Characterization of nitrifying bacteria communities of soils from different ecological regions of China by molecular and conventional methods

Soil nitrification rate is very different among soil types, as a result of differences in physical and chemical properties. Little is known about the composition of the nitrifying bacteria community. In this investigation, three soils (fluvo-aquic soil, permeable paddy soil and red earth) from different geo-ecological regions in China were characterized for their nitrification activities and their nitrifying bacteria communities determined either by molecular approaches or by conventional culture methods. A 28-day long-term soil incubation showed that the maximum nitrification potential was found in the fluvo-aquic soil with almost 100% of inorganic N present as NO₃^–-N, while the minimum nitrification potential was in red earth with only a 4.9% conversion rate from ammonium into nitrate. There was no relationship between nitrification potential and numbers of nitrifiers in the soil. The conventional most probable number (MPN) method could enumerate ammonia oxidizers, but failed in enumerating nitrite oxidizers. Therefore, we used an MPN-PCR procedure which gave a convincing nitrite oxidizer count result, instead of MPN-diphylamine. Soils were characterized by denaturing gradient gel electrophoresis (DGGE) of DNA extracted from soils and amplified using a primer specific for the 16S rRNA gene and/or for the amoA gene. The DGGE columns of the three soils differed from each other. There were two similar bands present in DGGE columns of the fluvo-aquic and permeable paddy soils, but no similar band was found in DGGE columns of the red earth. The sequence of amoA indicated that all ammonia oxidizers in these soils were grouped into Nitrosospira clusters 1 and 3, and each soil had a common band similar to the other soils and a special band which differed from the other soils.     

Numbers of autotrophic nitrifiers and nitrification in fertilized forest soil

The most probable number (MPN) method was used to estimate how numbers of autotrophic nitrifiers in Myrtillus-type and Calluna-type pine forest soils in southern Finland were affected by seven different fertilization treatments. No NH⁺₄ oxidizers and only a few hundred NO₂^?1 oxidizers g^?1 of soil were found in unfertilized organic (O) horizons. Ammonium nitrate and nitroform (ureaformaldehyde) had hardly any effect on the nitrifiers. Urea, alone or applied together with apatite + biotite or with apatite + biotite + micronutrients, increased numbers of NH₄⁺ and NO₂^? oxidizers. Wood ash, alone or with apatite, also had a stimulative effect. The effects of the stimulative fertilizers were less in the A₂ horizon than in the O horizon. The MPN counts were considerably affected by the duration of incubation: counts of NH₄⁺ oxidizers kept increasing for at least 8 weeks and counts of NO₂^? oxidizers for at least 15 weeks. These MPN counts were compared with earlier results from incubation experiments on the same soils to find out how they reflect changes in soil nitrification after fertilization.     

Soil pH and plant diversity drive co-occurrence patterns of ammonia and nitrite oxidizer in soils from forest ecosystems

In this study, we investigated how co-occurrence patters of ammonia and nitrite oxidizers, which drive autotrophic nitrification, are influenced by tree species composition as well as soil pH in different forest soils. We expected that a decline of ammonia oxidizers in coniferous forests, as a result of excreted nitrification inhibitors and at acidic sites with low availability of ammonia, would reduce the abundance of nitrite-oxidizing bacteria (NOB). To detect shifts in co-occurrence patterns, the abundance of key players was measured at 50 forest plots with coniferous respectively deciduous vegetation and different soil pH levels in the region Schwäbische Alb (Germany). We found ammonia-oxidizing archaea (AOA) and Nitrospira-like NOB (NS) to be dominating in numbers over their counterparts across all forest types. AOA co-occurred mostly with NS, while bacterial ammonia oxidizers (AOB) were correlated with Nitrobacter-like NOB (NB). Co-occurrence patterns changed from tight significant relationships of all ammonia and nitrite oxidizers in deciduous forests to a significant relationship of AOB and NB in coniferous forests, where AOA abundance was reduced. Surprisingly, no co-occurrence structures between ammonia and nitrite oxidizers could be determined at acidic sites, although abundances were correlated to the respective nitrogen pools. This raises the question whether interactions with heterotrophic nitrifiers may occur, which needs to be addressed in future studies.     

Potential mineralization and nitrification in volcanic grassland soils in Chile

Abstract

A proportion of the nitrogen (N) applied to grasslands as organic or inorganic fertilizers can be lost to water courses as nitrate and to the atmosphere as nitrous and nitric oxides. Volcanic soils from Chile are not generally prone to leaching, possibly due to net immobilization of nitrate and/or ammonium, and/or due to inhibition of nitrification by either chemical or physical processes. In laboratory studies we found large mineralization potentials in soils from three different Chilean soils after 17 weeks of incubation, totalling 215 and 254 mg kg^?1 dry soil for two Andisols and 127 mg kg^?1 dry soil in an Ultisol. Nitrification occurred after a short period, and was lowest in the Ultisol. In addition, microbial analysis showed nitrifiers to be present in all three soils. Adsorption of ammonium was two-fold stronger than for nitrate, ranging from 29 to 180 kg N ha^?1. The highest potential for N adsorption in the 0–60 cm soil profile was with the Ultisol (398 kg N ha^?1), but was similar in both Andisols (193 and 172 kg N ha^?1, respectively). The combination of ammonium retention together with delayed nitrification could account for the low leaching rates in these soils.     

Role of nitrite and nitric oxide in the processes of nitrification and denitrification in soil: Results from N tracer experiments

Recent research has proven soil nitrite to be a key element in understanding N-gas production (NO, N₂O, N₂) in soils. NO is widely accepted to be an obligatory intermediate of N₂O formation in the denitrification pathway. However, studies with native soils could not confirm NO as a N₂O precursor, and field experiments mainly revealed ammonium nitrification as the source of NO. The hypothesis was constructed, that the limited diffusion of NO in soil is the reason for this contradiction. To test this diffusion limitation hypothesis and to verify nitrite and NO as free intermediates in native soils we conducted through-flow (He/O₂ atmosphere) ¹⁵N tracer experiments using black earth soil in an experimental set up free of diffusion limitation. All of the three relevant inorganic N soil pools (ammonium, nitrite, nitrate) were ¹⁵N labelled in separate incubation experiments lasting 81 h based on the kinetic isotope method. During the experiments the partial pressure of O₂ was decreased in four steps from 20% to about 0%. The net NO emission increased up to 3.7 μg N kg⁻¹ h⁻¹ with decreasing O₂ partial pressure. Due to the special experimental set up with little to no obstructions of gas diffusion, only very low N₂O emission could be observed. As expected the content of the substrates ammonium, nitrate and nitrite remained almost constant over the incubation time. The ¹⁵N abundance of nitrite revealed high turnover rates. The contribution of nitrification of ammonium to the total nitrite production was approx. 88% under strong aerobic soil conditions but quickly decreased to zero with declining O₂ partial pressure. It is remarkable that already under the high partial pressure of 20% O₂ 12 % of nitrite is generated by nitrate denitrification, and under strict anaerobic conditions it increases to 100%. Nitrite is present in two separate endogenous pools at least, each one fed by the nitrification of ammonium or the denitrification of nitrate. The experiments clearly revealed that nitrite is almost 100% the direct precursor of NO formation under anaerobic as well as aerobic conditions. Emitted N₂O only originated to about 100% from NO under strict anaerobic conditions (0-0.2% O₂), providing evidence that NO is a free intermediate of N₂O formation by denitrification. To the best of our knowledge this is the first time that NO has been detected in a native soil as a free intermediate product of N₂O formation at denitrification. These results clearly verify the “diffusion limitation” hypothesis.     

Factors Affecting Nitrification in Soils

Abstract

Nitrification in soil converts relatively immobile ammonium‐nitrogen (N) to highly mobile nitrate‐N (via nitrite), and this has implications for N‐use efficiency by agricultural systems as well as for environmental quality, especially in situations where the potential for loss of soil or added N is high following nitrate formation. The literature on various physical, environmental, and chemical factors and their interactions on nitrification in soil is reviewed and discussed with examples from natural and agro‐ecosystems. Among the various factors, soil matrix, water status, aeration, temperature, and pH have strong influence on nitrification. The information on factors that influence nitrification is useful when developing strategies for regulating nitrification in soils by employing chemical or biological nitrification inhibitors.     

Nitrification Inhibitor,Fertilizer Rate,and Temperature Effects on Nitrous Oxide Emission and Nitrogen Transformation in Loamy Sand Soil

An incubation study investigated the effects of nitrification inhibitors (NIs), dicyandiamide (DCD), and neem oil on the nitrification process in loamy sand soil under different temperatures and fertilizer rates. Results showed that NIs decreased soil nitrification by slowing the conversion of soil ammonium (NH₄⁺)-nitrogen (N) and maintaining soil NH₄⁺-N and nitrate (NO₃^?)-N throughout the incubation time. DCD and neem oil decreased soil nitrous oxide (N₂O) emission by up to 30.9 and 18.8%, respectively. The effectiveness of DCD on reducing cumulative soil N₂O emission and retaining soil NH₄⁺-N was inconsistently greater than that of neem oil, but the NI rate was less obvious than temperature. Fertilizer rate had a stronger positive effect on soil nitrification than temperature, indicating that adding N into low-fertility soil had a greater influence on soil nitrification. DCD and neem oil would be a potential tool for slowing N fertilizer loss in a low-fertility soil under warm to hot climatic conditions.     

Effect of fulvic and humic acids on nitrificationPart 1: In vitro production of nitrite and nitrate

Abstract

Mixed opinions exist on the effect of organic matter on nitrification in soils as well as the lack of data on the effect of fulvic (FA) and humic (HA) acids on this biochemical process. This in vitro investigation was conducted to study the effect of FA and HA on oxidation of NH⁺ ₄ and NO^‐ ₂ by soil nitrifiers and on the delay period (t') and maximum nitrification rate (K_max). Soil extracts containing an ammonium‐oxidizer population were incubated in vitro for 3 weeks at 25C in the presence of (NH₄)₂SO₄and 0 to 320 mg FA or HA/L at pH 7.0 or 8.0. A similar experiment was conducted with soil extracts containing a nitrite‐oxidizer population, but with KNO₂ as the N source. An additional experiment was conducted with the nitrite‐oxidizer extracts for the determination of t’ and K_max values. Nitrite production tended to increase linearly as a result of FA and HA treatments from 0 to 320 mg/L at pH 7.0 or 8.0. Fulvic acid appeared to be more effective than HA in increasing the oxidation process. Differences in pH had only a slight effect. On the other hand, nitrate production was decreased linearly by FA or HA treatments from 0 to 320 mg/L which provided some justification for reports of lower nitrate, but higher nitrite concentrations in soils high in organic matter content. Humic acid treatments increased the delay period (t¹), and at the same time decreased the maximum nitrification rate (K_max). The latter suggests that in the presence of HA more time is required to reach a maximum nitrification rate.     

Simultaneous nitrification and diffusion in soil

Darrah et al. (1985b, 1986) presented a model that predicted the distribution of ammonium and nitrate in a column of soil, following the addition of ammonium chloride as a band of fertilizer to one end of the column. Ammonification and nitrification, the inhibition of nitrification by high levels of fertilizer addition and the simultaneous diffusion of ammonium and nitrate were modelled. By simplifying algorithms concerned with the calculation of the concentration of ammonium in solution and the diffusion of solutes in the column, up to eight-fold savings in the time required to run the model could be made without significantly affecting the accuracy of the predictions. This simplified version was used to test the effect of a ±25% change in the values of the main input parameters. The distribution of ammonium was mainly influenced by parameters affecting the diffusive flux of solutes and the concentration of ammonium in the soil solution; that of nitrate was influenced by parameters affecting the growth, activity and inhibition of the nitrifiers. In general, the model was most sensitive to changes in the maximum specific growth rate of the nitrifiers, while almost no effect was observed when the affinity constant for ammonium oxidation was varied. Given the present state of knowledge of the range of parameter values in different soils, it is necessary to measure all the input parameters tested, with the possible exception of the diffusional impedance factor, in order to obtain accurate predictions from the model.     

Effect of cropping systems on nitrification in soils

Abstract

Limited information is available about the effect of cropping systems and N application on nitrification potential of soils. This study was conducted to evaluate nitrification rates of soils that have been under long‐term cropping systems at three sites in Iowa. Each experiment consisted of three cropping systems (continuous corn, corn‐soybean‐corn‐soybean, and corn‐oats‐meadow‐meadow) and two fertilizer treatments: untreated (0 N) and treated (+ N) with ammonium or ammonium‐forming fertilizers (180 or 200 kg ha/yr) before corn. The rate of nitrification was studied at 30°C. Results showed that, although soil pH decreased in the plots treated with ammoniacal fertilizers before corn in the cropping system, the rate of nitrification was significantly greater in N‐treated than in untreated plots, suggesting that fertilization with ammonium or ammonium‐forming fertilizers either increased the microbial populations responsible for nitrification in soils and/or that such treatments increased the efficiency of the nitrifiers by inducing the enzymes responsible for conversion of NH⁴⁺ to NO₃‐. The results suggest that continuous application of ammonium or ammonium‐forming fertilizer could enhance the nitrification rate and increase the potential of contamination of groundwater with nitrate.     

Fluctuations in the abundance of ammonia oxidizers and the contents of inorganic nitrogen in solarized soil

Solarization makes a great impact on the abundance of ammonia oxidizers and nitrifying activity in soil. To elucidate fluctuations in the abundance of ammonia oxidizers and nitrification in solarized soil, copy numbers of amoA gene of ammonia-oxidizing bacteria (AOB) and archaea (AOA), viable number of ammonia oxidizers and inorganic nitrogen contents were investigated in greenhouse experiments. The copy number of amoA gene and the viable number of ammonia oxidizers were determined by the quantitative polymerase chain reaction and most probable number methods, respectively. Abundance of AOB based on the estimation of amoA gene copy numbers and viable counts of ammonia oxidizers was decreased by the solarization treatment and increased during the tomato (Solanum lycopersicum L.) cultivation period following the solarization. Effect of solarization on the copy number of amoA gene of AOA was less evident than that on AOB. The proportion of nitrate in inorganic nitrogen contents was declined by the solarization and increased during the tomato cultivation period following the solarization. Positive correlations were found between the proportion of nitrate in inorganic nitrogen content and the copy number of bacterial or archaeal amoA gene or the viable number of ammonia oxidizers; the copy number of bacterial amoA gene showed a strong correlation with the viable number of ammonia oxidizers. The present study revealed influences of solarization on the fluctuation in the abundance of ammonia oxidizers and dynamics of inorganic nitrogen contents in soil and the results indicate that the determination of amoA gene of AOB is possibly a quick and useful diagnostic technique for evaluating suppression and restoration of nitrification following solarization.     

Active methanotrophs suppress nitrification in a humisol

Summary The coexistence of chemoautotrophic nitrifiers and methanotrophs in a cultivated humisol was investigated. Under laboratory conditions which supported the growth and activity of methanotrophs, the nitrifiers were partially or completely inhibited. The inhibition was related to a competition for available oxygen and a high assimilatory requirement for inorganic nitrogen by the Methanotrophs. Dissolved methane concentrations as high as 250 M had no direct effect on the oxidation of ammonium. Simultaneous nitrification and methane oxidation was observed only if relatively high levels of ammonium and oxygen were maintained. Coupled nitrification-assimilatory/dissimilatory nitrate reduction resulted from the high oxygen demand of the actively growing methanotrophs. This study suggests that the potential competitive effects of methanotrophs may influence nitrification by chemoautotrophic nitrifiers in certain environmental systems.     

Relationships between soil moisture content and nitrous oxide production during nitrification and denitrification

Summary The effect of soil water content [60%–100% water-holding capacity (WHC)] on N₂O production during autotrophic nitrification and denitrification in a loam soil was studied in a laboratory experiment by selectively inhibiting nitrification with a low C₂H₂ concentration (2.1 Pa). Nitrifiers usually produced more N₂O than denitrifiers. During an initial experimental period of 0–6 days the nitrifiers produced more N₂O than the denitrifiers by a factor ranging from 1.4 to 16.5, depending on the water content and length of incubation. The highest N₂O production rate by nitrifiers was observed at 90% WHC, when the soil had become partly anaerobic, as indicated by the high denitrification rate. At 100% WHC there were large gaseous losses from denitrification, while nitrification losses were smaller except for the first period of measurement, when there was still some O₂ remaining in the soil. The use of 10 kPa C₂H₂ to inhibit reduction of N₂O to N₂ stimulated the denitrification process during prolonged incubation over several days; thus the method is unsuitable for long-term studies.     

Umsetzung von Cyanamid,Harnstoff und Ammonsulfat in Abhängigkeit von Temperatur und Bodenfeuchtigkeit

Transformation of cyanamide, urea and ammonium sulfate as influenced by temperature and moisture of soil The conversion of cyanamide, urea and ammonium sulfate solutions to nitrate was investigated in a sandy silt loam (pH 6.2) in relation to temperature and soil moisture conditions. 1. Cyanamide was transformed to urea within 1–5 days. Increasing temperature (2°–100°C) accelerated the breakdown, whereas high moisture conditions (120 % of total water capacity) decreased transformation. 2. The hydrolysis of urea to ammonia took place within 5–10 days even at 2°C regardless of whether cyanamide or urea was added. Low soil moisture (40 % of total water capacity) and high temperature (up to 50°) accelerated the breakdown. 3. Following urea application (20 mg N) there was a transient formation of up to five times more nitrite (0.5 mg NO₂-N) as compared with cyanamide or ammonium sulfate treatments. 4. Clear differences were observed in the rates of nitrification. The rate was greater for urea than for cyanamide and ammonium sulfate. The formation of nitrate began at 2°C, with an optimum between 20° and 30°C. Under flooded conditions (120 % of total water capacity) and low temperature the rate of nitrification was slow. At higher temperatures rapid denitrification took place.     

Nitrifier dominance of Arctic soil nitrous oxide emissions arises due to fungal competition with denitrifiers for nitrate

Arctic soils emit nitrous oxide, which is a potent greenhouse gas and also represents an important loss of nitrogen to oligotrophic Arctic ecosystems. However, little is known about the temperature sensitivity of nitrous oxide release in Arctic soils or the organisms mainly responsible for it. We investigated controls on nitrous oxide emissions in an Arctic soil across a typical temperature range (between 4 and 13 °C) on Truelove Lowland, Devon Island, Canada (75°40′N 84°35′W) at two different moisture contents. When fertilized with ammonia or nitrate, nitrous oxide emissions and temperature dependence of nitrous oxide emissions were insensitive to soil moisture content but linked to nitrification rates. Stable isotope analysis revealed that nitrous oxide was predominantly released by nitrifiers. However, nitrous oxide emissions were not linked to nitrifier prevalence with an insignificant (P < 0.219) increase in amoA genes and a (P < 0.01) decrease in archaeal nitrifiers. In contrast, denitrifier nosZ prevalence was 10,000 times greater than that of nitrifiers and was related to nitrous oxide emission potential when soils were fertilized with nitrate. Manipulating water-filled pore space should have changed the pattern of N₂O emissions. We used selective inhibitors to further explore why denitrification did not occur under field conditions when we manipulated water-filled pore space or when we used ¹⁵N analysis. When fungi were inhibited in the soil, nitrous oxide emissions from denitrifiers increased with no change in nitrous oxide released by nitrifiers. When fungi were active in the soil, there was little available nitrate but when fungi were inhibited, available soil nitrate increased over the incubation period. The dominance of nitrifiers in nitrous oxide emissions from Arctic soils under field conditions is linked to the competition for nitrate between fungi and denitrifiers.     

The relative contribution of nitrifiers to autotrophic nitrification across a pH-gradient in a vegetable cropped soil

Purpose

Microbial nitrification plays an important role in nitrogen cycling in ecosystems. Nitrification is performed by ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), and nitrite-oxidizing bacteria (NOB) including complete ammonia oxidizers. However, the relative importance of nitrifiers in autotrophic nitrification in relation to soil pH is still unclear.

Materials and methods

Combining DNA-based stable isotope probing (SIP) and molecular biological techniques, we investigated the abundance, structure, and activity of AOA, AOB, and NOB along a pH-gradient (3.97–7.04) in a vegetable cropped soil.

Results and discussion

We found that AOA abundance outnumbered AOB abundance and had a significantly negative relationship with soil pH. The abundances of NOB Nitrospira 16S rRNA, nxrB gene, and Nitrobacter nxrA gene were affected by soil pH. Incubation of soil with ¹³CO₂ and DNA-SIP analysis demonstrated that significant ¹³CO₂ assimilation by AOA rather than by AOB occurred in the acidic soils, whereas the labeled ¹³C level of AOA was much less in the neutral soil than in the acidic soils. There was no evidence of ¹³CO₂ assimilation by NOB except for Nitrobacter with NxrB gene at pH 3.97. Phylogenetic analysis of AOA amoA gene in the ¹³C- and ¹²C-labeled treatments showed that the active AOA mainly belonged to Nitrososphaera in the acidic soils.

Conclusions

These results suggested that the main performer of nitrification was AOA in the acidic soils, but both AOA and AOB participated in nitrification in the neutral soil with low nitrification activity. NOB Nitrospira and Nitrobacter did not grow in the soils with pH 4.82–7.04 and other populations of NOB were probably involved in nitrite oxidation in the vegetable cropped soil.

     

Indications for passive rather than active release of natural nitrification inhibitors in Brachiaria humidicola root exudates

Plants have the ability to suppress microbial nitrification process through secondary metabolites released from their root exudates or/and leaf litter. For decades, grasses were suggested to control nitrification process, and recently, Brachiaria humidicola accession 26159 (BH) as a tropical and subtropical grass has been shown to reduce nitrification rates under laboratory and soil conditions. In this study, experiments were conducted under controlled conditions in nutrient solution culture to investigate whether the reported release of natural nitrification inhibitors from root exudates of BH is an active or passive phenomenon. So different variables such as N-form (nitrate vs. ammonium), collecting medium (distilled water vs. 1 mM NH₄Cl) and collecting period (6 vs. 24 hrs) were included to study the hypothesis. Results showed when root exudates were collected in distilled water there was no nitrification inhibition activity for all ammonium and nitrate grown plants. However, when collection was done in a medium containing 1 mM NH₄Cl, root exudates showed significant nitrification inhibition activity similar to results obtained by Subbarao et al. The observed nitrification inhibition activity had a positive correlation to ammonium treatment particularly in collection medium, probably due to root cells damage induced by low pH and membrane depolarization under ammonium nutrition. This was more supported by application of shoot homogenates of NH₄⁺, NO₃^? or NH₄NO₃ grown plants that showed significant nitrification inhibition activity compared to distilled water and DMPP controls in a bioassay test, independent of N-form. Potassium concentrations in root exudates (as a result of potassium leakage) were found to increase in root washings of plants, which were grown with ammonium, particularly when root exudates were collected in 1 mM NH₄Cl solution. In addition, higher electric conductivity of root washings after collection of root exudates in ammonium containing medium (low pH) and also in nitrate containing medium which adjusted to pH 3 by applying H₂SO₄, strongly suggest that release of natural nitrification inhibitors from root exudates of B. humidicola may not be an active process, but instead it is rather a passive phenomenon by ammonium induced root physicochemical damages.     

不同水分模式对山东茶园土壤氮素动态的影响

以山东茶园土壤为研究对象,采用室内好气培养法,分析了恒定湿润和干湿交替模式下土壤氮素转化特征。结果表明:(1)至培养结束时,恒湿模式下60%WHC处理土壤净矿化量和净硝化量较高;脲酶和亚硝酸还原酶活性较强。20%WHC处理下土壤净矿化速率、净硝化速率严重受到抑制。(2)干湿交替模式下复水后土壤净矿化量、净硝化量以及酶活性得到增强,并出现"脉冲"式变化。(3)2种模式下氮素损失均为N_2O排放量大于NH_3挥发量。N_2O排放量与土壤含水量呈正比,NH_3挥发量与土壤含水量呈反比。干湿交替均增强土壤N_2O和NH_3排放量。(4)结构方程模型(SEM)揭示土壤含水量通过直接或间接作用影响土壤氮素转化(p0.001),脲酶显著影响恒湿模式下土壤氮素转化(p0.001),而亚硝酸还原酶在2种模式下均显著负影响氮素转化(p0.001)。研究结果有助于更好地调节茶园生态系统中土壤管理及氮肥的使用。     

Effects of nitrogen deposition on animal-mediated nitrogen mobilization in coniferous litter

Summary In microcosm studies the organic layers of coniferous forest soils show high nitrate and low ammonium mobilization, in accord with the presence of high numbers of autotrophic nitrifiers. The fungivorous collembolan Tomocerus minor (Lubbock) increases ammonium mobilization, probably through its excretion products, and has an indirect effect on nitrate mobilization. An input of N seems to have a negative effect on the number of nitrifiers and on nitrate mobilization; a decrease in N mobilization in the presence of T. minor is probably due to stimulation of microbial growth, which has an immobilizing effect.     

Simultaneous nitrification and diffusion in soil. II. The effects at levels of ammonium chloride which inhibit nitrification

The physicochemical and microbiological changes occurring in a fine sandy loam soil following the application of ammonium chloride were followed experimentally and with a simulation model. Two levels of ammonium addition were used corresponding to application rates of 37 and 143 kg ha^?1. The measured concentration profiles of ammonium nitrate and pH, which developed in soil columns as a result of the diffusion and simultaneous nitrification of the added NH₄⁺, were measured at different incubation times. The measured profiles suggested that nitrification was inhibited at the site of application of the ammonium salt. This inhibition was attributed to an effect of increased osmotic pressure or chloride ion in the soil. A simulation model was developed to account for the inhibition by examining and testing two hypotheses about the response of nitrifiers to a fluctuating osmotic pressure. These were the irreversible inhibition model, which assumed that exposure to high osmotic pressures irreversibly inactivated a portion of the nitrifier population, and the reversible inhibition model, which assumed that the nitrifiers would recover after exposure to high osmotic pressures. The model included terms for the adsorption equilibria of NH⁴⁺ and soil acidity with the soil solid phase, and the influence of other ions on the rate of diffusion of each diffusing ion. The inputs to the model were based on parameters obtained independently of the diffusion experiments. Good agreement was found between experimental and predicted concentration profiles for both models although the reversible inhibition model gave the better simulation of the data.