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A field and laboratory study of nitrite accumulation under arable soil conditions was carried out using a considerable number of nitrogen fertilizers applied at various seasons of the year. The results show that nitrite will accumulate under neutral or alkaline soil conditions provided the rate of fertilization is heavy enough to produce a considerable concentration of the ammonium ion. Under field conditions, especially where the soils are not warm enough for rapid biological activity, the nitrite produced may persist for several months. For example, nitrite accumulating from urea applied in January (43 ppm N as NO₂) persisted into May (31 ppm N). Amounts of nitrite up to 90 ppm N were produced under favorable conditions. More nitrite was produced from urea than any other nitrogen fertilizer. This is considered due to the high ammonia and pH conditions resulting from application of this material. In none of the field studies was there any evidence of nitrite accumulation from denitrification though occasionally heavy fertilization with nitrate fertilizer supplemented with straw was immediately followed by heavy rains or irrigation. From these studies, it appears that wherever two conditions occur, namely, fairly high ammonium concentrations and neutral or alkaline soil conditions, more or less nitrite accumulation may be expected.

By both Pseudomonas strains the N2 and N2O production depended on the “physiological condition” of the bacteria. The N2O formation by strain C 31 in mineral media depended also on the precultivation medium. The bacteria produced N2O besides N2, if precultivated in Nutrient Broth + KNO3. Precultivated in a mineral medium, however, they produced only N2. The anaerobic gas production of Pseudomonas aeruginosa strain C 31 and strain N.C.I.B 8704 was determined under hydrogen atmosphere. The results are as follows: In Nutrient Broth + 0.35% KNO3 nitrate was respirated quantitatively to N2 and N2O by the two strains. On the contrary, in Nutrient Broth + 0.5% KNO3 nitrate was not completely consumed by strain C 31. In nitrate-ammonium-medium we succeeded to recover quantitatively the given nitrogen compounds (nitrate, nitrite, ammonium, organic N), partially in the denitrification products, after the end of the experiment, but not in the experiment with nitrate-medium. In nitrate-medium strain C 31 produced a much higher amount of nitrite (up to 73 mg N/l) as in nitrate-ammonium-medium (up to 213 μg N/l). The influence of the ammonium ions is discussed.

SUMMARY: Micrococcus denitrificans is capable of carrying out assimilatory and dissimilatory nitrate reduction, though only the assimilatory process occurs under both aerobic and anaerobic conditions. Aeration affects the dissimilatory activity (reduction of nitrate to nitrogen) of a growing culture in at least three ways: (a) it prevents the adaptive formation of the system, (b) it partially represses any further synthesis if the system is already present, and (c) it inhibits the activity of the preformed system completely. To some extent these effects of oxygen are reflected in the control which it exerts upon the organism's content of nitrate reductase (the enzyme responsible for the initial reduction of nitrate to nitrite) and upon its activity during growth. Ammonium ions partially inhibit the transformation of nitrate into cell nitrogen but have no detectable effect on the nitrate reductase activity of crude extracts of this organism.

The fate of added nitrogen in submerged soils was studied using 15N-labelled ammonium sulfate and alanine. After 8 weeks of incubation 25 and 22%, respectively, of nitrogen from ammonium sulfate and alanine were recovered in the soil. Under the experimental conditions used nitrogen added to presubmerged soils was lost rapidly outside of the soil-water system, regardless of whether the nitrogen was organic or inorganic. Fractionation studies revealed that the amount of tagged N incorporated into exchangeable ammonium, residual fractions, volatilized as NH3 and chemically fixed nitrogen was not enough to account for the nitrogen loss. The nitrogen loss was attributed to nitrification and subsequent denitrification during the incubation period. The effect of N-Serve [2-chloro-6-(trichloromethyl)pyrimidine] on nitrification of 15N-labelled (NH4)2SO4-in submerged soils was studied. About 15% more nitrogen was recovered from non-presubmerged soils, and less nitrate was accumulated in presubmerged soils...

Immobilization of fertilizer nitrogen and mineralization of soil organic nitrogen were measured in three soils receiving ammonium sulfate or calcium nitrate labelled with the N15 tracer. In the presence of added straw deficient in nitrogen the rate of nitrogen immobilization was strongly temperature dependent, whereas the total quantity of nitrogen eventually incorporated in the organic fraction was little affected by temperature. Nitrifying bacteria were able to compete effectively for ammonium with the heterotrophic population in all three soils. Little remineralization of tagged nitrogen occurred, suggesting progressive stabilization of immobilized nitrogen. Ammonium-nitrogen was immobilized in greater quantity than nitrate-nitrogen in all soils. An appreciable fraction of the ammonium-nitrogen was fixed within the crystal lattice of clay minerals in two of the soils. The data indicate that this clay-fixed ammonium was completely unavailable to nitrifying bacteria or other microorganisms. Absolute mineralization and immobilization rates were calculated for periods of maximum activity at the beginning of the experiments. The highest mineralization rate observed was 12 ppm nitrogen per day, compared with a concurrent immobilization rate of 16 ppm per day. The data indicate that turnover rates of soil organic nitrogen are relatively low, and suggest that when nitrogen fertilizers are applied, part of the nitrogen remains unavailable to plants for many years.

Experiments are reported in which a study was made of the absorption of NH4(-) and NO3(-)N and plant growth by cotton seedlings. The seedlings were grown in solution, sand, and soil cultures. The solution cultures were used in a series designed to study the influence of various concentrations of NH4(-) and NO3(-)N on the absorption of these ions. The effect of the age of the seedlings, reaction of the culture solution, and length of the absorption period on the relative amounts of NH4(-) and NO3(-)N removed from the culture solutions were studied. Results of a study of the NH4(-) and NO3(-)N content and the pH value of sap from plants at different ages are included. The results of the investigation may be briefly summarized as follows: NH4(-)N was used in larger amounts than NO3(-)N by the young seedlings until they were from 3 to 5 weeks old. After this age more NO(3)-N was absorbed. The data show that both NH4- and NO3(-)N were absorbed in large amounts when the plants were 4 to 8 weeks old. Both growth and fruiting of plants were largest when both forms of nitrogen were present. NH4(-)N absorption increased as the acidity of the culture solution decreased; the absorption of NO(3)-N was only slightly affected by the reaction of the solution used. The highest total nitrogen absorption usually occurred at pH 6.0. Total nitrogen absorption was greatest when both forms of nitrogen were present. The growth and fruiting of cotton in field plats agreed well with that of the culture solutions. Data are presented which indicate that nitrogen in the seed is available to the sprouts and young seedlings in the NH4 form. A theoretical discussion of the observed absorption phenomenon is included.

Some features of enzyme activities and microbial growth kinetics are reviewed and applied to sequential biochemical reactions in soil. Transformations of some soluble nitrogen compounds are discussed in terms of time of fluid flow downward in soil and a mathematical model is analyzed in such a way as to predict concentration-depth profiles of urea, ammonium, nitrite and nitrate. The intermediate compounds tend to reach maximal amounts in the profile whereas urea declines and nitrate, in the absence of denitrification, accumulates with depth. Although the model is general, it is limited to fallow soils or to laboratory soil columns. Nevertheless, the model suggests that much more needs to be learned about the influence of microenvironments on soil microbes and soil metabolism before any realistic analysis of dynamic profiles can be achieved.

Paddy fields are kept in a flooded condition for at least three months during the growing period of rice plants. The chemical characteristics of soils in paddy fields, therefore, are remarkably different from these of upland fields. Many soil constituents undergo great changes during the flood period. The soil can be understood as a redox system (1, 17). Japanese workers (3) have found oxidized and reduced layers in the submerged soil, which cause the soil to lose nitrogen by nitrification of ammonium and denitrification of nitrate and/or nitrite.

The investigation shows that when water-logged soils were treated with nitrogenous manures and fertilizers like oil cake, water hyacinth, ammonium sulphate and sodium nitrate, a large volume of nitrogen evolved as gas. This loss began 7–12 days after application of different treatments and was quicker from inorganic fertilizers than from organic manures. Besides loss as gas, a large amount of nitrogen was also lost in the drainage. The results suggest that if fertilizers like ammonium sulphate and sodium nitrate are applied to rice-fields by top dressing during the active vegetative growth of the crop, the loss nitrogen as gas will probably be greatly minimized.

SUMMARY Biddulphia aurita, a centric diatom, can grow on either nitrate, nitrite, or ammonia as its sole nitrogen, source. Cells remove ammonium nitrogen from the medium 2.3–2.4 times faster than either nitrate or nitrite nitrogen and, when grown for 24 hr in the ammonium medium, contain higher levels of non-protein nitrogen than cells grown in the nitrate or nitrite medium for the same period of time. The nitrogenous compounds in the nonprotein nitrogen fraction from cells grown in the nitrate, nitrite, or ammonium medium contain the same level of soluble-free amino nitrogen, combined amino nitrogen, and ammonium nitrogen. The high level of soluble nonprotein nitrogen in the medium of the cells grown in the ammonium medium is due to soluble amide nitrogen which represents 18% of the total soluble nitrogen present in these cells, whereas it represents only 2% in cells from the nitrite medium, and its level is negligible in cells from the nitrate medium. Cells grown in the nitrate medium have both nitrate- and nitrite-reductase activity. Cells grown in the nitrite medium have only nitrite-reductase activity in significant levels, while cells grown in the ammonium medium lack both enzymes.

Fixed ammonium nitrogen is the dominant inorganic form of nitrogen in the scdimcnts of Lake Ontario. The fixed ammonium nitrogen concentration is around 300 ,ug/g of sediment at the top of a 10-m core and increases gradually to 525 pg at 150 cm, below which it remains constant. As nitrification is precluded in the rcduccd sediments below 3 cm, the ammonium ion is tither fixed within the scdimcnt clay lattices or migrates upward in the sediment interstitial waters. The uniform fixed and exchangcablc ammonium concentrations below 150 cm in the core indicate that the sediment is saturated with rcspcct to ammonium fixation, and the dccrcasc in thcsc concentrations above 150 cm in the core suggest that equilibrium is not attained with the ammonium ion. The dccpcr scdimcnts probably do not regenerate nitrogen to Lake Ontario, but most of the nitrogen relcascd to the hypolimnion by the sediments is from nitrification, denitrification, and ammonification reactions at the sediment-water intcrfacc. A minimum of 20% of the organic nitrogen input to the scdimcnts is rcgcneratcd to the lake from the top G cm of sediments. About 90% of the nitrogen in the surface muds is organic: 28-46% as amino acid-N, 4-7s as hcxosamine-N, and 21-3170 as hydrolyzable unidentified-N, From 29-57s of the total nitrogen could not be accounted for as amino acids, hcxosamincs, fixed and cxchangeablc amnonia, nitrate, and nitrite in the surface sediments.

Rice cv. IR5 was grown in pots in waterlogged soil with varying levels of K, Na, Mg, Ca, NH4, NO3, Cl, PO4 and SO4. Where NO3 but no NH4 was supplied, denitrification resulted in N shortage and restricted N uptake. With NH4, uptake of N was determined by plant growth and capacity to incorporate N. When KNO3 was replaced by NH4Cl, plant growth was less than where (NH4)2SO4 was the replacement salt; this was related to the greater accumulation of Cl ions in the tissues and a consequent marked fall in carboxylates to a very low level indicative of nutritional stress. (Abstract retrieved from CAB Abstracts by CABI’s permission)

A study of the transformation of applied nitrogen in submerged rice soils, using N15 tagged compounds is reported. A part of the nitrogen applied as (N15H4)2SO4 and N15H4Cl was immobilized within ten days of submergence, due to microbial immobilization and the proportion of this remained fairly constant for a seventy-five day period of submergence. Fraction of the tagged nitrogen recovered in the ammonium fraction decreased with incubation. Application of extraneous inorganic nitrogen appeared to stimulate the mineralization of soil organic nitrogen. About 23–24% of the applied nitrogen was not accounted for at the end of seventy-five days of incubation. This loss is attributed to the oxidation of ammonium nitrogen to the nitrate form in the surface layer of the soil with subsequent leaching and denitrification in the reduced sub-surface zone.

The fate of top-dressed nitrogen (ammonium sulphate) enriched with 15N was investigated in the paddy field. The amounts of nitrogen applied were 30 kg per hectare in 1969 and 25 kg per hectare in 1970, respectively. The earlier was the top-dressing at the growth stage of rice plant applied, the longer stayed the ammonia derived from the top-dressed fertilizer in the plowed layer. This fact is approximately consistent with the duration of absorption of top-dressed nitrogen by rice plant. After harvest only 3 to 10 per cent of top-dressed nitrogen was found in the plowed layer. Since the drainage of the paddy soil was very poor, it was assumed that almost all of the unrecovered top-dressed nitrogen was lost by denitrification. The recoveries of top-dressed nitrogen were higher for the plots applied at the young panicle formation stage than for other ones. The earlier was the top-dressing applied, the longer the duration of top-dressed nitrogen uptake by the plant. And the later was the top-dressing applied, the higher the absorption rate of nitrogen by the plant. The amounts of soil nitrogen abosrbed by the plant were greater in all the plots with top-dressing nitrogen than in those without top-dressing. Therefore, the gross recovery of nitrogen in the plant was greater than the recovery calculated from the tracer.

An appreciable loss of labeled nitrogen occurred in flooded soils exposed to atmospheric oxygen. Nitrogen added as ammonium was apparently nitrified in the aerobic surface layer of soil and then diffused downward into the underlying anaerobic zone where it was denitrified and lost from the system. Denitrification in flooded soil and swamp sediments makes a significant contribution to the elemental ni- trogen returned to the atmosphere. Although severe losses of nitrogen from flooded soils as a result of the alternate aerobic and anaerobic conditions caused by periodic flooding and drying have been recognized for years (Russell 1961), the magnitude and mech- anism of denitrification loss in continuously flooded soil systems have not been well established. Denitri- fication in continuously flooded soils apparently oc- curs as a result of the development of a thin aerobic soil layer overlying a thicker anaerobic soil layer. The soil is usually capable of consuming much more ox- ygen than the amount that diffuses through the flood water, and consequently oxygen penetrates only a short distance into the soil before being reduced. Nitrate formed in the surface aerated layer leaches or diffuses downward into the anaerobic layer where it is denitrified. Our report describes a study of the loss of 15N-labeled nitrogen from a continuously flooded soil. Samples of soil (100 g Crowley silt loam) in bot- tles received 100 ppm ammonium nitrogen contain- ing 10.228 atom percent 15N excess and were incu- bated at 30'C for periods up to 120 days. The specific treatments used were: * s~~~~~b~

IT has been reported1 that when a virgin soil is brought under cultivation, the percentage of total nitrogen usually falls off with time up to a limiting value. A loss of nitrogen from soils on the addition of nitrogenous compounds, especially when the conditions are favourable for oxidation, has also been observed by different investigators; but this phenomenon has been explained satisfactorily by Dhar and collaborators2, who have postulated that this type of nitrogen loss in soils is chiefly due to the formation and decomposition of the unstable ammonium nitrite produced in the processes of ammonification and nitrification. There is also the possibility of the reaction of nitrous acid on amines, amides and amino-acids which may sometimes be present in the soil or formed in the decomposition of soil organicsubstances. It has also been reported that soil loses nitrogen in the form of nitrogen gas much less when manured with sodium or potassium nitrate than when treated with ammonium sulphate, urea or other organic nitrogenous manures.

AT this Laboratory, investigations into the decomposition of soil organic matter have been concerned with the effect of sulphur and nitrogen content on decay. To simplify the problem in its preliminary stages, and to gain better control of variables, the decomposition of mixtures of pure organic substances was studied.

Oat straw and alfalfa hay were decomposed in the presence of added inorganic nitrogen under differing conditions of incubation. Changes in organic, ammonium, and oxidized forms of nitrogen were determined. With plant material in shallow (3 mm) layers and at two-thirds moisture saturation, there was loss of nitrogen in decomposition of alfalfa, but not in decomposition of oat straw. Some influences of pH, moisture content, pore size, added energy-rich materials, kind of inorganic nitrogen, and of addition of germicidal materials on the occurrence and extent of denitrification are reported. Vigorous bacterial activity and an alkaline reaction were necessary for any extensive denitrification from added nitrate. An active fungus flora was commonly encountered in decompositions in which there were no appreciable losses of nitrogen. Nitrite toxicity appeared to be an important ecological factor in preventing bacterial growth and biological denitrification in acid substrates.

Two New York soils were incubated under conditions permitting control of the partial pressure of oxygen in a stream of gas passing continuously through the soil sample. Ammonium and nitrate salts and organic materials were added in various amounts and combinations and distribution of inorganic nitrogen determined at the end of each experiment. In two experiments the N¹⁵ tracer was used to determine accurately the fate of added nitrate. Immobilization of added inorganic nitrogen occurred whether or not ample nitrogen was available to the soil population, probably due to preferential assimilation. Reduction of nitrate to ammonia was almost negligible at all oxygen concentrations, though slightly higher under anaerobic conditions than otherwise. Denitrification of added nitrate was inversely related to partial pressure of oxygen, but was of appreciable magnitude even under fully aerobic conditions. No evidence of nitrogen loss from the soil was obtained when ammonium salts were added. The experimental findings are compatible with the theory that nitrous oxide in the earth's atmosphere arises from the soil, and that denitrification is of major importance in the nitrogen cycle.