Acceleration by montmorillonite of nitrification in soil

A soil naturally containing montmorillonite (M) was amended with 10% M and sequentially perfused with glycine, with fresh glycine being added every 16–17 d after nitrification of the previously added glycine-nitrogen had reached a plateau. In some systems, the old perfusates were replaced each time with a fresh glycine solution; in others, the initial perfusate was not replaced but only adjusted each time to the original 200 ml volume and a comparable glycine concentration (140 μg NH₂-N/ml). The incorporation of M enhanced the rates of heterotrophic degradation of glycine and subsequent autotrophic nitrification, but these stimulatory effects decreased with each successive perfusion. The reasons for these decreases are not known, but they did not appear to be related to inorganic nutrition, as perfusion with a mixed cation solution after five perfusion cycles did not significantly enhance nitrification in either the check or M-amended soils during three subsequent perfusions with glycine. The enhancement of nitrification by M appeared to be a result, in part, of the greater buffering capacity of the M-amended soil, as indicated by lesser reductions in the pH of perfusates from the M-amended soil, by titration curves of the soils, and by the greater and longer stimulation of nitrification in the cheek soil amended with 1% CaCO₃, which had a greater buffering capacity than did M. The stimulation by CaCO₃ may also have been partially the result of providing CO₂ for the autotrophic nitrifyers. Significant concentrations of nitrite accumulated only in perfusates from soil amended with CaCO₃. Air-drying and remoistening the soils enhanced nitrification of subsequently added glycine, especially in the check soil. The importance of pH-mediation, of the production of inhibitors, and/or of feed-back inhibition was indicated by the lower rate and extent of nitrification in systems wherein the perfusates were not replaced between successive additions of glycine. Although the results of these studies confirmed previous observations that M enhances the rate of nitrification in soil, the mechanisms responsible for this stimulation are still not known.     

Influence of Environmental Factors on Antagonism of Fungi by Bacteria in Soil: Nutrient Levels

The addition of 0.25, 0.5, or 1.0% glucose to a soil (K) amended with either 6% kaolinite (K6K) or montmorillonite (K6M) or the adjustment of the C/N ratio of the soils from 23/1 to 10/1 with NH₄NO₃ eliminated the inhibition of Aspergillus niger by Serratia marcescens, regardless of whether the fungus and bacterium were inoculated into the same or separate sites in the soils. The adjustment of the C/N ratio to 15/1 or of the C/P ratio from 1,000/1 to 100/1 with KH₂PO₄ did not eliminate the antagonism. However, with the higher glucose and NH₄NO₃ amendments, S. marcescens died out in the K and K6K (but not in the K6M) soils, apparently due to reductions in pH that resulted from the increased metabolism induced by added nutrients. In soils amended with CaCO₃, S. marcescens did not die out, but the inhibition of A. niger by S. marcescens or Agrobacterium radiobacter was eliminated or reduced by the addition of glucose, but not of NH₄NO₃, and was influenced by the clay mineralogy and pH of the soils. When NH₄NO₃ was added to the soils adjusted with CaCO₃ to pH values above 6.0, growth of A. niger was inhibited, regardless of whether bacteria were present or not, as a result of the volatilization of NH₃. Bacillus cereus and another species of Bacillus did not inhibit A. niger under any of the environmental conditions. There was a direct correlation between the degree of inhibition and the rate of glucose utilization by the various bacteria, indicating that the antagonism of A. niger by some bacteria in soil was the result primarily of a competition for carbon and that this competition was influenced by other environmental factors, such as pH and clay mineralogy.     

Ability of nitrapyrin,dicyandiamide and acetylene to retard nitrification in a mineral and an organic soil

Laboratory experiments were conducted to evaluate the efficacy of nitrapyrin, dicyandiamide (DCD) and acetylene (C₂H₂) as nitrification inhibitors in a silt loam and oragnic soil with and without added NH₄. Nitrapyrin (8 μg/g soil) and DCD (20 μg/g soil) were very effective in retarding nitrification of NH₄−N in the silt loam soil during 14 days of aerobic incubation at 30°C. However neither nitrapyrin, (20 μg/g soil) nor DCD (20 or 100 μg/g soil) were effective in retarding NO₃ production in the organic soil not amended with NH₄. Dicyandiamide was moderately effective in retarding nitrification (39% inhibition) at 100 μg/g concentration but nitrapyrin at 20 μg/g rate had little effect (8% inhibition) on nitrification in the organic soil amended with NH₄. In a separate experiment C₂H₂ was a very effective inhibitor in both soils when present in the flask atmosphere at 0.1% or 1% (v/v).     

Effects of the invasive plant Mikania micrantha H.B.K. on soil nitrogen availability through allelopathy in South China

Allelopathy has been regarded as a mechanism for successful exotic plant invasion. However, it is not clear if and what effects of allelopathic substances may exert on soil nutrient. The exotic plant Mikania micrantha H.B.K. (M. micrantha) has invaded many forests in south China, and recent studies have suggested it has allelopathic potential for other plants and soil microbial community. Thus, we hypothesized that M. micrantha could influence soil nutrients and N transformation through allelopathy. We measured total C and N, NO₃ ⁻, NH₄ ⁺ and pH of the soil beneath M. micrantha and the adjacent open soil, and then measured the above soil properties after treating soil with 3 concentrations of aqueous extracts of M. micrantha (T₁: 0.005 g ml⁻¹; T₂: 0.025 g ml⁻¹; T₃: 0.100 g ml⁻¹). In addition, a bioassay was conducted to determine the allelopathic potential of the soil beneath M. micrantha. The results showed that M. micrantha significantly affected soil nutrients and N transformation. Soil beneath M. micrantha had inhibitory effects on seed germination and seedling growth of test plant, and had significantly higher C, N, ammonia, net nitrification rate than those of open soil. The plant extracts decreased soil pH, and T₁ decreased it the most, and it increased soil C and N, and T₁ represented the greatest increase in both C and N. The extracts also increased both NO₃ ⁻ and NH₄ ⁺ in soil, whereas no significant difference existed among the 3 extract treatments. Compared to the water control, the soil net mineralization rate was higher under T₁, while lower under T₂ and T₃. However, the extracts increased the soil nitrification rates under all the treatments (T₁, T₂ and T₃). Our results suggest that the water soluble allelochemicals of M. micrantha improve soil nutrient availability, through which the invasive plant M. micrantha may successfully invade and establish in new habitats.     

Nitrification in soils of secondary vegetational successions from Eucalyptus forest and grassland to cool temperate rainforest in Tasmania

Rates of nitrification in well drained granitic soils from forest stands and grassland of differing successional status and from beneath isolated individuals of several tree species were compared in a series of laboratory experiments. Fresh samples were perfused with distilled water or nutrient solution for 10 to 14 weeks at 20°C. The following treatments were applied to the soils singly and in combination: 200 and 400 g N g^–1 as (NH₄)₂SO₄; 100 g P g^–1 as KH₂PO₄; 4000 g CaCO₃ g^–1; inoculation of non-nitrifying soil with nitrifying soil; perfusion of nitrifying soil with leachate from non-nitrifying soil.Nitrification was absent or occurred at only a low rate in many soils; it generally increased as succession proceeded from nature grassland or eucalypt forest towards climax temperate rainforest, but decreased in mature climax forests. However, the influence of individual tree species was often paramount. Nitrification was stimulated by disturbance of a stand by disease. A possible inhibitor of nitrification in a rainforest soil could not be removed by leaching with water, nor transferred via the leachate to a nitrifying soil. Addition of P was without effect on either total amount of nitrate produced or on net mineralisation of soil N, but sometimes increased the rate of nitrification of added ammonium. Non-nitrifying rainforest soil of pH 4.3 was induced to nitrify only after addition of (NH₄)₂SO₄, inoculation with a nitrifying soil, and addition of CaCO₃ to raise pH by 3 units. However, once nitrification had commenced it could continue with little change in rate while pH decreased to a value of 3.4.It was concluded that rate of nitrification is dependent upon the presence of particular tree species in a stand, upon its history of disturbance, and hence in part upon the stand's successional status. It is not limited by pHper se within the range found in these soils, although an increase in pH may be necessary to initiate nitrification. In some soils the rate of nitrification may be limited by the level of ammonium substrate, and nitrifiers are virtually absent from others. Overall microbial activity is limited by lack of utilisable carbon substrate.     

Fertilization influences nitrogen dynamics in soils under Dalbergia sissoo

The influence of added ammonium, phosphorus, potassium, and gypsum on net nitrogen mineralization was studied in soil beneath a six-year-old plantation of the N₂-fixing tree Dalbergia sissoo in Pakistan. Soil with and without amendments was placed in polyethylene bags and incubated, buried in the soil, for 30 days. After that time the soil was analyzed and net ammonium and nitrate production and net nitrogen mineralization were calculated. The addition of ammonium stimulated nitrification indicating that the process was substrate limited. The inhibition of nitrification by Nitrapyrin showed that the process is autotrophic in these soils. Gypsum addition lowered soil pH from 8.0 to 7.2 and significantly stimulated ammonification, nitrification and net nitrogen mineralization. The addition of potassium more than tripled the soil K:Na ratio. Net ammonium and nitrate production and net nitrogen mineralization all increased in this treatment. The addition of phosphorus had no significant effect on soil nitrogen dynamics.     

Enhanced Photodegradation of PNP on Soil Surface under UV Irradiation with TiO2

Photodegradation of p-nitrophenol (PNP) on soil surface was investigated to explore the photochemical remediation of soil polluted by nitrophenols. Soil samples spiked with PNP were irradiated by UV light with and without the addition of TiO ₂ . The addition of 0.5–2 wt% TiO ₂ enhanced PNP photodegradation with approximately 1.36 times increase in apparent rate of PNP disappearance. Soil moisture, humic acid and soil pH were important factors influencing the rate of PNP photodegradation. Increase in soil moisture improved the degradation significantly, whereas humic acid reduced the degradation rate. Changes in soil pH resulted in different degradation rates, and higher degradation efficiencies were observed under alkaline condition.     

Soil and fertilizer nitrogen transformations under alternate flooding and drying moisture regimes

Summary A study of changes in NH₄ ⁺ and NO₃ ^––N in Maahas clay amended with (NH₄)₂SO₄ and subjected to 4 water regimes in the presence and absence of the nitrification inhibitor N-Serve (Nitrapyrin) showed that the mineral N was well conserved in the continoous regimes of 50% and 200% (soil weight basis) but suffered heavy losses due to nitrification-denitrification under alternate drying and flooding. N-Serve was effective in minimizing these losses.Another incubation study with 3 soils showed that after 10 cycles of flooding and drying (either at 60°C or 25°C), the ammonification of soil N was enhanced. Nitrification of soil as well as fertilizer NH₄ ⁺ was completely inhibited upto 4 weeks by the treatments involving drying at high temperature. Flooding and air drying at 25°C, on the other hand, enhanced ammonification of soil N but retarded nitrification. These treatments, however, enhanced both ammonification and nitrification of the applied NH₄ ⁺ fertilizer N. Under flooded conditions rate of NH₄ ⁺ production was faster in soils that were dried at 60°C or 25°C and then flooded as compared to air dried soils.It is concluded that N losses by nitrification-denitrification and related N transformations may be considerably altered by alternating moisture regimes. Flooding and drying treatments seem to retard nitrification of soil N but conserve that of fertilizer NH₄ ⁺ applied after these treatments.     

The contribution of nitrogen transformation processes to total N2O emissions from soils used for intensive vegetable cultivation

The rapid expansion of intensively farmed vegetable fields has substantially contributed to the total N₂O emissions from croplands in China. However, to date, the mechanisms underlying this phenomenon have not been completely understood. To quantify the contributions of autotrophic nitrification, heterotrophic nitrification, and denitrification to N₂O production from the intensive vegetable fields and to identify the affecting factors, a ¹⁵N tracing experiment was conducted using five soil samples collected from adjacent fields used for rice-wheat rotation system (WF), or for consecutive vegetable cultivation (VF) for 0.5 (VF1), 6 (VF2), 8 (VF3), and 10 (VF4) years. Soil was incubated under 50% water holding capacity (WHC) at 25°C for 96 h after being labeled with ¹⁵NH₄NO₃ or NH ₄ ¹⁵ NO₃. The average N₂O emission rate was 24.2 ng N?kg^?1 h^?1 in WF soil, but it ranged from 69.6 to 507 ng N?kg^?1 h^?1 in VF soils. Autotrophic nitrification, heterotrophic nitrification and denitrification accounted for 0.3–31.4%, 25.4–54.4% and 22.5–57.7% of the N₂O emissions, respectively. When vegetable soils were moderately acidified (pH, 6.2 to ?≥?5.7), the increased N₂O emissions resulted from the increase of both the gross autotrophic and heterotrophic nitrification rates and the N₂O product ratio of autotrophic nitrification. However, once severe acidification occurred (as in VF4, pH?≤?4.3) and salt stress increased, both autotrophic and heterotrophic nitrification rates were inhibited to levels similar to those of WF soil. The enhanced N₂O product ratios of heterotrophic nitrification (4.84‰), autotrophic nitrification (0.93‰) and denitrification processes were the most important factors explaining high N₂O emission in VF4 soil. Data from this study showed that various soil conditions (e.g., soil salinity and concentration of NO ₃ ^- or NH ₄ ⁺ ) could also significantly affect the sources and rates of N₂O emission.     

Binding of nitrite-N on polyphenols during nitrification

Summary The conversion of substantial amounts of ammonia nitrogen into organic nitrogen as a direct result of nitrification at neutral pH-values, was monitored in soil suspensions amended with ammonium nitrogen. The influence of the chemolithotrophic nitrifying bacteria was verified by applying nitrapyrin as a selective inhibitor in control experiments. In addition, the role of phenolic compounds was examined by adding α naphthol. The factors influencing the nitrification processi.e. pH, NH ₄ ⁺ −N, NO ₂ ⁻ −N, NO ₃ ⁻ −N were measured during a 60 days incubation period. Nitrification started to be active after 5 and 10 days in the normal and the naphthol spiked soil suspensions respectively; it was inhibited in the nitrapyrin controls. Parallel with nitrification, formation of organic nitrogen was observed. The humic matter fractions were extracted and analyzed by I.R. spectroscopy which revealed the valence vibration ranges of nitro and nitroso groups fixed in different positions on aromatic compounds, both for normal and naphthol spiked samples. High resolution gas chromatography combined with mass spectroscopic analysis indicated the formation of nitrosonaphtholes. In addition a novel organic nitro compound was identifiedi.e. an azido nitro benzene. No nitrogen was fixed in the samples treated with nitrification inhibitor. A mechanism for the fixation of nitrite nitrogen during nitrification is proposed.     

Biodegradation of trichloroethylene and toluene by indigenous microbial populations in vadose sediments

The unsaturated subsurface (vadose zone) receives significant amounts of hazardous chemicals, yet little is known about its microbial communities and their capacity to biodegrade pollutants. Trichloroethylene (TCE) biodegradation occurs readily in surface soils; however, the process usually requires enzyme induction by aromatic compounds, methane, or other cosubstrates. The aerobic biodegradation of toluene and TCE by indigenous microbial populations was measured in samples collected from the vadose zone at unpolluted and gasoline-contaminated sites. Incubation at field moisture levels showed little activity on either TCE or toluene, so samples were tested in soil suspensions. No degradation occurred in samples suspended in water or phosphate buffer solution; however, both toluene and TCE were degraded in samples suspended in mineral salts medium. TCE degradation depended on toluene degradation, and little loss occurred under sterile conditions. Studies with specific nutrients showed that addition of ammonium sulfate was essential for degradation, and addition of other mineral nutrients further enhanced the rate. Additional studies with vadose sediments amended with nutrients showed similar trends to those observed in sediment suspensions. Initial rates of biodegradation in suspensions were faster in uncontaminated samples than in gasolinecontaminated samples, but the same percentages of chemicals were degraded. Biodegradation was slower and less extensive in shallower samples than deeper samples from the uncontaminated site. Two toluene-degrading organisms isolated from a gasoline-contaminated sample were identified as Corynebacterium variabilis SVB74 and Acinetobacter radioresistens SVB65. Inoculation with 10⁶ cells of C. variabilis ml^–1 of soil solution did not enhance the rate of degradation above that of the indigenous population. These results indicate that mineral nutrients limited the rate of TCE and toluene degradation by indigenous populations and that no additional benefit was derived from inoculation with a toluene-degrading bacterial strain. Correspondence to: K.M. Scow     

Urease gene‐containing Archaea dominate autotrophic ammonia oxidation in two acid soils

The metabolic traits of ammonia‐oxidizing archaea (AOA) and bacteria (AOB) interacting with their environment determine the nitrogen cycle at the global scale. Ureolytic metabolism has long been proposed as a mechanism for AOB to cope with substrate paucity in acid soil, but it remains unclear whether urea hydrolysis could afford AOA greater ecological advantages. By combining DNA‐based stable isotope probing (SIP) and high‐throughput pyrosequencing, here we show that autotrophic ammonia oxidation in two acid soils was predominately driven by AOA that contain ureC genes encoding the alpha subunit of a putative archaeal urease. In urea‐amended SIP microcosms of forest soil (pH 5.40) and tea orchard soil (pH 3.75), nitrification activity was stimulated significantly by urea fertilization when compared with water‐amended soils in which nitrification resulted solely from the oxidation of ammonia generated through mineralization of soil organic nitrogen. The stimulated activity was paralleled by changes in abundance and composition of archaeal amoA genes. Time‐course incubations indicated that archaeal amoA genes were increasingly labelled by ¹³CO₂ in both microcosms amended with water and urea. Pyrosequencing revealed that archaeal populations were labelled to a much greater extent in soils amended with urea than water. Furthermore, archaeal ureC genes were successfully amplified in the ¹³C‐DNA, and acetylene inhibition suggests that autotrophic growth of urease‐containing AOA depended on energy generation through ammonia oxidation. The sequences of AOB were not detected, and active AOA were affiliated with the marine Group 1.1a‐associated lineage. The results suggest that ureolytic N metabolism could afford AOA greater advantages for autotrophic ammonia oxidation in acid soil, but the mechanism of how urea activates AOA cells remains unclear.     

放牧对滇西北高原纳帕海沼泽化草甸湿地土壤氮转化的影响

选择位于滇西北高原纳帕海国际重要湿地内的典型沼泽化草甸湿地为研究对象,采用原位土柱室内控制实验法研究了放牧干扰(猪翻拱扰动和牲畜践踏)对沼泽化草甸湿地土壤氮转化的影响。研究结果表明,放牧活动显著提高了沼泽化草甸湿地表层土壤的容重和pH值,降低了土壤含水率、TOC、TN和NH_4~+-N含量,而对NO_3~--N含量影响不显著。放牧干扰下沼泽化草甸湿地土壤的矿化速率和硝化速率均表现为猪翻拱扰动样地(ZG)牲畜践踏样地(JT)对照样地(CK);表现为ZGJTCK。放牧干扰促进了沼泽化草甸湿地土壤的矿化和硝化作用,猪的翻拱活动比牲畜践踏活动对土壤氮矿化和硝化作用的促进作用更显著。放牧干扰下沼泽化草甸湿地土壤的反硝化速率表现为ZGCKJT,猪的翻拱活动促进了土壤N_2O气体的排放,而牲畜践踏活动抑制了土壤N_2O气体的排放。相关性分析表明,受放牧干扰的沼泽化草甸湿地土壤的矿化和硝化速率均与土壤容重、pH呈显著正相关,与土壤含水率、NH_4~+-N、TOC、TN含量呈显著负相关;反硝化速率与TOC含量呈显著负相关。     

Oil bioremediation in a high and a low phosphorus soil

Phosphorus (P) content may influence bioremediation of soils contaminated with crude oil. A soil testing high in plant available P (Weswood, 194 mg P kg^?1 soil) and one testing low in plant available P (Lufkin, 2 mg P kg^?1 soil) were selected for laboratory experiments on oil biodegradation. Plant available P content was determined using acidified ammonium acetate at pH 4.2 as the soil extractant. Soils were amended with 3, 6, and 9% crude oil by weight and incubated for 120 d at 25°C. Treatments consisted of a factorial arrangement, with soil, N, P, and oil concentration as factors. Addition of P without N generally did not enhance biodegradation. Addition of N without P approximately tripled the quantity of oil degraded. Addition of P and N together did not increase biodegradation of oil more than addition of N alone when oil concentration was 3%. At 6 and 9% oil concentrations, CO₂ evolution increased for both soils by adding P and N together in comparison to adding N alone, and total petroleum hydrocarbon (TPH) bio‐degradation increased by 30% for the Weswood soil by 60 d and at least 25% for the Lufkin soil by 30 d. The quantity of plant‐available P or total P in soil was not very useful in predicting need for supplemental P. Addition of P to soil to enhance oil degradation was only beneficial for oil concentrations above 3% and the positive effect for higher concentrations was transitory.     

Nitrogen mineralization and nitrification during incubation of East Pakistan ‘tea’ soils in relation to pH

Summary A study was made of the effects of increasing pH, by addition of varying levels of calcium carbonate, on N-mineralization and nitrification during aerobic incubation (30°C for 12 weeks) of two tea soils (original pH 4.1 and 4.2) from East Pakistan. Mineral-N (NH₃- plus NO₃-N) accumulation increased with pH in both soils. In the low-flat soil maximum nitrate accumulation occurred at pH 5.0, whilst at the higher pH levels mineral-N accumulated mainly as ammonia-N. In the high-flat soil nitrate accumulation increased considerably with pH; mineralized-N was accounted for largely as ammonia at pH 5.0 or less, and almost entirely as nitrate at higher pH levels. Results are discussed in relation to possible occurrence of heterotrophic nitrification in these soils.     

The response of seedlings of two dipterocarp species to nutrient additions and ectomycorrhizal infection

An experiment was conducted to investigate the effect of ectomycorrhizal infection on growth and nutrient uptake, especially of P and K of dipterocarp seedlings.Hopea helferi (Dyer) Blanco andHopea odorata Roxb. seedlings were grown in a sandy loam soil given a basal dressing. Nutrient treatments were unamended soil (NIL), amended soil with the addition of P and K (F), amended soil without P but with K (-P), amended soil without K but with P (-K), amended soil without P or K addition (-PK). Seedlings grown in the amended soil treatments showed foliar symptoms suggestive of calcium deficiency. Ectomycorrhizal infection appeared to improve shoot Ca concentration and relieved the foliar symptoms. Ectomycorrhizal infection inH. odorata plants increased shoot P concentration and increased shoot and total dry weight to the same or greater extent than those of uninfected plants growing on P amended soil.H. helferi showed a positive response to ectomycorrhizal infection in shoot, root and total dry weight in all nutrient treatments but no response to the nutrient treatments themselves.     

Influence of the phenolic compound bearing species Ledum palustre on soil N cycling in a boreal hardwood forest

The effects of the understory shrub Ledum palustre on soil N cycling were studied in a hardwood forest of Interior Alaska. This species releases high concentrations of phenolic compounds from green leaves and decomposing litter by rainfall. Organic and mineral soils sampled underneath L. palustre and at nearby non-Ledum sites were amended with L. palustre litter leachates and incubated at controlled conditions. We aimed to know (i) whether L. palustre presence and litter leachate addition changed net N cycling rates in organic and mineral soils, and (ii) what N cycling processes, including gross N mineralization, N immobilization and gross N nitrification, were affected in association with L. palustre. Our results indicate that N transformation rates in the surface organic horizon were not affected by L. palustre presence or leachate addition. However, mineral soils underneath L. palustre as well as soils amended with leachates had significantly higher C/N ratios and microbial respiration rates, and lower net N mineralization and N-to-C mineralization compared to no Ledum and no leachates soils. No nitrification was detected. Plant presence and leachate addition also tended to increase both gross N mineralization and immobilization. These results suggest that soluble C compounds present in L. palustre increased N immobilization in mineral soils when soil biota used them as a C source. Increases in gross N mineralization may have been caused by an enhanced microbial biomass due to C addition. Since both plant presence and leachate addition decreased soil C/N ratio and had similar effects on N transformation rates, our results suggest that litter leachates could be partially responsible for plant presence effects. The lower N availability under L. palustre canopy could exert negative interactions on the establishment and growth of other plant species.     

Effect of simazine,lindane and ceresan on soil respiration and nitrification rates

Summary The effect of three pesticidesviz, simazine, lindane and ceresan upto 100-fold of field application was studied on organic matter mineralization as well as on nitrification rates in Delhi alluvial soil. The normal rate of simazine (2 ppm) did not influence CO₂ evolution but higher concentrations suppressed it. Lindane and ceresan inhibited CO₂ production from soil by normal concentrations but the same CO₂ production was enhanced in berseem roots treated soil upto 10 ppm of ceresan.The nitrification of ammonium sulphate in soil due to these pesticides was impaired significantly for varying periods. The inhibitory effect of higher concentrations was marked only upto 3 weeks, subsequently the rate of nitrification was restored. Among the nitrifiers, Nitrobacter were more susceptible as compared to Nitrosomonas.     

Nitrification and utilization of ammonium and nitrate during oil bioremediation at different soil water potentials

Bioremediation of petroleum spills requires aerobic soil conditions and readily available N, which may be susceptible to leaching. Our objectives were to determine the influence of soil water potential on nitrification in the presence of crude oil, the toxicity of oil to NHj‐oxidizing bacteria, and the preferences of microorganisms for NH⁺ ₄ or NO^? ₃. A Weswood clay loam was amended with crude oil to contain 0, 5, and 10% by soil dry weight, and N was added to achieve C:N ratios of 90:1 and 120:1. Soil water potentials were maintained at ‐0.02, ‐0.1, and ‐1.0 kJ/kg or allowed to fluctuate between ‐0.02 and ‐3 kJ/kg. Concentrations of NH⁺ ₄ and NO₃ ^?were measured during an incubation period of 40 d. Nitrification in soil not amended with oil was rapid at water potentials of ‐0.02 and ‐0.1 kJ/kg but inactive at a water potential of ‐1.0 kJ/kg. Oil reduced nitrification rates and populations of NH⁺ ₄‐oxidizing bacteria. Little NO^? ₃ accumulated when the C:N ratio was 120:1, but when the C:N ratio was 90:1, up to 150 μg of NO₃‐N/g of soil accumulated at a soil water potential of ‐0.02 kJ/kg. Soil water potential in the range used did not greatly influence the extent of oil bioremediation but significantly influenced nitrification. Ammonium was preferentially used over NO^? ₃ by microorganisms during oil bioremediation. Nitrate accumulation from urea applied to stimulate oil bioremediation was low when N application matched requirements for oil bioremediation, and nitrification was restricted by controlling soil water content.