The release,diffusion and nitrification of nitrogen in soils surrounding sulfur-coated urea granules

Summary We have compared sulfur-coated urea granules (SCU) with ammonium sulfate granules (AS) in regard to nitrogen (N) release, diffusion, nitrification and the effect of irrigation. In the experiments plastic containers were filled with six layers of soil, separated from each other by fine nylon cloths. The fertilizer granules were placed between the two central layers, and irrigation was simulated by application of tap water to the uppermost layer. Nitrogen release from the SCU was slow, and after three months, 29.5% of the applied N remained in the granules. At the end of the experiment there was a deficit of 37.1% N in the case of the AS granules, while there was virtually none with the SCU. Throughout the experiment, N from SCU remained at a relatively even level, while 95% of the N applied as AS had disappeared after irrigation. Nitrification was rapid in both cases.     

Microclimatic controls of nitrogen mineralization and nitrification in shortgrass steppe soils

Summary The depth distributions of rates of net nitrogen mineralization and nitrification were measured in a series of field and laboratory incubations. Field studies suggested that the highest rates of mineralization and nitrification occurred in the surface 2.5 cm such that forty to sixty percent of the N mineralization in 20-cm soil column occurred in the surface 2.5cm. Some upward nitrate movement occurred but laboratory studies suggested that surface rates were not an artifact of nitrate mobility alone. Microclimatic data indicate that either dew or upward movement and condensation of soil water vapor may drive biological activity at the soil surface. High rates of N mineralization even in dry horizons were sustained as long as water was stored within the 0-to 20-cm depth. High rates of nitrification were found in all incubations, and field measurements showed NO ₃ ⁻ to be the predominant form of inorganic N, despite previous characterization of the shortgrass steppe as an NH ₄ ⁺ -dominated system.     

Leaching and transformation of urea in dry and wet soils as affected by irrigation water

Summary The leaching losses of urea in dry and wet soils as affected by the irrigation water, were studied in 90 cm long and 5 cm diameter plexiglass columns. In dry sandy loam soil urea leached with irrigation water and peaks of urea were observed with water front but in sandy soil the wetting front moved faster leaving the urea peak behind. In initially wet soils the urea peak did not coincide the wetting front, following infiltration and after redistribution for 24 hours of infiltration. More urea was recovered in sandy soil than in sandy loam soil following infiltration and redistribution for 24 hours. Urea leached only upto 30 cm with 5 and 7.5 cm irrigation water whereas with 10 and 15 cm irrigation water profiles urea penetrated to deeper layers. The sub-surface application of urea did not change the leaching behaviour of urea. Urea behaves as a non-reacting ion like Cl with respect to its movement with soil-water before it is hydrolysed.     

Distinguishing sources of base cations in irrigated and natural soils: evidence from strontium isotopes

Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) of soil solids, soil cation extracts, irrigation water and plant material are used to determine strontium and therefore cation sources and fluxes in irrigated and natural soil–plant systems. Strontium isotopes of soil solids from four soil profiles (two irrigated vineyard soils and two 'natural' profiles from nearby reserves) show large differences between soil horizons with depth. These differences are not reflected in ⁸⁷Sr/⁸⁶Sr ratios of soil labile cations, which show both little variation down-profile and much lower ratios than soil solids. In the undisturbed, natural soil profiles, labile cation ⁸⁷Sr/⁸⁶Sr ratios are close to the ratio of modern seawater, indicating that solutes of marine origin from precipitation are the primary input of strontium (and calcium) to the labile cation pool. In the irrigated soil profiles, ⁸⁷Sr/⁸⁶Sr ratios of labile cations are consistently shifted towards that of the irrigation water. Mass-balance calculations using ⁸⁷Sr/⁸⁶Sr ratios of the different inputs to the labile cation pool indicate more than 90% of labile strontium is derived from precipitation solutes in unirrigated soils, and up to 44% from irrigation water solutes as an additional source in irrigated soils. The ⁸⁷Sr/⁸⁶Sr ratios of grapes grown in irrigated soils match precisely with those of the labile soil cations, demonstrating that cation nutrients are drawn wholly from the labile cation pool and have the same mix of precipitation, irrigation, and soil solid sources. The ⁸⁷Sr/⁸⁶Sr ratios of grapes grown in the irrigated soils may therefore vary over time depending on (1) the changing mix of irrigation water and local precipitation and (2) potential change to irrigation water. These findings suggest limitations to the use of strontium isotopes in the tracing of grapes and wines to their soil of origin.     

Experimental determination of nitrogen kinetic isotope fractionation: Some principles; illustration for the denitrification and nitrification processes

Summary A few principles relative to the presentation and use of nitrogen stable isotopic data are briefly reviewed. Some classical relationships between the isotope composition of a substrate undergoing a single-step unidirectional reaction, are introduced. They are illustrated through controlled experiments on denitrification in a soil, and through nitrification by pure cultures ofNitrosomonas europaea. In the latter case, the isotope fractionation is calculated from the isotopic composition of the residual substrate, then of the product and the result is shown to be statistically the same for the two procedures. The isotopic enrichment factor for denitrification is −29.4±2.4‰ at 20°C, and −24.6±0.9‰ at 30°C; for nitrification this factor is −34.7±2.5‰ under the experimental conditions employed.     

Post-treatment of fish canning effluents by sequential nitrification and autotrophic denitrification processes

In this research study a nitrifying/autotrophic denitrifying system was used for the post-treatment of an effluent coming from an anaerobic digester treating the wastewater produced in a fish canning industry. The nitrifying reactor achieved 100% of ammonia oxidation into nitrate. The effluent from this unit was fed to the autotrophic denitrifying reactor which treated a maximum sulphide loading rate (SLR) of 200 mg S^2?/L d with removal percentages of 100% and 30% for sulphide and nitrate, respectively. The low nitrate removal efficiency is attributed to sulphide limitations.The operational costs of this system were estimated as 0.92 €/kg N_removed, lower than those for conventional nitrification/denitrification processes. For nitrogen removal the SHARON/anammox processes is the cheapest option. However the combination of nitrification and autotrophic denitrification (using elemental sulphur) processes would present a better operational stability compared to the SHARON/anammox system.     

Nitrification and nitrogen accumulation in the early stages of primary succession on Mt. Fuji

The relationship between nitrification potential and nitrogen accumulation was studied in an early successional sere on Mt. Fuji. Soil organic nitrogen accumulated with the invasion ofPolygonum cuspidatum and successively withMiscanthus oligostachyus and other species. Laboratory incubation experiments showed a higher nitrification potential at theM. oligostachyus state. The numbers of nitrifying bacteria increased with the progress of succession. No significant difference in nitrate reductase activity was found between pioneer and succeeding species. The soil solution at theM. oligostachyus stage contained a lower level of nitrate than rainwater, while that of the bare ground and theP. cuspidatum stage contained a higher nitrate level than rainwater. It was concluded that the high nitrate levels in the soil solution of the bare ground and theP. cuspidatum stage were due to lower nitrate-absorbing activity, leading to loss of nitrogen with precipitation, while the lower nitrate levels at theM. oligostachyus stage when higher nitrification activity occurred were due to higher nitrate-absorbing activity, preventing net loss of nitrogen from the ecosystem.     

Molecular analysis of the nitrogen cycle in deep-sea microorganisms from the Nankai Trough: genes for nitrification and denitrification from deep-sea environmental DNA

Nitrification and denitrification are bacterial functions, which are important for the global nitrogen cycle. Thus, it is important to study the diversity and distribution of bacteria in the environment, which are involved in the nitrogen cycle on the earth. Ammonia monooxygenase encoded by the amoA gene and nitrite reductase encoded by nirK or nirS are essential enzymes for nitrificaton and denitrification, respectively. These genes can be used as markers for the identification of organisms in the nitrogen cycle. In this study, we identified amoA (42 clones) and nirS (98 clones) genes in parallel from samples recovered from the deep-sea of the Nankai Trough. Genes for nirK could not be amplified from these samples. The obtained amoA sequences were not so closely related to those of amoA genes from previously isolated environmental organisms and those of genes from environmental DNAs. On the other hand, the nirS genes sequenced showed some relationship to some extent with the latter genes. However, some of the newly sequenced genes formed clusters, which contained no previously identified genes on a phylogenetic tree. These are likely present in specific denitrifiers from the deep-sea. The results of this study further suggest that nitrifiers and denitrifiers live in the same area of the Nankai Trough and the nitrogen cycle exists even in the deep-sea.     

Decline of soil nitrogen mineralization and nitrification during forest conversion of evergreen broad-leaved forest to plantations in the subtropical area of Eastern China

We examined soil nitrogen (N) mineralization and nitrification rates, and soil and forest floor properties in one native forest: evergreen broad-leaved forest (EBLF), one secondary shrubs (SS), and three adjacent plantation forests: Chinese fir plantation (CFP), bamboo plantation (BP) and waxberry groves (WG) in Tiantong National Forest Park, Eastern China. All forests showed seasonal dynamics of N mineralization and nitrification rates. Soil N mineralization rate was highest in EBLF (1.6 ± 0.3 mg-N kg⁻¹ yr⁻¹) and lowest in CFP (0.4 ± 0.1 mg-N kg⁻¹ yr⁻¹). Soil nitrification rate was also highest in EBLF (0.6 ± 0.1 mg-N kg⁻¹ yr⁻¹), but lowest in SS (0.02 ± 0.01 mg-N kg⁻¹ yr⁻¹). During forest conversion of EBLF to SS, CFP, BP and WG, soil N mineralization rate (10.7%, 73%, 40.3% and 69.8%, respectively), soil nitrification rate (94.9%, 32.2%, 33.9% and 39%, respectively), and soil N concentration (50%, 65.4%, 78.9% and 51.9%, respectively) declined significantly. Annual soil N mineralization was positively correlated with total C and N concentrations of surface soil and total N concentration of forest floor, and negatively correlated with soil bulk density, soil pH and C:N ratio of forest floor across the five forests. Annual soil nitrification was positively correlated with total C concentration of surface soil and N concentration of forest floor, and negatively correlated with soil bulk density and forest floor mass. In contrast, annual soil nitrification was not correlated to pH value, total N concentration, C:N ratio of surface soil and total C concentration and C:N ratio of forest floor.     

Land use and organic carbon content of some subtropical soils

Summary The assumption that the organic matter content of tropical forest soils is oxidized to atmospheric carbon dioxide when these soils are converted to agricultural use was tested using results of soil surveys in Puerto Rico (1940's, 1960's, and 1980's). Results showed that under intensive agricultural use, soil carbon in the top 18 cm of soil was about 30–37 Mg/ha, regardless of climatic conditions. Reduced intensity of agricultural use resulted in an increase of soil carbon in the order of 0.3–0.5 Mg.ha⁻¹. yr⁻¹ over a 40-yr period. Rates of soil carbon accumulation were inversely related to the sand content of soils. The relation between rates of soil carbon accumulation and climate or soil texture were better defined at higher soil carbon content. Soils under pasture accumulated soil carbon and often contained similar or greater amounts than adjacent mature forest soils (60–150 Mg/ha in the top 25 or 50 cm). Soils in moist climates exhibited greater variations in soil carbon content with changes in land use (both in terms of loss and recovery) than did soils in dry climates. However, in all life zones studied, the recovery of soil carbon after abandonment of agriculture was faster than generally assumed. Low carbon-to-nitrogen ratios suggested that intensively used soils may be stable in their nutrient retention capacity. The observed resiliency of these soils suggested that their role as atmospheric carbon sources has been overestimated, while their potential role as atmospheric carbon sinks has been underestimated.     

Actual and potential nitrogen fixation ofAlbizia rhizobium symbioses as affected by nitrate

Actual nitrogen fixation of root nodules of differentAlbizia-rhizobium symbioses, was compared with the potential nitrogen fixation of isolated bacteroids. The potential nitrogen fixation exceeded actual nitrogen fixation in all symbionts. After addition of nitrate the actual nitrogen fixation decreased more than did potential nitrogen fixation in effective symbiosis, whereas in a less effective symbiosis, the actual and potential nitrogen fixation increased as a result of better photosynthate supply to the roots and nodules. As confirmed by correlation analysis, the nitrogen fixation and photosynthetic yield of suboptimum symbioses were relatively enhanced by dressing with inorganic nitrogen fertilizer.     

Nitrogen losses in puddled soils as affected by timing of water deficit and nitrogen fertilization

Erratic rainfall in rainfed lowlands and inadequate water supply in irrigated lowlands can results in alternate soil drying and flooding during a rice (Oryza sativa L.) cropping period. Effects of alternate soil drying and flooding on N loss by nitrification-denitrification have been inconsistent in previous field research. To determine the effects of water deficit and urea timing on soil NO₃ and NH₄, floodwater NO₃, and N loss from added ¹⁵N-labeled urea, a field experiment was conducted for 2 yr on an Andaqueptic Haplaquoll in the Philippines. Water regimes were continuously flooded, not irrigated from 15 to 35 d after transplanting (DT), or not irrigated from 41 to 63 DT. The nitrogen treatments in factorial combination with water regimes were no applied N and 80 kg urea-N ha^–1, either applied half basally and half at 37 DT or half at 11 DT and half at 65 DT. Water deficit at 15 to 35 DT and 41 to 63 DT, compared with continuous soil flooding, significantly reduced extractable NH₄ in the top 30-cm soil layer and resulted in significant but small (<1.0 kg N ha^–1) soil NO₃ accumulations. Soil NO₃, which accumulated during the water deficit, rapidly disappeared after reflooding. Water deficit at 15 to 35 DT, unlike that at 41 to 63 DT, increased the gaseous loss of added urea N as determined from unrecovered ¹⁵N in ¹⁵N balances. The results indicate that application of urea to young rice in saturated or flooded soil results in large, rapid losses of N (mean = 35% of applied N), presumably by NH₃ volatilization. Subsequent soil drying and flooding during the vegetative growth phase can result in additional N loss (mean = 14% of applied N), presumably by nitrification-denitrification. This additional N loss due to soil drying and flooding decreases with increasing crop age, apparently because of increased competition by rice with soil microorganisms for NH₄ and NO₃.     

Evaluation of pilot-scale modified A2O processes for the removal of nitrogen compounds from sewage

In this study, pilot-scale experiments for the removal of nitrogen from sewage obtained from a county Y sewer system were performed using modified A₂O processes. Using this approach, the total amount of nitrogen discharged during denitrification of the influent was average 38.6 mg/L and a level of average 10.8 mg/L was maintained throughout the denitrification process, which resulted in an average removal efficiency that was greater than 72%. The nitrogen components in the effluent water consisted of 22% ammonia nitrogen, 6% nitrite nitrogen and 72% nitrate nitrogen, reaching a nitrification efficiency of 94%. In conclusion, since these advanced treatment methods, which involve modified A₂O processes, were successfully employed to remove nitrogen from sewage discharge, they hold promise for wide spread use by treatment plants.     

An evaluation of carbon disulphide as a sulphur fertilizer and as a nitrification inhibitor

There was little release of extractable SO₄-S during four weeks from CS₂ applied by injecting into two S-deficient soils. In this incubation experiment, the rate of CS₂ was 30 μg S g⁻, placement was injection at 9 cm depth, soil temperature was 20°C, and soil moisture tension was 33 kPa. The yield of barley forage after seven weeks in the greenhouse showed only small increases from 10 or 30 μg S g⁻¹ of CS₂ as compared to Na₂SO₄, on the two soils. While CS₂ supplied little plant available S in the short term, it was an effective inhibitor of nitrification. In the laboratory, or in the field, the injection of CS₂ (with N fertilizers) at a point 9 cm into the soils either stopped or reduced nitrification. In one laboratory experiment, 35 μg of CS₂ g⁻¹ of soil with urea reduced nitrification for at least four weeks; and in another experiment 20 μg of CS₂ g⁻¹ of soil with aqua NH₃ nearly or completely inhibited nitrification at 20 days. In two field experiments, 3 and 12 μg of CS₂ g⁻¹ of soil (or 6 and 24 kg ha⁻¹) with aqua NH₃ inhibited nitrification from October to the subsequent May. In addition, CS₂ reduced the amount of ammonium produced from the soil N, both in these two field experiments and in the laboratory experiments. That is to say, CS₂ injected at a point, inhibited both nitrification and ammonification. In other field experiments, CS₂ at a rate of 10 kg ha⁻¹ was injected in bands 9 cm deep with urea in October, and by May there was still reduced nitrification. Less than half of the fall-applied urea alone was recovered as mineral N, but with the application of CS₂ the recovery was increased to three-quarters. The yield and N uptake of barley grain was increased where fall-applied banded urea or aqua NH₃ received banded CS₂, (NH₄)₂CS₃, or K₂CS₃. The average increase in yield from fall-applied fertilizer, from inhibitor with fall-applied fertilizer, and from spring-applied fertilizer was 800, 1370, and 1900 kg ha⁻¹, respectively. In the same order, the apparent % recovery of fertilizer N in grain was 24, 42, and 60.     

Pea responses to saline stress is affected by the source of nitrogen nutrition (ammonium or nitrate)

The effect of the source of nitrogen nutrition (ammonium or nitrate), onthe response of pea plants to a moderate saline stress (30 mMNaCl)was studied. Growth declined under saline stress but nitrate-fed plants wereless sensitive to salinity than ammonium-fed plants. This different sensitivitywas due mainly to a better maintenance of root growth in nitrate-fed plants.Organic nitrogen content decreased significantly in roots of ammonium-fedplants. Water relations changed slightly under saline stress leading to adecrease in stomatal conductance, which was correlated to a decline in carbonassimilation rates regardless of nitrogen source. Salinity affects the uptakeofseveral nutrients in a different way, depending on the nitrogen source. Thus,chloride was accumulated mainly in nitrate-fed plants, displacing nitrate,whereas sodium was accumulated mainly in ammonium-fed plants, especially inroots, displacing other cations such as ammonium and potassium. It is concludedthat the nitrogen source (ammonium or nitrate) is a major factor affecting pearesponses to saline stress, plants being more sensitive when ammonium is thesource used. The different sensitivity is discussed in terms of a competitionfor energy between nitrogen assimilation and sodium exclusion processes.     

Growth of ectomycorrhizal mycelia and composition of soil microbial communities in oak forest soils along a nitrogen deposition gradient

Deciduous forests may respond differently from coniferous forests to the anthropogenic deposition of nitrogen (N). Since fungi, especially ectomycorrhizal (EM) fungi, are known to be negatively affected by N deposition, the effects of N deposition on the soil microbial community, total fungal biomass and mycelial growth of EM fungi were studied in oak-dominated deciduous forests along a nitrogen deposition gradient in southern Sweden. In-growth mesh bags were used to estimate the production of mycelia by EM fungi in 19 oak stands in the N deposition gradient, and the results were compared with nitrate leaching data obtained previously. Soil samples from 154 oak forest sites were analysed regarding the content of phospholipid fatty acids (PLFAs). Thirty PLFAs associated with microbes were analysed and the PLFA 18:2ω6,9 was used as an indicator to estimate the total fungal biomass. Higher N deposition (20 kg N ha⁻¹ y⁻¹ compared with 10 kg N ha⁻¹ y⁻¹) tended to reduce EM mycelial growth. The total soil fungal biomass was not affected by N deposition or soil pH, while the PLFA 16:1ω5, a biomarker for arbuscular mycorrhizal (AM) fungi, was negatively affected by N deposition, but also positively correlated to soil pH. Other PLFAs positively affected by soil pH were, e.g., i14:0, a15:0, 16:1ω9, a17:0 and 18:1ω7, while some were negatively affected by pH, such as i15:0, 16:1ω7t, 10Me17:0 and cy19:0. In addition, N deposition had an effect on the PLFAs 16:1ω7c and 16:1ω9 (negatively) and cy19:0 (positively). The production of EM mycelia is probably more sensitive to N deposition than total fungal biomass according to the fungal biomarker PLFA 18:2ω6,9. Low amounts of EM mycelia covaried with increased nitrate leaching, suggesting that EM mycelia possibly play an important role in forest soil N retention at increased N input.     

Botryococcus braunii carbon/nitrogen metabolism as affected by ammonia addition

Carbon metabolism in photosynthesizing and respiring cells of Botryococcus braunii was radically changed by the presence of 1 mM NH₄Cl in the medium, when the so-called resting state previously had been subjected to a nitrogen-deficient medium. Ammonia addition to the algae photosynthesizing with ¹⁴C-labelled HCO ₃ ^- almost completely inhibited the synthesis of ¹⁴C-labelled botryococcenes and other hexane-extractable compounds, and also inhibited the formation of insoluble compounds; however, it resulted in a large increase in the synthesis of alanine, glutamine, other amino acids, and especially of 5-aminolevulinic acid. Total CO₂ fixation decreased about 60% and O₂ evolution decreased more than 50%.CO₂ fixation in the dark with ammonia present led to labelled products derived from phosphoenolpyruvate carboxylation, such as glutamine, glutamate, and malate. Respiratory uptake of O₂ increased by about 70%.The inhibition of terpenoid synthesis and increased synthesis of C₅ amino acids by Botryococcus upon ammonia addition indicates 1) a diversion of acetyl coenzyme A from synthetic pathways leading to terpenoids and 2) increased operation of pathways leading to the synthesis of amino acids, especially 5-aminolevulinic acid, a precursor to chlorophyll biosynthesis.This work was supported in part by the Office of Energy Research, Office of Basic Energy Sciences, Biological Energy Research Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098, in part by a grant from SOHIO, and, in part, by a grant from the Japan-U.S. Cooperative Science Program (The Japan Society for the Promotion of Science, National Science Foundation, Division of International Programs)     

Response of coastal phytoplankton to ammonium and nitrate pulses: seasonal variations of nitrogen uptake and regeneration

Seasonal variation in uptake and regeneration of ammonium and nitrate in a coastal lagoon was studied using ¹⁵N incorporation in particulate matter and by measuring changes in particulate nitrogen. Uptake and regeneration rates were two orders of magnitude lower in winter than in summer. Summer uptake values were 2.8 and 2.2 mol N.l^–1.d^–1 for ammonium and nitrate, respectively. Regeneration rates were 2.9 and 2.1 mol N.l^–1.d^–1 for ammonium and nitrate respectively. Regeneration/uptake ratios were often below one, indicating that water column processes were not sufficient to satisfy the phytoplankton nitrogen demand. This implies a role of other sources of nitrogen, such as macrofauna (oysters and epibionts) and sediment. Phytoplankton was well adapted to the seasonal variations in resources, with mixotrophic dinoflagellates dominant in winter, and fast growing diatoms in summer. In winter and spring, ammonium was clearly preferred to nitrate as a nitrogen source, but nitrate was an important nitrogen source in summer because of high nitrification rates. Despite low nutrient levels, the high rates of nitrogen regeneration in summer as well as the simultaneous uptake of nitrate and ammonium allow high phytoplankton growth rates which in turn enable high oyster production.     

Nitrogen-removal bioreactor capable of simultaneous nitrification and denitrification for application to industrial wastewater treatment

A bioreactor system with 30 packed gel envelopes was installed in a thermal power plant for the removal of nitrogen from ammonia-containing desulfurization wastewater. Each envelope consisted of double-sided plate gels containing Nitrosomonas europaea and Paracoccus denitrificans cells with an internal space in between for injecting an electron donor. The envelope can remove ammonia from wastewater in a single step. When the wastewater was continuously treated with the bioreactor system, it removed 95.0% of the total nitrogen in the inlet, and the total nitrogen concentration in the outlet was below 9.0 mg L⁻¹. The maximum nitrogen removal rate was 6.0 g day⁻¹ per square meter of the gel area. The maximum utilization efficiency of the injected ethanol for denitrification was 98.4%, and the total organic carbon concentration in the outflow was maintained at a low level. Since the bioreactor system could use the electron donor effectively, it was not necessary to use an additional aerobic tank to remove the electron donor and a settling tank to segregate the surplus sludge containing bacteria from wastewater. Our concept of using packed gel envelopes would be highly effective for constructing a simple and efficient nitrogen removal system capable of simultaneous nitrification and denitrification.     

Community composition of ammonia-oxidizing bacteria and archaea in rice field soil as affected by nitrogen fertilization

Little information is available on the ecology of ammonia-oxidizing bacteria (AOB) and archaea (AOA) in flooded rice soils. Consequently, a microcosm experiment was conducted to determine the effect of nitrogen fertilizer on the composition of AOB and AOA communities in rice soil by using molecular analyses of ammonia monooxygenase gene (amoA) fragments. Experimental treatments included three levels of N (urea) fertilizer, i.e. 50, 100 and 150 mg N kg⁻¹ soil. Soil samples were operationally divided into four fractions: surface soil, bulk soil deep layer, rhizosphere and washed root material. NH₄⁺-N was the dominant form of N in soil porewater and increased with N fertilization. Cloning and sequencing of amoA gene fragments showed that the AOB community in the rice soil consisted of three major groups, i.e. Nitrosomonas communis cluster, Nitrosospira cluster 3a and cluster 3b. The sequences related to Nitrosomonas were predominant. There was a clear effect of N fertilizer and soil depth on AOB community composition based on terminal restriction fragment length polymorphism fingerprinting. Nitrosomonas appeared to be more abundant in the potentially oxic or micro-oxic fractions, including surface soil, rhizosphere and washed root material, than the deep layer of anoxic bulk soil. Furthermore, Nitrosomonas increased relatively in the partially oxic fractions and that of Nitrosospira decreased with the increasing application of N fertilizer. However, AOA community composition remained unchanged according to the denaturing gradient gel electrophoresis analyses.