Induction of enhanced CH4 oxidation in soils: NH4 inhibition patterns

The influence of NH₄⁺ on microbial CH₄ oxidation is still poorly understood. Therefore, the influence of NH₄Cl and (NH₄)₂SO₄ on CH₄ oxidation was studied in soils at the different stages of the induction of enhanced methanotrophic activity. After a brief peak in the methanotrophic activity, a steady state was observed in which NH₄⁺ inhibited CH₄ oxidation at low CH₄ concentrations, and stimulated CH₄ oxidation at high concentrations. Chloride did not strongly inhibit CH₄ oxidation during this phase. During a second phase methanotrophic activity increased again. Ammonium no longer stimulated CH₄ oxidation, and Cl⁻ became an important source of uncompetitive inhibition. It was hypothesized that type I methanotrophs dominated during the first, soil-N-dependent phase while N₂-fixing type II methanotrophs dominated the second, soil-N-independent phase.     

Methane oxidation in arable soil as inhibited by ammonium, nitrite, and organic manure with respect to soil pH

 The effects of inorganic N and organic manure, applied to a loamy arable soil, on CH₄ oxidation were investigated in laboratory incubation experiments. Applications (40 mg N kg^–1) of NH₄Cl, (NH₄)₂SO₄, and urea caused strong instantaneous inhibition of CH₄ oxidation by 96%, 80%, and 84%, respectively. After nitrification of the added N the inhibitory effect was not fully reversible, resulting in an residual inhibition of 21%, 16%, and 25% in the NH₄Cl, (NH₄)₂SO₄, and urea treatments, respectively. With large NH₄ ⁺ applications [240 mg N kg^–1 as (NH₄)₂SO₄] the residual inhibition was as high as 64%. Exogenous NO₂ ^– (40 mg NO₂ ^–-N kg^–1) initially inhibited CH₄ oxidation by 84%, decreasing to 41% after its oxidation. Therefore, applied NO₂ ^– was a more effective inhibitor of CH₄ consumption than NH₄ ⁺. Temporary accumulation of NO₂ ^– during nitrification of added N was small (maximum: 1.9 mg NO₂ ^–-N kg^–1) and thus of minor importance with respect to the persistent inhibition after NH₄ ⁺ or urea application. CH₄ oxidation after NaNO₃ (40 mg N kg^–1) and NaCl addition did not differ to that of the untreated soil. The effect of organic manures on CH₄ oxidation depended on their C/N ratio: fresh sugar beet leaves enhanced mineralization, which caused an instantaneous 20% inhibition, whereas after wheat straw application available soil N was rapidly immobilized and no effect on CH₄ oxidation was found. The 28% increase in CH₄ oxidation after biowaste compost application was not related to its C/N ratio and was probably the result of an inoculation with methanotrophic bacteria. Only with high NH₄ ⁺ application rates (240 mg N kg^–1) could the persistent inhibitory effect partly be attributed to a pH decrease during nitrification. The exact reason for the observed persistent inhibition after a single, moderate NH₄ ⁺ or urea application is still unknown and merits further study. Received: 31 October 1997     

Relationships between ammonia oxidizers and N2O and CH4 fluxes in agricultural fields with different soil types

Nitrous oxide (N₂O) is a greenhouse gas that contributes to the destruction of stratospheric ozone, and agricultural soil is an important source of N₂O. Aerobic soils are sinks for atmospheric methane (CH₄), a greenhouse gas. Ammonia monooxygenase (AMO) can oxidize CH₄, but CH₄ is mostly oxidized by methane monooxygenase (MMO), and CH₄ oxidation by AMO is generally negligible in the soil. We monitored the N₂O and CH₄ fluxes after urea application in fields containing different soils using an automated sampling system to determine the effects of environmental and microbial factors on the N₂O and CH₄ fluxes. The soil types were Low-humic Andosol (Gleyic Haplic Andosol), yellow soil (Gleyic Haplic Alisol) and gray lowland soil (Entric Fluvisol). Cumulative N₂O emissions from the yellow soil were higher than those from other soil types, although the difference was not significant. The CH₄ uptake level by Andosol was one order of magnitude higher than that by other soils. There were significant relationships between the ammonia oxidation potential, AOB and AOA amoA copy numbers, and the CH₄ uptake. In contrast, the gene copy numbers of methane-oxidizing bacteria (MOB) pmoA were below the detection limit. Our results suggested that the AMOs of AOB and AOA may have more important roles than those previously considered during CH₄ oxidation in agricultural soils treated with N fertilizers.     

Recovery of nitrification and production of NO and N2O after exposure of soil to acetylene

Incubation of soil under low partial pressures of acetylene (10 Pa) is a widely used method to specifically inhibit nitrification due to the suicide inhibition of ammonium monooxygenase (AMO), the first enzyme in NH₄ ⁺ oxidation by nitrifying bacteria. Although the inhibition of AMO is irreversible, recovery of activity is possible if new enzyme is synthesized. In experiments with three different soils, NH₄ ⁺ concentrations decreased and NO₃ ^– concentrations increased soon after acetylene was removed from the atmosphere. Recovery of NO production started immediately after the removal of acetylene. The release rates of NO and N₂O were higher in soil samples which were only preincubated with 10 Pa acetylene than in those which were kept in the presence of 10 Pa acetylene. In the permanent presence of 10 Pa acetylene, NH₄ ⁺ and NO₃ ^– concentrations stayed constant, and the release rates of NO and N₂O were low. These low release rates were apparently due to processes other than nitrification. Our experiments showed that the blockage of nitrification by low (10 Pa) acetylene partial pressures is only reliable when the soil is kept in permanent contact with acetylene. Received: 17 July 1996     

Evaluation of Ammonia Recovery from a Laboratory Static Diffusion Chamber System

Ammoniacal fertilizers are susceptible to ammonia (NH₃) volatilization, and multiple methods have been introduced to quantify loss. Methods to quantify differences in NH₃ loss are important for evaluating the effectiveness of treatments. Recent research hypothesized that opening chamber enclosures resulted in nitrogen (N) loss (16–30%). Thus, the recovery efficiency of static diffusion chambers used in laboratory experiments was investigated. Chambers with a sand–calcium carbonate (CaCO₃) mixture received ammonium-N (NH₄-N) solutions. Three time intervals were used to determine if variation in enclosure opening influenced recovery. Acid trap percent recovery and a mass balance approach were used. No differences in cumulative NH₃ volatilization were measured from either acid traps or using a mass balance approach. No differences were measured in percent recovery based on N application rate, sample interval, or their interaction, and mean percent N recovery was 99.0%. Thus, diffusion chambers can be reliably used to measure differences in NH₃ volatilization.     

Structure and function of the oxidation products of polyphenols and identification of potent lipoxygenase inhibitors from Fe-catalyzed oxidation of resveratrol

Polyphenols have recently attracted much attention as potent antioxidants and related bioactive substances. These potent antioxidative polyphenols are very oxidizable due to their chemical properties, and their oxidation products must accumulate in the oxidizing foods when they are contained as the active ingredients. In this investigation, 30 polyphenols and related phenolics were oxidized with oxygen in the presence of a catalytic amount of Fe ions. Piceatannol, catechin, epicatechin, hydroxytyrosol, carnosol, and carnosic acid were oxidized very quickly. Sinapic acid, caffeic acid, chlorogenic acid, rosmarinic acid, gallic acid, propyl gallate, α-tocopherol, quercetin, and nordihydroguaiaretic acid were moderately oxidized. Protocatechuic acid, syringic acid, taxifolin, resveratrol, gentisic acid, secoisolariciresinol, and ellagic acid were oxidized for 19-20 days; however, their oxidation was very slow and did not complete. The other phenolics were not oxidized. The obtained oxidation products were next subjected to a lipoxygenase inhibition assay and the results compared to those of the corresponding phenols. Very interestingly, the oxidation product from resveratrol showed a high inhibitory activity, whereas resveratrol itself had no activity and its oxidation efficiency was low. To clarify the inhibition principle of the oxidation product, an LC-MS analysis was carried out on the oxidation product. The analytical results showed that they are the oligomeric and degraded compounds of resveratrol. Among them, the structures of three dimeric compounds were successfully identified, and their activity data clarified that the closed ring dimers were potent lipoxygenase inhibitors, whereas the opened ring dimer was not. It should be noted that resveratrol had almost no lipoxygenase inhibitory activity, contrary to some researchers' findings.     

Contributions of ammonia-oxidizing archaea and bacteria to nitrification in Oregon forest soils

Ammonia oxidation, the first step of nitrification, is mediated by both ammonia-oxidizing archaea (AOA) and bacteria (AOB); however, the relative contributions of AOA and AOB to soil nitrification are not well understood. In this study we used 1-octyne to discriminate between AOA- and AOB-supported nitrification determined both in soil-water slurries and in unsaturated whole soil at field moisture. Soils were collected from stands of red alder (Alnus rubra Bong.) and Douglas-fir (Pseudotsuga menziesii Mirb. Franco) at three sites (Cascade Head, the H.J. Andrews, and McDonald Forest) on acidic soils (pH 3.9–5.7) in Oregon, USA. The abundances of AOA and AOB were measured using quantitative PCR by targeting the amoA gene, which encodes subunit A of ammonia monooxygenase. Total and AOA-specific (octyne-resistant) nitrification activities in soil slurries were significantly higher at Cascade Head (the most acidic soils, pH < 5) than at either the H.J. Andrews or McDonald Forest, and greater in red alder compared with Douglas-fir soils. The fraction of octyne-resistant nitrification varied among sites (21–74%) and was highest at Cascade Head than at the other two locations. Net nitrification rates of whole soil without NH₄⁺ amendment ranged from 0.4 to 3.3 mg N kg⁻¹ soil d⁻¹. Overall, net nitrification rates of whole soil were stimulated 2- to 8-fold by addition of 140 mg NH₄⁺-N kg⁻¹ soil; this was significant for red alder at Cascade Head and the H.J. Andrews. Red alder at Cascade Head was unique in that the majority of NH₄⁺-stimulated nitrifying activity was octyne-resistant (73%). At all other sites, NH₄⁺-stimulated nitrification was octyne-sensitive (68–90%). The octyne-sensitive activity—presumably AOB—was affected more by soil pH whereas the octyne-resistant (AOA) activity was more strongly related to N availability.     

Influence of Salinity on Nitrogen Transformations in Soil

Laboratory experiments were carried out to study the influence of various salinity levels [1 (control), 9 (medium), 17 (high), and 27 dS m^–1(strong)] on nitrogen (N) transformations in soil fertilized with urea and ammonium sulfate. Generally, soil salinization affected the normal pathway of N transformations. The results showed that salinity (medium to high) inhibited the second step of nitrification, causing nitrite (NO₂ ^?) accumulation in soil. The inhibition was more severe in cases of high level of salinity. The greatest salinity level caused inhibition of even the first step of nitrification, leaving more ammonium (NH₄)-N accumulation in soil. Severity in nitrification inhibition was observed with increase in salinity and rate of N application, which declined with time. Ammonium accumulation with increased salinity caused N losses in the form of ammonia (NH₃) volatilization. After 14 days, the NH₃ losses were 1.4-, 2-, and 5-fold greater at 9, 17, and 27 dS m^–1 than that of the control (1 dS m^–1). After 42 days, the losses reached up to 6-fold more than the control at the greatest salinity level. Initially (up to 14 days), NH₃ losses were more from urea than from ammonium sulfate, whereas at the later stages (42 days), the losses were almost equal from both the fertilizers. The overall results revealed significant adverse effects of salinity on N transformations in soil.     

Suppression of methane oxidation in aerobic soil by nitrogen fertilizers,nitrification inhibitors,and urease inhibitors

Concentrations of CH₄, a potent greenhouse gas, have been increasing in the atmosphere at the rate of 1% per year. The objective of these laboratory studies was to measure the effect of different forms of inorganic N and various N-transformation inhibitors on CH₄ oxidation in soil. NH ₄ ⁺ oxidation was also measured in the presence of the inhibitors to determine whether they had differential activity with respect to CH₄ and NH ₄ ⁺ oxidation. The addition of NH₄Cl at 25 g N g^-1 soil strongly inhibited (78–89%) CH₄ oxidation in the surface layer (0–15 cm) of a fine sandy loam and a sandy clay loam (native shortgrass prairie soils). The nitrification inhibitor nitrapyrin (5 g g^-1 soil) inhibited CH₄ oxidation as effectively as did NH₄Cl in the fine sandy loam (82–89%), but less effectively in the sandy clay loam (52–66%). Acetylene (5 mol mol^-1 in soil headspace) had a strong (76–100%) inhibitory effect on CH₄ consumption in both soils. The phosphoroamide (urease inhibitor) N-(n-butyl) thiophosphoric triamide (NBPT) showed strong inhibition of CH₄ consumption at 25 g g^-1 soil in the fine sandy loam (83%) in the sandy clay loam (60%), but NH ₄ ⁺ oxidation inhibition was weak in both soils (13–17%). The discovery that the urease inhibitor NBPT inhibits CH₄ oxidation was unexpected, and the mechanism involved is unknown.     

Methane oxidation,nitrification, and counts of methanotrophic bacteria in soils from a long-term fertilization experiment (”︁Ewiger Roggenbau” at Halle)

Aerobic soils are important sinks for atmospheric methane. CH₄ oxidation, mediated mainly by methanotrophic bacteria, is the responsible process, which is strongly inhibited by ammonium accessible for nitrification. An inhibitory effect immediately after fertilization as well as a long-term effect exists, which results from repeated ammonium applications and which is independent from the actual concentration of NH₄⁺-N in soil. This long-term effect could be caused by a shift in the microbial population of the soil. Thus, with soil samples from long-term fertilization treatments of the field experiment ”︁Ewiger Roggenbau” at Halle (Germany) incubation studies were conducted to investigate the interference between CH₄ oxidation and nitrification and to determine the cell numbers of methanotrophic bacteria. Including the treatments PK, NPK, and farmyard manure, which were established in 1878, a close negative correlation between CH₄ oxidation and net nitrification was found (r = —0.92). The CH₄ oxidation rates, determined with an initial concentration of 10 μl CH₄ l^—1, varied between 6.7 and 1.1 μg C kg^—1 d^—1 in the PK and NPK treatment, respectively. After application of NH₄Cl a strong inhibition of CH₄ oxidation occurred, which was 91%, 88%, 81%, and 63% in the treatments PK, NPK, FYM, and U (unfertilized), respectively. After a lag-phase of 2 to 3 weeks an incubation with high CH₄ concentrations (20 Vol.% CH₄) could induce CH₄ oxidizing activity in the NPK treatments under continuous rye or maize cropping. An increase of up to 40 times in comparison to the control under atmospheric CH₄ (2 μl CH₄ l^—1) was observed. A negative correlation (r = —0.74) existed between the CH₄ oxidation rates of the soils without recently applied NH₄⁺ and the numbers of methanotrophic bacteria, determined with the ”︁most probable number” method (MPN). Thus, the MPN technique is not suitable to characterize the physiologically active population of methanotrophic bacteria in soils, which oxidize CH₄ in the atmospheric concentration range. The results of this study suggest that in aerobic arable soils methanotrophic bacteria and not nitrifiers are responsible for CH₄ oxidation.     

Treatment of High Ammonium–Nitrogen Wastewater from Composting Facilities by Air Stripping and Catalytic Oxidation

Composting municipal wastewater sludge may generate composting wastewater (acid washer water and tunnel wastewater) with high ammonium–nitrogen (NH₄–N) concentration; this kind of wastewater is usually generated in a rather small daily amount. A procedure of air stripping with catalytic oxidation was developed and tested with pilot-scale and full-scale units for synthetic disposal of the high NH₄–N wastewaters from composting facilities. In air stripping, around 90% NH₄–N removal efficiency was reliably achieved with a maximum of 98%. A model to describe the stripping process efficiency was constructed, which can be used for process optimization. After catalytic oxidation, the concentrations in the outlet gas were acceptable for NH₃, NO_X, NO₂, and N₂O, but the NH₃ and N₂O concentrations limited the feasible loading range. The treatment costs were estimated in detail. The results indicate that air stripping with the catalytic oxidation process can be applied for wastewater treatment in composting facilities.     

A simple biomonitor for measuring ammonia deposition in rural areas

Summary Knowledge of the contribution of NH₃ to the total deposition of N in rural areas is sparse, because the determination of NH₃ deposition is costly and labour-intensive. A simple biomonitor consisting of barley plants grown in pots with an inert growth medium is therefore proposed for estimating total N deposition, including NH₃. The rise in total N of the plant-soil system reflects the deposition. The biomonitor was tested near a dairy farm. Different N contents in the green biomass reflected differences in deposition, and the deposition correlated very well with NH₃ levels in the area and in two background stations. The biomonitors were placed above the crop, the measurements thus representing total N at the edge of a plant community. In 1 month the deposition in the NH₃ plume was 8 kg N ha^–1.     

Methane oxidation in a temperate coniferous forest soil: effects of inorganic N

Methane oxidation rates were measured in soils obtained from a coniferous forest in northern England. The effects of depth and added K⁺ (K₂SO₄), NH₄⁺ ((NH₄)₂SO₄) and NO₃⁻ (KNO₃) on potential CH₄ oxidation were investigated in a series of laboratory incubations. The humus (H) layer soil showed much greater CH₄ oxidation rates than the other soil layers, with maximal rates of 53 and 226 ng CH₄ gdw⁻¹ h⁻¹ when incubated with initial 10 and 1000 μl CH₄ l⁻¹, respectively. Additions of the solutes K⁺, NH₄⁺ and NO₃⁻ showed differing degrees of inhibition on CH₄ oxidation, which varied with the initial CH₄ concentration, the ion added, and the ion concentration. In general, inhibition by the ions was slightly greater for incubations with an initial concentration of 1000 μl CH₄ l⁻¹ than for 10 μl CH₄ l⁻¹ under otherwise identical conditions. For K⁺ and NH₄⁺ treatments, inhibitory rates were usually less than 15%, but at high K⁺ and NH₄⁺ concentrations inhibition could reach 50%, the inhibitory effects of NH₄⁺ were consistently slightly greater than those of K⁺ at the same concentration. In marked contrast to NH₄⁺, NO₃⁻ showed a very strong inhibitory effect. Added NO₃⁻ and NO₂⁻ produced via added NO₃⁻ reduction in anaerobic ‘microsites’ are probably toxic to CH₄-oxidizing bacteria. These results, together with those from other reports, suggest that NO₃⁻ may have a greater importance in the inhibition of CH₄ oxidation in forest soils than that attributed to NH₄⁺ and needs to be investigated in a wide range of soil types from various forests.     

Short-term effects of nitrogen on methane oxidation in soils

 The short-term effects of N addition on CH₄ oxidation were studied in two soils. Both sites are unfertilized, one has been under long-term arable rotation, the other is a grassland that has been cut for hay for the past 125 years. The sites showed clear differences in their capacity to oxidise CH₄, the arable soil oxidised CH₄ at a rate of 0.013 μg CH₄ kg^–1 h^–1 and the grassland soil approximately an order of magnitude quicker. In both sites the addition of (NH₄)₂SO₄ caused an immediate reduction in the rate of atmospheric CH₄ oxidation approximately in inverse proportion to the amount of NH₄ ⁺ added. The addition of KNO₃ caused no change in the rate of CH₄ oxidation in the arable soil, but in the grassland soil after 9 days the rate of CH₄ oxidation had decreased from 0.22 μg CH₄ kg^–1 h^–1 to 0.13 μg CH₄ kg^–1 h^–1 in soil treated with the equivalent of 192 kg N ha^–1. A ¹⁵N isotopic dilution technique was used to investigate the role of nitrifiers in regulating CH₄ oxidation. The arable soil showed a low rate of gross N mineralisation (0.67 mg N kg^–1 day^–1), but a relatively high proportion of the mineralised N was nitrified. The grassland soil had a high rate of gross N mineralisation (18.28 mg N kg^–1 day^–1), but negligible nitrification activity. It is hypothesised that since there was virtually no nitrification in the grassland soil then CH₄ oxidation at this site must be methanotroph mediated. Received: 31 October 1997     

稻田养萍模式下不同施氮量对稻田氨挥发及红萍生物固氮作用的影响

红萍对水体铵态氮浓度较为敏感，稻田放养红萍模式下，红萍的生物固氮作用及其抑制氨挥发的作用对不同施氮量的响应未知。红萍为水生蕨藻共生体，具有很强的生物固氮能力。红萍可作为优质绿肥放养于稻田，以替代部分化学氮肥，起到节能减排的效应。为明确稻田养萍模式下不同施氮量对红萍生物固氮作用和田间氨挥发的影响，采用盆栽试验设置了0、75、150、225、300kg/hm²共5个施氮(以纯N量计)水平，监测了稻田放养红萍和水稻单种各处理的氨挥发量、生物固氮速率和水稻产量。结果表明：(1)同一施氮水平下，稻田放养红萍可显著降低氨挥发日通量峰值及氨挥发总量。在施氮量为225 kg/hm²时，稻田放养红萍对氨挥发总量的抑制作用最大，与水稻单种相比，抑制幅度可达83.2%。(2)红萍的生物固氮速率及固氮总量与施氮量呈线性负相关关系，随施氮量的增加，固氮速率和固氮量逐渐降低，施氮量300 kg/hm²并放养红萍处理得到的固氮速率及总量同不施氮肥不养萍处理之间无显著差异。(3)与不养萍处理相比，放养红萍组各处理的水稻产量都明显增加，其中施氮量为225...     

The effect of nitrification inhibitors on the nitrous oxide (N2O) release from agricultural soils—a review

The use of nitrification inhibitors (NI) is a technique which is able to improve N fertilizer use efficiency, to reduce nitrate leaching and to decrease the emission of the climate‐relevant gas N₂O simultaneously, particularly in moderately fertilized agricultural systems adapted to plant N demand. The ammonia monooxygenase (AMO) is the first enzyme which is involved in the oxidation of NH$ _4^+ $ to NO$ _3^ - $ in soils. The inhibition of the AMO by NIs directly decreases the nitrification rate and it reduces the NO$ _3^- $ concentration which serves as substrate for denitrification. Hence, the two main pathways of N₂O production in soils are blocked or their source strength is at least decreased. Although it has been shown that archaea are also able to oxidize NH₃, results from literature suggest that the enzymatic activity of NH₃ oxidizing bacteria is the most important target for NIs because it was much stronger affected. The application of NIs to reduce N₂O emissions is most effective under conditions in which the NI remains close to the N ‐ fertilizer. This is the case when the NI was sprayed on mineral ‐ N fertilizer granules or thoroughly mixed with liquid fertilizers. Most serious problems of spatial separation of NI and substrate emerge on pasture soils, where N₂O hotspots occur under urine and to a lesser extent under manure patches. From the few studies on the effect of different NI quantities it seems that the amount of NI necessary to reduce N₂O emissions is below the recommendations for NI amounts in practice. NIs can improve the fertilizer value of liquid manure. For instance, the addition of NIs to slurry can increase N uptake and yield of crops when NO$ _3^ - $ ‐ N leaching losses are reduced. It has clearly been demonstrated that NIs added to cattle slurry are very effective in reducing N₂O as well as NO emissions after surface application and injection of slurry into grassland soils. In flooded rice systems NIs can reduce CH₄ emission significantly, whereas the effect on CO₂ emission is varying. On the other hand, as an effect of the delay of nitrification by NIs, NH₃ emission might increase when N fertilizers are not incorporated into the soil. As compared to other measures NIs have a high potential to reduce N₂O emissions from agricultural soils. Further, no other measure has so consistently been proofed according its efficiency to reduce N₂O emissions. From the published data [Akiyama et al. ( 2010 ) and more recent data from the years 2010–2013; 140 data sets in total] a reduction potential of approx. 35% seems realistic; however, further measurements in different management systems, particularly in regions with intense frost/thaw cycles seem necessary to confirm this reduction potential. These measurements generally should cover a whole annual cycle.     

Short-term population dynamics of ammonia oxidizing bacteria in an agricultural soil

Ammonia oxidizing bacteria (AOB) control the rate limiting step of nitrification, the conversion of ammonia (NH₄⁺) to nitrite (NO₂⁻). The AOB therefore have an important role to play in regulating soil nitrogen cycling. Tillage aerates the soil, stimulating rapid changes in soil N cycling and microbial communities. Here we report results of a study of the short term responses of AOB and net nitrification to simulated tillage and NH₄⁺ addition to soil. The intensively farmed vegetable soils of the Salinas Valley, California, provide the context for this study. These soils are cultivated frequently, receive large N fertilizer inputs and there are regional concerns about groundwater N concentrations. An understanding of N dynamics in these systems is therefore important. AOB population sizes were quantified using a real-time PCR approach. In a 15 day experiment AOB populations, increased rapidly following tillage and NH₄⁺ addition and persisted after the depletion of soil NH₄⁺. AOB population sizes increased to a similar degree, over a 1.5-day period, irrespective of the amount of NH₄⁺ supplied. These data suggest selection of an AOB community in this intensively farmed and C-limited soil, that rapidly uses NH₄⁺ that becomes available. These data also suggest that mineralization may play an especially important role in regulating AOB populations where NH₄⁺ pool sizes are very low. Methodological considerations in the study of soil AOB communities are also discussed.     

Nitrification: Interference by phenolic compounds

Effects of several phenolic acids, tea seed cotyledon, and tea waste (factory waste) powder on the rate of nitrification in aerated solution cultures were investigated. Both phenolic acids and tea seed cotyledon powder reduced the amount of nitrate (NO₃) produced from ammonium (NH₄) or nitrite (NO₂) oxidation significantly. When phenolic acids or tea seed powder were mixed with aerated NH₄ or NO₂ solutions under sterile (abiotic) conditions separately, there was a considerable loss of both NH₄‐N and NO₂‐N. It was concluded that the inhibitory effects of phenolic compounds on nitrification reported in literature are mainly due to fixation of NH₄ and volatilization of NO₂ by such chemicals resulting in a lower availability of substrates to nitrifying bacteria.     

Influence of iron pyrites and dicyandiamide on nitrification and ammonia volatilization from urea applied to loess brown earths (luvisols)

Laboratory incubation study showed that iron pyrites retarded nitrification of urea-derived ammonium (NH₄ ⁺), the effect being greatest at the highest level (10000 mg kg^–1 soil). Nitrification inhibition with 10000 mg pyrite kg^–1 soil, at the end of 30 days, was 40.3% compared to 55.9% for dicyandiamide (DCD). The inhibitory effect with lower rates of pyrite (100–500 mg kg^–1) lasted only up to 9 days. Urea+pyrite treatment was also found to have higher exchangeable NH₄ ⁺-N compared to urea alone. DCD-amended soils had the highest NH₄ ⁺-N content throughout. Pyrite-treated soils had about 7–86% lower ammonia volatilization losses than urea alone. Total NH₃ loss was the most with urea+DCD (7.9% of applied N), about 9% more than with urea alone. Received: 11 November 1995     

Frontiers in the microbial processes of ammonia oxidation in soils and sediments

Purpose

Two recent discoveries in nitrogen (N) cycling processes, i.e., archaeal ammonia oxidizers and anaerobic ammonia (ammonium) oxidation (anammox), have triggered great interest in studying microbial ammonia oxidation processes. The purpose of this review is to highlight recent progress in ammonia oxidation processes in soils and sediments and to propose future research activities in this topic.

Results and discussion

Aerobic ammonia oxidation and anammox processes are linked through the production and consumption of nitrite, respectively, thereby removing the reactive N (NH₄ ⁺, NO₂ ^?, NO₃ ^?) from soil and sediment ecosystems. Ammonia-oxidizing microorganisms are widely distributed in soils and sediments, and increasing evidence suggests that ammonia-oxidizing archaea and bacteria are functionally dominant in the ammonia oxidation of acid soils and other soils, respectively. The widespread occurrence and great variation in the abundance of anammox bacteria indicate their heterogeneous distribution and niche differentiation. Therefore, the worldwide distribution of both microbial groups in nature has stimulated researchers to investigate the physiology and metabolism of related groups, as well as appraising their contribution to N cycling.

Conclusions

We summarized the current progress and provided future perspectives in the microbiology of aerobic and anaerobic ammonia oxidation in soils and sediments. With increasing concern and interest in soil and sediment ammonia oxidation processes, studies in the microbial mechanisms underlying nitrification and anammox, as well as their interactions, are essential for understanding their contribution to the loss of N either through nitrate leaching or N-related gas emissions.