or acetylene. These results suggest that acid-tolerant or acidophilic autotrophic nitrification occurs in acid tea soils in Japan.

Effects of urea fertilization and liming on nitrification in cerrados soils (Brazil)

Abstract Nitrification is a microbiological process involved in the production of nitrate, which is important from the agricultural and environmental view points due to its mobility in soil. Also, in this form it is easily reduced by the denitrifying bacteria to gases which are released from the soil. Nitrification in soil is controlled by several environmental factors such as pH, mineral nutrition, temperature, and moisture (Haynes 1986). Nitrification has been found to be minimal at a pH value below 5.0 (Dancer et al. 1973; Gilmour 1984). However, nitrification can occur in some acid soils at a pH value below 4.5 (Weber and Gainey 1962; Weier and Gilliam 1986). Although nitrification in acid soils may be associated with the activity of autotrophic nitrifying bacteria (Hankinson and Schmidt 1988; Hayatsu 1993), heterotrophic microorganisms may also be involved in the nitrification process (Schimel et al. 1984; Stroo et al. 1986). In the Cerrados soils that are strongly acidic, information about the microorganisms involved in nitrification is limited.

Nitrous oxide production and nitrification in acidic soil from a dutch coniferous forest

The production of nitrous oxide and nitrification were studied using aerobic incubations of soil sampled from the F-layer of a nitrogen-saturated Douglas fir stand (The Netherlands). The nitrous oxide production (3 ng N 2O-N g −1 dry soil h −1) in the acid soil (pH 3.7) could be inhibited by 90% using low (3 Pa) or high (2.5 kPa) acetylene concentrations. The experiments with aerobic soil slurries showed that N;0 production at pH 4 was 4–8 times higher than at pH 6. Also the nitrification rate was higher at pH 4 than at pH 6. The ratio of N 2O-N to NO 3 −-N was higher at pH 4 (0.33–0.47%) than at pH 6 (0.04–0.18%). Acetylene (2.5 kPa) or nitrapyrin (10μgml −1) inhibited N 2O (40–96%) and NO 3 − production (79–100%) in the soil slurries indicating the importance of chemolithotrophic bacteria. The results showed that N 2O is produced in a nitrogen-saturated acid forest soil in North Western Europe even if well-aerated. The aggregated ammonium oxidizers ( Nitrosospira) that are responsible for ammonium oxidation in these acid soils may have importance in N 2O production. The possible significance of the aggregation of Nitrosospira cells for N 2O production is discussed.