Comparing the effects of paper pulp and lime additions to acid tropical soils using batch experiments

Paper mill residuals may beneficially be used to improve the fertility of tropical acid soils. The effects of paper pulp on soil pH, exchangeable Al and soil solution composition of three acid tropical soils were compared with the effects of equivalent rates of lime in two batch experiments. Paper pulp was more effective than lime in increasing soil pH. However, both amendments were equally effective in decreasing exchangeable Al. Paper pulp and lime similarly influenced the composition of the soil solution by increasing soil solution pH, dissolved organic carbon, inorganic carbon, NO₃, SO₄, Ca and Mg. The supply of nitrate by the soil, however, was reduced in paper pulp treatments compared to lime treatments. Nitrate had a major role in controlling nutrient concentrations in the soil solution. Reduced NO₃ concentrations in paper pulp treated soils compared to limed soils could therefore result in lower nutrient availability and limited losses by leaching.     

采前营养液处理对水培小白菜硝酸盐积累的影响

开展了在采收前减少小白菜营养液中的氮量或在去除硝态氮的基础上加入渗调离子以试图降低小白菜的硝酸盐积累的试验。结果表明,在去除营养液中的硝态氮后,在营养液中加入Cl-、SO42-、苹果酸根离子、山梨酸根离子、乙酸根离子是降低溶液培养小白菜硝酸盐积累的有效措施,但处理后采收的时间不可推迟太久。而只减少营养液中氮肥用量会使产量迅速下降,硝酸盐含量降低却缓慢,加入渗调离子可缓解因去除氮肥引起的小白菜产量的下降。     

Mineral nutrition of chickpea plants supplied with NO3 or NH4 N

Chickpea plants (Cicer arietinum L cv. ILC 195) were grown for 24 days in water culture under two regimes of nitrogen nutrition (NO₃ or NH₄‐N) with or without Fe. For plants fed with NO₃‐N, Fe stress severely depressed fresh weight accumulation and chlorotic symptoms of Fe‐deficiency developed rapidly. Little difference in growth occurred in the NH₄‐fed plants, whether or not Fe was withheld, with no visual evidence of Fe‐deficiency indicating a beneficial effect of NH₄ in depressing the symptoms of Fe chlorosis. Typical pH changes were measured in the nutrient solution of the control plants in relation to nitrogen supply, increasing with NO₃ and decreasing with NH₄‐nutrition. With both forms of nitrogen, plants acidified the nutrient solution in response to Fe‐stress. Under NH4‐nutrition, acidification was enhanced by withholding Fe. In the NO₃‐fed plants the uptake of all nutrients was reduced by the stress but proportionally NO³‐ and K⁺ were most affected. Total anion uptake was depressed more than that of cation uptake. For the NH₄‐fed plants withholding Fe resulted in an increased uptake of all ions except NH₄ ⁺ which was depressed. Regardless of the form of N‐supply, when Fe was withheld from the nutrient solution the net H⁺ efflux calculated from the (C‐A) uptake values was closely balanced by the OH” added to the nutrient solution to compensate for the pH changes. Evidence of accumulation of organic acids in the Fe‐stressed plants was found, especially in the NO₃‐fed plants, indicating a role for these internally produced anion charges in balancing cation charge in relation to the depression of NO₃ ^‐ uptake associated with Fe‐stress.     

Isolation,distribution, and characterization of peach seedling nitrate reductase 1

Nitrate reductase (NR) was extracted from leaf, root, and stem tissue of ‘Lovell’ peach seedlings [Prunus persica (L.) Batsch] grown for 8 weeks in nutrient solution containing 15 mM nitrate. Enzyme activity of NR in leaf, stem, and root tissue was 10.20: 0.07: 0.04 nM N0₂/min/g tissue extracted, respectively. When seedlings wee transferred to nutrient solution containing 15 mM NH₄, NR activity was not detected after 72 hours. The enzyme was specific for NADH and had a pH optimum of 7.5. The K_m for NO₃ was 1.3 x 10–3 M and the rate of reaction remained linear for 45 min. Enzyme activity of leaf tissue was dependent on NO₃ concentration in the nutrient solution. At NO₃ concentrations of 15, 7.5, 1.5, and 0.15 mM, the NR activity was 22.8, 16.2, 13.8, and 2.2 nM NO₂/mg protein/hr.     

Potassium recirculation in tomato plants in relation to potassium supply

The Ben Zioni ‐ Dijkshoorn hypothesis that NO₃ uptake by roots is regulated by NO₃ assimilation in the shoot was tested using the tomato plant. Plants were grown at three K levels and the fate of anion charge from NO₃,^‐ and SO₄ ^2‐. assimilation followed in its distribution between organic acid anion accumulation and HCO₃ ^‐ efflux into the nutrient medium. For the high K treatment almost all of this charge was directed towards organic acid accumulation with HCO₃ ^‐ efflux accounting for only 3% of the total charge. On the other hand for plants supplied at the low K level, a substantial proportion of the anion charge was excreted as HCO₃ ^‐ (32%).

Xylem sap analyses and NO₃ reductase assay results indicated that in the tomato plant the upper plant parts constituted the major site for NO₃ reduction. The quantitatively most important ionic constituents in the sap were K⁺, Ca²⁺ and NO₃ ^‐.

Results have been presented that indicate that when K is in short supply in the nutrient medium, K recycling occurs within the plant to facilitate the upward transport of NO₃ ^‐ from root to shoot.     

In vivo nitrate reductase activity in carex pseudocyperus L.: The influence of nitrate ‐ ammonium concentration ratios and correlation with growth

Abstract

The influence of the nitrate nutritional status and increasing ammonium concentrations on the nitrate reductase activity of shoots and roots of Carex pseudocyperus L. was investigated. The activity of this enzyme was correlated with the relative growth rates of the plant. Nitrate reductase activity was determined by a modified in vivo test (1). A specially developed test system allowed a large amount of samples to be handled easily.

The optimization procedure of the incubation buffers led to different assay conditions for the shoot and the root, respectively. Enzyme activity in the shoot was dependent on the length of the incubated leaf pieces. Incubation had to take place under dark, anaerobic conditions.

Enzyme activity was influenced by an evident diurnal rhythm with an optimum six hours after starting illumination, so that harvesting occurred always at that day time.

Increasing nitrate concentrations of up to 2.5mM NO₃‐ in the nutrient solution induced an increasing nitrate reductase activity in the shoot. The enzyme activity of the root was already fully induced at 1mM NO₃ ^?. A nitrate concentration above 5mM NO₃ ^? inhibited enzyme activity in shoots as well as in roots. The addition of increasing amounts of ammonium to a solution containing 2mM NO₃ ^? led to a significant inhibition of the enzyme activity in both parts of the plant.

Relative growth rates of the shoot, as a function of increasing nitrate concentrations in the nutrient solution, were highly positively correlated to the corresponding nitrate reductase activity, but only a slight, negative correlation was observed between these two parameters in the root.     

Effects of Molybdenum,Nickel, and Nitrogen Sources on the Mineral Nutrition and Growth of Rice Plants

Upland rice plants, cultivar ‘IAC 202,’ were grown in nutrient solution until full tillering. Treatments consisted of ammonium nitrate (AN) or urea (UR) as nitrogen (N) source plus molybdenum (Mo) and/or nickel (Ni): AN + Mo + Ni, AN + Mo ? Ni, AN ? Mo + Ni, UR + Mo + Ni, UR + Mo ? Ni, and UR ? Mo + Ni. The experiment was carried out to better understand the effect of these treatments on dry‐matter yield, chlorophyll, net photosynthesis rate, nitrate (NO₃ ^?‐N), total N, in vitro activities of urease and nitrate reductase (NR), and Mo and Ni concentrations. In UR‐grown plants, Mo and Ni addition increased yield of dry matter. Regardless of the N source, chlorophyll concentration and net photosynthesis rate were reduced when Mo or Ni were omitted, although not always significantly. The omission of either Mo or Ni led to a decrease in urease activity, independent of N source. Nitrate reductase activity increased in nutrient solutions without Mo, although NO₃ ^?‐N increased. There was not a consistent variation in total N concentration. Molybdenum and Ni concentration in roots and shoots were influenced by their supply in the nutrient solution. Molybdenum concentration was not influenced by N sources, whereas Ni content in both root and shoots was greater in ammonium nitrate–grown plants. In conclusion, it can be hypothesized that there is a relationship between Mo and Ni acting on photosynthesis, although is an indirect one. This is the first evidence for a beneficial effect of Mo and Ni interaction on plant growth.     

Low phosphate nutrition alters bean plants’ ability to assimilate and translocate nitrate

Bean plants (Phaseolus vulgaris L.) were cultured for 10 or 18 days on phosphate sufficient (+P) or phosphate deficient (‐P) nutrient medium. Nitrate and phosphate distribution between shoot and root, nitrate uptake, and nitrate reductase activity (NR activity, in vivo and in vitro) in root and leaves was estimated. The decrease in Pi concentration in leaves and roots led to decreased rate of NO₃ uptake and increased NO₃ accumulation in roots, accompanied by alterations in NO₃ distribution between shoot and roots. Nitrate reductase activity estimated in vitro was twice higher than estimated in vivo and both in +P and ‐P plants was lower in the roots than in the shoots. The decrease of NR activity in ‐P plants was more pronounced in the roots and after 2 weeks of phosphate starvation it was about 40% lower as compared with the control. The depression in nitrate uptake may be the result of feedback inhibition due to accumulation of nitrate in the roots. The increased NO₃ concentration in root tissue may be explained by decreased NR activity and lower transport of nitrate from roots to shoot.     

Growth and chloroplast ultrastructure of two citrus rootstock seedlings in response to ammonium and nitrate nutrition 1

Abstract

It is important to understand the differential response of citrus rootstock to various rates of nitrogen (N) forms in order to evaluate the tree response to N availability under different production conditions. In this study, the effects of N sources (NH₄‐N or NO₃‐N), and rates (5, 15, 45, and 135 mg N L^?1) on two citrus rootstock seedlings (Swingle citrumelo, SC; Cleopatra mandarin, CM) growth (110 d) and N concentrations in various parts of seedling were investigated in a nutrient solution experiment. The effects of N nutrition on the chloroplast ultrastructure of leaves were examined at the end of the experiment. Rootstock and N sources significantly influenced the growth of leaves, stems, and roots. The growth of all seedling parts of both rootstocks decreased with an increase in concentration of NH₄‐N in the solution. In contrast, there was a positive relationship between the seedling growth and the concentration of NO₃‐N in the nutrient solution with marked response observed as the NO₃‐N concentration increased from 5 to 15 mg L^?1 The experiment demonstrated a distinct growth suppression effect with an increase in concentration of NH₄‐N in the nutrient solution, particularly SC rootstock. Chloroplast ultra‐structure of the leaves showed evidence of injury of the seedlings which received N entirely as NH₄ ⁺ form, but the injury was not seen when the seedlings received N as the NO₃‐N form. The disruption of chloroplast ultrastructure increased with increased rate of NH₄‐N. The most conspicuous characteristic of ammonium toxicity was the massive accumulation of strands granules and phytoferritin which is clearly an evidence of NH₃ toxicity. The results are important for understanding the implications of N source on seedling growth and chloroplast structure of citrus leaves.     

Influence of Ammonium and Nitrate Nutrition on Plant Growth and Zinc Accumulation by Indian Mustard

ABSTRACT

The source of nitrogen (N) used in soil fertility practices affects plant growth, nutrient absorption, and the availability of nutrients. Consequently, the potential of plants to extract zinc (Zn) from soils may be increased by controlling the ratio of NH₄ ⁺ to NO₃ ^? to maximize growth and Zn accumulation. The objectives of this research were to determine the effects of Zn supply and different molar ratios of NH₄ ⁺ to NO₃ ^? on growth and Zn accumulation in Indian mustard (Brassica juncea Czern.). In a factorial experiment with solution culture, Indian mustard (accession 182921) was supplied with two concentrations of Zn (0.05 and 4.0 mg L^?1) in combination with six N treatments with different molar percentage ratios of NH₄ ⁺ to NO₃ ^? (0:100, 10:90, 20:80, 30:70, 40:60, and 50:50) for three weeks. Zinc supplied at 0.05 mg Zn L^?1 represented a common concentration of Zn in solution culture, whereas 4.0 mg Zn L^?1 was excessive for plant nutrition. If the supply of Zn in solution was excessive, plants developed symptoms of foliar chlorosis, which became severe if plants were supplied with 80% of N as NO₃ ^?. Supplying high proportions of NO₃ ^? in the nutrient medium stimulated Zn accumulation, whereas increasing proportions of NH₄ ⁺ (up to 50% of the total N) enhanced shoot growth. The pH of nutrient solutions generally decreased with increasing proportion of NH₄ ⁺ in solutions and with increased Zn supply. The Zn phytoextraction potential of Indian mustard was maximized, at about 15 mg Zn plant^?1, if plants received 10% of the total N as NH₄ ⁺ and 90% as NO₃ ^?.     

Ammonium increases the net rate of sodium influx and partitioning to the leaf of muskmelon

The influx and partitioning of sodium (Na) is controlled by potassium (K)/Na selectivity/exchange mechanisms. Since ammonium‐nitrogen (NH₄‐N) has been shown to inhibit K absorption and K/Na selectivity/exchange mechanisms control Na influx and partitioning, our objective was to observe if NH₄‐N affects Na influx and partitioning in muskmelon. Muskmelon (Cucumis melo L.) were grown in a pH controlled nutrient solution with 100 mM NaCl in a complete nutrient solution containing either 10 mM nitrate‐nitrogen (NO₃‐N) or NH₄‐N. With NH₄‐N, Na accumulation and partitioning to the leaf blade increased while K absorption was almost completely inhibited. A second study omitted K to simulate the inhibition of K absorption by NH₄‐N and monitored Na accumulation and partitioning as K was depleted in the plant. Sodium accumulation and partitioning to the leaf increased as K decreased in the plant, mirroring the effect of NH₄‐N. Roots appeared healthy in both studies. Our work indicates that at a given level of NaCl stress, NO₃‐N reduces the level of stress experienced by muskmelon plants through reducing the net rate of Na influx and transport to the sensitive leaf blade, not by reducing chloride (Cl) absorption, thereby permitting these plants to “avoid”; this stress.     

Influence of nitrate and chloride on the adsorption and transport of sulphate in soils

The adsorption and desorption of SO₄ was investigated as a function of KCI and KNO₃ concentration using soils with contrasting surface-charge properties. In the net negatively-charged soils, additions of C1 or NO₃ of up to about 0.05–0.10 M increased the adsorption of SO₄ but at higher concentrations adsorption decreased. In contrast, adsorption by the net positively-charged soils decreased with concentration increase over the entire range (0 to 1 M) investigated. The effects of CI and NO₃ on the adsorption of SO₄ were practically identical. The different pattern of SO₄ adsorption in the two groups of soils in response to addition of KCI or KNO₃ can at least partly be explained in terms of the effect of electrolyte on soil pH. The depression in pH of net negatively-charged soils induced by an indifferent electrolyte favours adsorption of SO₄; but, because pH changes in the opposite direction in positively-charged soils, SO₄ adsorption decreases in these soils. The distribution of a pulse of 35S-labelled SO₄ in soil columns after leaching with KCI solutions, ranging in concentration from 0 (H₂O) to 0.10 M, clearly reflected the manner in which electrolyte concentration affected the adsorption of SO₄. The distribution of ³⁵S was reasonably well simulated using the general transport equation combined with the Freundlich equation to describe the adsorption/desorption of SO₄. In contrast to other inorganic anions (H₂PO₄ and OH) applied in agricultural practice, C1 and NO₃ may have beneficial effects on the S economy of many soils by decreasing leaching losses of SO₄.     

Changes in growth and nitrogen assimilation in barley seedlings under cadmium stress

Barley (Hordeum vulgare L. cv. Martin) plants grown in solution culture, were exposed to increasing cadmium (Cd) concentration (0, 5, 10, 25, 50, and 100 μM) for a duration of 12 days. The sequence of important biochemical steps of nitrate (NO₃) assimilation were studied in roots and shoots as a function of external Cd concentration. Cadmium uptake in roots and shoots increased gradually with Cd concentration in the medium. This Cd accumulation lowered substantially root and shoot biomass. The nitrate reductase (NR, EC 1.6.6.1) and nitrite reductase (NiR, EC 1.6.6.4) activities declined under Cd stress. Concurrently, tissue NO₃ contents and xylem sap NO₃ concentration were also decreased in Cd‐treated plants. These results suggest that Cd could exert an inhibitory effect on the assimilatory NO₃ reducing system (NR and NiR) through a restriction of NO₃ availability in the tissues. We therefore examined, in short‐term experiments (12 h), the impact of Cd on NO₃ uptake and the two reductases in nitrogen (N)‐starved plants that were pretreated or not with Cd. It was found that Cd induced inhibition of both NO₃ uptake and activities of NR and NiR, during NO₃ induction period. The possible mechanisms of Cd action on NO₃ uptake are proposed. Further, in Cd‐grown plants, the glutamine synthetase (GS, EC 6.3.1.2) showed a decreasing activity both in shoots and roots. However, increasing external Cd concentration resulted in a marked enhancement of glutamate dehydrogenase (NADH‐GDH, EC 1.4.1.2) activity, coupled with elevated levels of ammonium (NH₄ in tissues. On the other hand, the total protein content in Cd‐treated plants declined with a progressive and substantial increase of protease activity in the tissues. These findings indicate that under Cd stress the usual pathway of NH₄ assimilation (glutamine synthetase/glutamate synthase) can switch to an alternative one (glutamate dehydrogenase). The changes in all parameters investigated were concentration‐dependent and more marked in roots than shoots. The regulation of N absorption and assimilation by Cd in relation to growth and adaptation to stress conditions are discussed.     

Response of wheat and maize to different nitrogen sources: II. Nitrate reductase and glutamine synthetase enzyme activities,and plastid pigment content

Glutamine synthetase and nitrate reductase enzyme activities occurred both in roots and leaves of maize (Zea mays L., hybrid Pioneer 3737) and wheat (Triticum aestivum L., cultivar Jantar) plants grown on different nitrogen (N) sources. Enzyme activities and plastid pigment content in maize plants were higher in the treatments with a mixture of nitrate (NO₃) and ammonium (NH₄) than with either N source alone. In wheat plants, plastid pigment content, nitrate reductase activity, and root glutamine snynthetase activity were higher in the treatments where NO₃ alone was applied to the nutrient medium.     

Changes in net proton release by roots of intact maize plants after local nutrient supply in a split root system

The effect of local nutrient supply to maize roots (Zea mays L. cv. Blizzard) on net proton release was studied using the split root technique (SRNS, SRCa) to compare plants that were cultivated with their roots completely in either nutrient solution (NS) or 0.1 mM CaSO₄ (Ca). Roots in NS released more protons than roots in Ca. This higher net proton release was associated with significantly higher ATP concentrations in the root tissue. Higher net proton release and ATP concentrations were also observed after a 4 h lag phase when 20 μM abscisic acid were exogenously applied to roots in 0.1 mM CaSO₄. It is suggested that higher metabolic activity in roots supplied with nutrients increased ATP concentrations and thus the substrate supply of the plasma membrane H⁺ ATPase. When only half of the root system was supplied with nutrient solution with the other half bathed in 0.1 mM CaSO₄, the roots in the SRNS compartment released significantly higher amounts of protons relative to the NS control plants. Conversely, roots in the SRCa compartment showed net proton uptake in contrast to the roots of control plants in 0.1 mM CaSO₄ which significantly acidified the root medium. These differences in proton release by roots in the split root system and control roots could not be explained in terms of differences in ATP concentrations. It is therefore suggested that an internal signal may lead to a modification of the plasma membrane H⁺ ATPase as shown earlier during plant adaptation to low pH in the root medium.     

Study on ammonium tolerance of cucumber plants

The influence of N form on xylem exudate and the guttation fluid concentration in cucumber plants was studied under greenhouse conditions. Plants were hydroponically grown with three NO₃:NH₄ ratios (100:0, 80:20, and 60:40) at a constant pH of 6.0 in the nutrient solutions. Plants supplied with 60:40 NO₃:NH₄ ratio displayed a significant decrease of NO₃‐N, total‐N, organic‐P, and Mn concentrations in the xylem sap and an increase of H₂PO₄‐P, SO₄‐S, Cl, B, and Zn concentrations. Potassium and Ca uptake in these plants was slightly reduced, indicating that pH control was an important factor for cationic nutrition in cucumber plants fed with NH₄. The major ions present in the nutrient solutions are concentrated in the xylem sap, particularly for NO₃, K, Ca, and Na. The NO₃:NH₄ ratio had a small effect on the ionic levels of the guttation fluid. The concentrations of all nutrients in the guttation fluid were substantially reduced, except for Cl, showing that the leaf tissues of cucumber plants remove the excess of Cl ion. Finally, in this study, secondary effects of N source on ion uptake and release were minimized by controlling nutrient solution pH.     

INFLUENCE OF SALINITY AND AMMONIUM: NITRATE RATIO ON GROWTH,PHOTOSYNTHESIS, FATTY ACID AND THE ACTIVITY OF ANTIOXIDATIVE ENZYMES IN CANOLA

The effects of sodium chloride (NaCl) salinity (0 and 200 mM) and ammonium (NH₄):nitrate (NO₃) ratios (100:0, 25:75, 50:50, and 75:25) on growth, photosynthesis, fatty acids and the activity of antioxidative enzymes were investigated in canola plants. Leaf area and fresh and dry weights of leaves were significantly reduced by the salinity. The reduction in vegetative characteristics varied in both salinized and unsalinized plants according to the NH₄:NO₃ ratios so that the lowest reduction was observed with the 50:50 (NH₄:NO₃) ratio. Increased NH₄ up to 50 percent (50:50) of total N, promotes the yield at both salinized and unsalinized plants. In both salinized and unsalinized plants, the increased NH₄ and NO₃ ratio in the nutrient solution reduced the photosynthetic (Pn) rate and stomatal conductance; however, the reduction in Pn rate was severely impaired at a higher ratio of NH₄ in the nutrient solution. In both salinized and unsalinized plants, the 75:25 ratio had the lowest potassium (K) and sodium (Na) content; however, the K/Na ratio was the highest in 50:50 ratio. An increase of NH₄ in the solution led to a significant increase in NH₄ content in both salinized and unsalinized plants. Salinity increased NH₄ content so that the salinized plant had nearly twice as high NH₄ content in the leaves. The activity of nitrate reductase was increased by increasing NH₄ from 0 to 50% and then reduced at a higher ratio of NH₄ in the solution. The activities of antioxidative enzymes increased in salinized plants regardless of the NH₄:NO₃ ratios. In salinized plants, the activities of superoxide dismutase and catalase enzymes were increased by 44.4 % and 97.5%, respectively. Within salinized and unsalinized treatments, the highest activities of all antioxidant were observed in 75:25 ratio, while they remained unchanged for all NH₄:NO₃ ratios. The increased NH₄ content in the solution increased the oil content and the maximum oil content in both salinized and unsalinized plant was obtained in both 50:50 and 75:25 ratios. The percentage of oleic acid was affected by both salinity and NH₄:NO₃ ratios. The ratios of NH₄:NO₃ had no effect on the protein content; however, salinity reduced the protein content by 20%.     

不同基因型小白菜硝酸盐积累差异及筛选研究

采用溶液培养方法研究了43种基因型小白菜在不同硝铵比条件下体内硝酸盐含量的变化情况。结果表明,不同基因型小白菜的硝酸盐含量差异较大,东妃青梗菜、上海白叶四月蔓和夏优高抗为低硝酸盐含量基因型小白菜,而高雄甜脆小白菜、宝大矮棋青和苏州青为高硝酸盐含量基因型小白菜。硝铵比50/50是最适宜筛选的氮素形态比例,而叶片中的硝酸盐含量也是最适宜的筛选指标。对上述6个基因型小白菜进一步分析结果表明,不同部位硝酸还原酶、根系硝态氮最大吸收速率以及亲和力这三个因素对小白菜各部位的硝酸盐积累都有显著的影响,但在不同部位,这三个因素所起的贡献率不同。     

Nutrient availability in FH horizon samples amended with fertilizer and lime

Abstract

A laboratory experiment designed to test the effect of additions of base cations and liming treatments on nutrient availability was carried out using the FH (Oe/Oa) horizon of an Orthic Ferro‐Humic Podzol (Humic Haplorthod) from a sugar maple (Acer saccharum Marsh) stand. Treatment with K₂SO₄ caused a decrease in pH of the leachates from samples incubated for five weeks while liming caused an increase in pH. Application of the base cation‐lime materials caused an increase in Ca, Mg, and K, as would be expected, however these materials also increased NH₄ and NO₃ concentrations suggesting that microbial activity and N dynamics were also affected.     

Distribution of bacteria in a continuous-flow nitrification column

Nutrient solution containing (NH₄)₂SO₄ was supplied at a constant rate to Nitrosomonas europaea and Nitrobacter agilis growing in a column packed with glass beads. Conversion of NH₄⁺ to NO₂^? and NO₃^? was incomplete indicating that growth of the bacteria was not nutrient limited. After 7 months the column was dismantled and the arrangement of the bacteria on the beads examined using a scanning electron microscope. Nitrifying bacteria were found only in the upper regions of the column. They occurred most commonly in monolayers, less commonly in layers of about 20 cells and rarely in piles of about 100 cells in depth. Further down the column the glass beads were covered in a layer of slime and no bacteria were seen. This suggests that growth of the nitrifiers was neither controlled by diffusion of metabolites through a microbial film nor limited by competition between the bacteria for space on the surface of the glass beads.