Nitrification, of Some Ammoniacal Fertilizers as Affected by Level of Fertilization and Soil Texture

Zhl. Bakt. II. Aht. 135 (1980), 589 - 598 [Faculty of Agriculture, Cairo University, and Desert Institute, Cairo]

Nitrification of Some Ammoniacal Fertilizers as Affected by Level of Fertilization and Soil Texture M. MONIE,

1.

HOSKY,

T.

EL-HADIDY,

and R.

EL-SHAHAWY

With 6 Figures

Summary Nitrification of increasing levels of three ammonium fertilizers was studied in five soils differing in texture. Soil texture, type of the NH4 + carrier, and its rate of application govern the magnitude of nitrification and the rate of accumulation and persistence of nitrite. The higher the dose of fertilization, the longer was the time required for its conversion to nitrate and the higher were the amounts of nitrite accumulated. Lower amounts of N0 2 -N were found in coarsetextured soil. Ammonium as nitrate favoured nitrite formation more than as hydroxide or sulphate. At higher levels of XH 4-N application, nitrification proceeded faster in heavy-textured soils.

Zusammenfassung Es wurde die Xitrifikation von drei Ammoniumdlingemitteln hei zunehmender Dosierung in fUnf verschiedenen Boden studiert. Bodenstruktur, NH4 + - Trager und dessen Anwendungsrate determinieren den U mfang del' Nitrifikation, die Akkumulationsrate und die Nitritpersistenz. Je hoher die Dlingemitteldosis war, urn so langere Zeit wurde fUr deren Umwandlung zu Nitrat benotigt und um so hoher waren die Nitritmengen. Geringere Mengen an N0 2 -N wurden in grobtexturierten Boden gefunden. Ammonium als NH 4N0 3 war fUr die Nitritbildung bessel' als in Form des Hydroxyds odeI' Sulfats. Bei Anwendung von hoheren NH 4-N-Dosen ging die Nitrifikation in schwereren Boden schneller vonstatten.

For improving soil productivity, there is an increasing use of ammonium fertilizers instead of fertilization with nitrates, which are liable to be lost from soil through leaching and denitrification. The chemolithotrophic nitrifying bacteria are peculiarly sensitive to a wide variety of environmental factors, and the concentration of NH4 +ions is considered one of the most important factors, determining the activity of this group of bacteria. Ammonium salts, when added to the soil, particularly as side dressing or when anhydrous ammonia is applied through injection, usually result in high concentrations of NH + ions in a localized area. The cations are distributed around the point of contact with the soil, forming a fertilizer band of decreasing concentrations towards the edge. Within this band there are certain concentrations more optimum for nitrification, while others are less suitable or even may inhibit the biological process. Since NH+ ~ ions tend to adsorb on the clay as well as on soil colloids as exchangeable cations, the diffusion of NH + 4 differs considerably according to the clay content of the soil. Soil texture, therefore, may play an important role in the magnitude of nitrification.

590

M. MONIB et al.

The present work was undertaken to study the effect of soil texture on the nitrification of three ammonium fertilizers, applied at increasing levels to represent a cross section of a fertilizer band.

Materials and Methods Fertile clay and sand soils were air·dried and crushed to pass a 2·mm sieve. Three other soils were prepared by mixing different portions of the two soils to obtain three textural classes, namely sandy clay, sandy clay loam, and sandy loam. Soils were distributed in glass jars (10 cm diameter and 15 cm height) at the rate of 300 g portions and autoclaved at 128 DC for one hour in three successive days. Standard solutions of ammonium sulphate, ammonium nitrate, or ammonium hydroxide were added to the soils in known amounts to obtain NH 4·N concentrations of 100, 200, 400, 600, or 800 ppm. Ammonium hydroxide was used in place of gaseous ammonia, since anhydrous ammonia is rapidly hydrated when applied to soils. This made it much easier to control the concentration of NH4-N applied. Each jar was inoculated with 3 % (v/w) fresh soil suspension (1: 10), as a source of mixed soil micro-organisms, plus 1 ml each of ammonium- and nitrite-oxidizing liquid bacterial cultures. This technique was applied instead of obtaining the required soil textures from different locations in nature. It is much preferable that the different strains of specific bacterial gropus, particularly the nitrifiers, be similar in all treatments in order to ensure that differences among soils in their nitrifying power are due to variations in texture and not due to differences in density or variations in the physiological characteristics of the naturally prevailing nitrifying bacteria. This also mimizes as much as possible the effect of any factor other than soil texture which may affect the nitrification process, such as the salt content, pH, organic matter, etc. Soil moisture was adjusted at 60 % WHC and incubated at 30 DC. Distilled water was added at intervals to compensate for loss by evaporation. Sufficient replicates were prepared for each treatment and at each sampling date, threE' jars, representing one soil treatment, were taken at random for the determination of the different forms of inorganic nitrogen. To obtain mixed cultures of ammonium- or nitrite-oxidizing bacteria, mineral salt broth (ALEXANDER and CLARK 1965), containing 0.5 % (NH4)2S04 or 0.1 % ~aN02' was inoculated with fertile soil and incubated "t 30 DC. Subculturing in the same media was made every three days up to one month, where each culture failed to bring about the specific reaction of the other.

Chemical analysis The different forms of inorganic nitrogen were extracted from soil, using 2 N KCl solution. Ammonium nitrogen was determined by alkaline distillation in presence of NaOH and then combined N0 2 - + N0 3 - were determined in the same aliquot in a second distillation, using Devarda's alloy. Nitrite nitrogen was determined colorimetrically in another soil portion (CHARLOT 1964) and in case of its presence, figures of N0 3 -N were obtained by difference.

Results The assessment of nitrification could be achieved by following up the decrease in NH\I-N as well as the increase in N0 2-N and N0 3-N. However, changes in one or more of these forms of nitrogen may occur, due to some processes other than nitrification, i.e. immobilization, denitrification or ammonia volatilization which could be ascertained by following up the periodical changes in total inorganic nitrogen. In case of using ammonium as sulphate or nitrate, only slight changes of ± 3 % were observed, while in case of NH 4 0H great losses in mineral nitrogen took place, particularly during the first two weeks which could be attributed to ammonia volatilization. Rate of volatilization was positively correlated with the level of NH 4 0H application and negatively correlated with the soil cation exchange capacity. In sand and sandy loam, 63-85 % and 56-79 %, respectively, of the mineral nitrogen were lost during the fir£t week against 10-55 % in the other three soils.

Nitrification of Some Ammoniacal Fertilizers

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Weeks Fig. 1. Decreases in NH 4 ,N and increases in N0 3 ,N in clay soil as affected by the level and type of ammoniacal fertilizers.

The periodical decrease in NH 4 - N was governed by the type of the fertilizer and its concentration in soil (Figs. 1-5). In presence of low concentration (100 ppm NH 4-N), nitrification of either (NH4)2S04 or NH 4 N0 3 proceeded at about the same rate, and within 4-6 weeks about 85 % of the added ammonium was oxidized. Increasing the level to 200 or 400 ppm NH 4 -N, prolonged time was required for oxidation, and differences between treatments due to the type of the fertilizer became pronounced. At both concentrations, 90 % or JIlore of the soil NH4-N was oxidizcd during 5-6 and 6-7 weeks, respectively, in case of (NH4)2S04 against 7-10 and 9-1~ weeks in case of NH 4 N0 3 additions. In all the previously JIlentioned concentrations, a sharp decrease in NH 4 -N took place during the earlier periods of the experilllent without the occurrence of a delay period. Only when sandy loam and sand soils were supplied with 400 ppm NH4-N in the forlll of NH 4 N0 3 , a period of about 9 or 10 weeks, characterized by a low nitrifieation rate, was recorded. Higher doses of fertilization (600 and 800 ppm NH 4-N) resulted in wide differences in the rate of nitrification, depending on the type of the fertilizer used: alllJlloniulIl as sulphate nitrified faster than as nitrate in all soils. In presence of the first fertilizer, a sharp drop in alllllloniulll took place during the first w('ck and the rate of oxidation continued thereafter at a relatively high rate. Oxidation power Imrdly differed from one soil to another, and 90 %

592

M. MONIB

et al.

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Fig. 2. Decreases in NH4-N and increases in N0 3 -N in sandy clay soil as affected by the level and type of ammoniacal fertilizers.

or more of NH4,N was oxidized during 6-8 weeks. Application of NH 4NO a at the same concentrations resulted in a prolonged period of about 13 weeks before a signi, ficant decrease in NH4,N took place. By this time only 9-15 and 6-13 % of the added fertilizer disappeared from soil at the fertilization rates of 600 and 800 ppm NH 4,N, respectively. There after, ammonium oxidation was markedly affected by the type of soil as well as by the concentration of the fertilizer applied. At the level of 600 ppm NH 4,N, about 90 % of the added ammonium was oxidized by the end of 20 weeks in the first four soils against 48 % in the sand soil. In presence of 800 ppm NH 4,N, the percentage decrease in ammonium in the clay, sandy clay, sandy clay loam, sandy loam, and sand soils was 85, 70, 60, 36, and 19, respectively. This result indicates that the soil texture affects the first step of the biological oxidation of ammonium nitrate when added in high concentrations. With respect to the nitrification of NH4,0H, it is evident that misleading trends will be obtained in case of following up the changes in soil ammonium, since variable amounts of this fertilizer volatilized. Accordingly, the periodical increase in N0 2,N plus NOa,N will give accurate informations about the nitrification of this fertilizer in the different soils. Taking in consideration the actual levels of the remaining ammonium and not the total amounts added, it could be stated that the oxidation of NH 4 0H was more rapid than the other two

Nitrification of Some Ammoniacal Fertilizers

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fertilizers when present in soils at equal amounts. There is also a tendency for nitrification to proceed rather rapid in light-textured soils. Nitrite nitrogen was invariably detected in all treatments and it seems probable that an interaction between three factors, namely soil texture, type of ammonium carrier, and its rate of application, governs the accumulation rate of N0 2-N and its persistence in soil. With respect to soil texture, it is evident from Fig. 6 that very low amounts of nitrite were detected in all treatments of the sand soil, if compared with those in the corresponding treatments of the other four soils. Less than 10 ppm N0 2-N were found when (NH4)2S04 or NH 40H were used, while in presence of NH 4N0 3 it hardly exceeded 50 ppm. With the exception of the soil which showed a characteristic pattcrn in nitrite formation, the effect of the type of fertilizer and its concentration in soil had a pronounced effect on nitrite formation and its rate of oxidation to nitrate. Lower levels of (NH4)2S04 (100 or 200 ppm NH 4-N) resulted in the production of 24-42 and 39-78 ppm N0 2-N, respectively. Such nitrite accumulation was limited to the first two weeks, followed by a sharp decrease; only about 5 ppm N0 2-N were detected in the 3-week period. A similar trend was obtained in presence of the same levels of NH4-N in the form of NH 40H, though comparatively lower amounts of nitrite were produced. Nitrite formation in presence of 100 ppm NH 4-N in the forIll of NH 4N0 3 was very low, but at 200 ppm NH4-N high amounts of N0 2 -N were produced (42-50 ppm) during the 4-week period and persisted for 39 Zbl. IJakt. II. Abt., 1M. 135

594

M. MONIB et al.

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Fig. 4. Decreases in NH4,N and increases in N0 3 ,N in sandy loam soil as affected by the level and type of ammoniacal fertilizers.

7-8 weeks before attaining a low value or to be completely oxidized to nitrate. The application of 400 ppm NH 4-N of any of the tested fertilizers resulted in the production of higher amounts ot nitrite. In the first four soils, nitrite formation increased with time to reach a peak, then tended to decline gradually. Maximum accumulation took place within the 4-week period when ammonium as sulphate or hydroxide was applied. Ranges from 108 to 128 and from 100 to 112 ppm N0 2-N were found, respectively, and by the end of the 7- or 8-week period nitrite disappeared from soils. One exception from this general trend was observed in sandy loam soil receiving NH 40H, where only small amounts of N0 2-N were detected during the first two weeks. This could be attributed to the fact that more than 70 % of the added fertilizer was lost through volatilization and the remaining amount, which hardly exceeded 100 ppm NH 4-N, was not high enough to result in the accumulation of big amounts of nitrite, comparable to those detected in the first three soils. The maximum accumulation rates of nitrite in presence of NH 4NO a ranged from 160 to 168 ppm N0 2-N in the first three soils against 44 ppm in the sandy loam. A longer period (10 or 11 weeks) was required for complete oxidation of nitrite. Differences in the magnitude of nitrite accumulation, resulting from the type of fertilizer used, became more pronounced when high levels of NH4-N were applied. At the rate of 600 ppm NH4-N in the form of (NH4)2S04' the maximum accumula-

Nitrification of Some Ammoniacal Fertilizers

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Ammonium sulphat~

Ammonium nitrate

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00

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Fig. 5. Decreases in NH4-N and increases in N0 3-N in sandy soil as affected by the level and type of ammoniacal fertilizers.

tion of N0 2-N took place during the 2-week period, where appreeiable amounts of about 250 ppm were found, representing 40 % of the added ammonium. In case of adding 800 ppm NH4-N of the samc fertilizer, the maximum accumulation of N0 2-N was found in the 4- or 5-week period, ranging from 330 to 440 ppm, which represents 40-55 % of the added ammonium. At either level of fertilization, gradual oxidation of nitrite took place, and by the 10- or 12-week period it was not longer detected. Although similar high nitrite accumulation rates existed in the first three soils receiving similar doses of ammonium in the form of hydroxide, yet rapid oxidation of nitrite was observed; it disappeared from soil by the end of 8 weeks. Nitrite accumulation in presence of 600 or 800 ppm NH4-N in the form of NH 4N0 3 showed another trend. Nitrite nitrogen in all soils hardly exceeded 30 ppm and disappeared within 5-7 weeks. Such low accumulation rate could be attributed to the rather slow rate of ammonium oxidation, hut was not due to a higher activity of the nitrite-oxidizing bacteria. Regarding nitrate formation, it is evident from Figs. 1-5 that the concentration of NH4-N in soils had a pronounced effect on the nitrification process; the higher the dose of the fertilizer applied, the longer the time needed for its conversion to nitrate. In all concentrations of (NH
596

M. MONIB et aI. added (ppm J

NHI. -N Ammonium SUlphate

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Fig. 6. Changes in N0 2 -N in clay, sandy clay, sandy clay loam, sandy loam, and sand soil as affected by type and level of ammoniacal fertilizers.

by volatilization, it is obvious that ammonium as hydroxide nitrified faster than as sulphate. Comparison between the two fertilizers on the basis of the actual amounts of NH 4-N, present in a given soil, revealed that complete oxidation of NH 4 0H to nitrate was rather rapid and nitrification approached completion in a relatively shorter time.

~itrification

of Some Ammoniacal Fertilizers

597

Lowest nitrification rates were always found when ammonium nitrate was used. At a concentration of 100 ppm NH4-N the nitrification rate of this fertilizer was more or less similar to that of ammonium sulphate, but at the concentration of 200 and 400 ppm, 2-4 and 3-7 weeks longer were required, respectively, for comparable amounts of NOa-N to be obtained, depending on the type of soil. Using the highest doses of ammonium nitrate (600 and 800 ppm NH4-N) lowered the nitrification rate in all soils. During the delay period which was extended for about 13 weeks, only 7-15 % of the added ammonium was recovered as nitrate. After such a period of low nitrate formation, NOg-N was produced in appreciable amounts in all soils, but it seems evident that the type of soil has a significant effect in that respect. By the end of the 20-week period, 90, 89, 89, 85, and 46 % of the added ammonium were converted to nitrate in the clay, sandy clay, sandy clay loam, sandy loam, and sand soil, respectively, when 600 ppm NH4-N of this fertilizer was applied. In case of supplementation with 800 ppm NH4-N the corresponding figures were 85, 70, 60, 36, and 20 %. This indicates that the interaction between soil type and the concentration of NH 4NO a governs the rate of nitrate formation. Soil texture has a pronounced effect on the magnitude of nitrate production from the three tested ammonium carriers. Complete oxidation of NH 4-N, particularly when present in high levels, was positively linked with the amount of sand fraction in soil, indicating that nitrification proceeds faster in light-textured soils than in heavy-textured ones.

Discussion The comparison between the three fertilizers revealed that the magnitude of nitrification followed the order NH 40H > (NH4)2S04 > NH 4NO g in all investigated soils which invariably have a soil reaction on the alkaline side of neutrality. Similar findings were obtained by BROADBENT et al. (1957) concerning acid and non-calcareous alkaline soils, but not in calcareous ones where ammonium as sulphate nitrified faster than as hydroxide. ENO and BLUE (1954 and 1957) who obtained similar results attributed the more rapid nitrification of NH 40H in acid than in neutral soils to the fact that the increase in pH of the acid soils enhance nitrate production. Amlllonium as either sulphate or nitrate nitrified similarly if added at low concentrations, but when the fertilization dose exceeds 200 ppm NH4-N significant differences in the rate of nitrification takes place; NH4N Oa nitrified rather slowly, particularly at the highest dose, accompanied by a more prolonged delay period. This could be attributed to the toxic effect of nitrate ions. Furthermore, the increase in osmotic pressure of soil solution, resulting from the addition of the fertilizer, llIay affect the activity of the nitrifying bacteria. The application of 800 ppm NH4-N in the form of XH 40H, (NH4)2S04 or NH 4NO g will increase the salt content of the soil by 0.20, 0.37, and 0.46 %, respectively. This may be one of the reasons contributing to the variations between the ammonium carriers in their nitrification. In connection with the effect of salts on nitrification, it may be expected that the harmful effect of the increased salinity might be more pronounced in the coarse-textured soils of low water holding capacity in which salts will be dissolved in smaller amounts of water in comparison to those in fine-textured ones. On the other hand, the increase of the clay content in soils resulting in higher cation-exchange capacities will favour the nitrification process (GOLDBERG and GAINEY 1955 and SMITH 1964). However, nitrification power in sand was comparatively higher than in soils rich in clay content. This may be due to the variations in aeration rates in soils in spite of that all soils were kept at 60 % WHC. It is possible that in heavy-textured soils the micro-environment may hecome essentially anaerohic or of low oxygen tem;ion, even though the soil is w~ll aerated by or-

598

M. MONIB et aI., Nitrification of Some Ammoniacal Fertilizers

dinary standards. Obviously such circumstances will occur more readily in clay than in sand. Small pores which are filled with water are practically prone to the development of anaerobic conditions, even though many of the large pores are filled with air. Nitrite formation, following the application of ammonium fertilizers under field conditions, is of great importance since nitrite is known to be highly toxic for the growing plants even when present in small amounts. The increasing level of any fertilizer was found to increase the magnitude of N0 2-N accumulation which supports the previous findings of BROADBENT et al. (1957), STOJANOVIC and ALEXANDER (1958), ALEEM and ALEXANDER (1960), and NAKOS and WOLCOTT (1972). This phenomenon was suggested to be due to the selective inhibition of ammonium to the nitrite-oxidizing bacteria. It is worth mentioning that presence of high levels of NH 4NO a inhibited the first step of nitrification for about 13 weeks. Thereafter, when oxidation took place at high rates, nitrite was rapidly oxidized to nitrate, indicating that Nitrobacter became acclimated to the prevailing high concentrations of NH4+ and NO aions. There was also a clear tendency for nitrite to accumulate in higher amounts in clay soil than in coarse-textured ones. Variations between the tested ammonium carriers in their nitrification rates may be of practical importance in choosing between the fertilizers, but it should be realized that two points should be taken into consideration. The first is that, although ammonium nitrate showed the lowest nitrification rate when applied at high doses, it is quite evident that the nitrogen content of this fertilizer is about double the amount in the other two fertilizers and under field conditions the nitrate portion of the fertilizer is taken into consideration. The second point is that the application of anhydrous ammonia results in great losses due to volatilization, especially in soils of low cation exchange capacity. References ALEEM, M.1. H., and ALEXANDER, M.: Nutrition and physiology of Nitrobacter ag'ilis. App!. Microbiol. 8 (1960), 80. ALEXANDER, M., and CLARK, F. E.: Nitrifying bacteria. In: Methods of Soil Analysis. Part 2. Chemical and Microbiological Properties. No.9 in the series of Agronomy. Am. Soc. Agron. Inc., Madison, Wisconsin, U.S.A. (1965). BROADBENT, F. E., TYLER, K. B., and HILL, G. N.: Nitrification of ammoniacal fertilizers in some California soils. Hilgardia 27 (1957), 247. CHARLOT, G.: Colorimetric Determination of Elements, Principles and Methods. Elsevier Publ. Co., N.Y. 1954. ENO, C. F., and BLUE, W. G.: The effect of anhydrous ammonia on nitrification and the microbiological population in sandy soils. Soil Sci. Soc. Amer. Proc. 18 (1954), 178. - - The comparative rate of nitrification of anhydrous ammonia, urea and ammonium sulfate in sandy soils. Soil Sci. Soc. Amer. Proc. 21 (1957), 392. GOLDBERG, S. S., and GAINEY, P. L.: Role of surface phenomena in nitrification. Soil Sci. 80 (1955),43. NAKOS, G. G., and WOLCOTT, A. R.: Bacteriostatic effect of ammonium on Nitrobacter agilis in mixed culture with Nitrosomonas europaea. Plant and Soil 36 (1972), 521. SMITH, J. H.: Relationships between soil cation·exchange capacity and the toxicity of ammonia to the nitrification process. Soil Sci. Soc. Amer. Proc. 28 (1964), 640. STOJANOVIC, B. J., and ALEXANDER, M.: Effect of inorganic nitrogen on nitrification. Soil Sci. 86 (1958), 208. Authors' addresses: Prof. Dr. M. MONIB and Prof. Dr. 1. HOSNY, Agricultural Microbiology Department, Faculty of Agriculture, Cairo University, Egypt, and Dr. T. T. EL·HADIDY and Dr. R. EL·SHAHAWI, Desert Institute, Mataria, Cairo, Egypt.