Nitrification of Ammonium Sulphate and Urea Fertilizers under Saline Condition

ZbI. Mikrobiol, 139 (1984),

343~347

[Faculty of Agriculture, Cairo University, Cairo, and Faculty of Agriculture, King Saud University, Saudi Arabia]

Nitrification of Ammonium Sulphate and Urea Fertilizers under Saline Condition R. M. EL-SHAHAwv and A. S. MASHHADY

Summary In normally productive sandy clay loam soil, nitrification of ammonium sulphate and urea as affected by salinity, was studied. Generally, increasing salinity delayed, suppressed or inhibited nitrification. Urea ammonification was slightly affected by salinity and the process appeared to be chemical rather than biological. N0 2-N accumulation was positively correlated with the level of salinity, i.e., as salinity increases N0 2-N increases and persisted for long time. The relative sensitivity of Nitrobacter to salinity was discussed.

Zusammenfassung In einem normal fruchtbaren Lehmboden wurde die Nit.rif'ikation von Ammonsulfat und Harnstoff unter dem EinfluJ3 einer Bodenversalzung untersucht. Steigende Salzkonzentrationen verzogerten, unterdriickten oder inhibierten die Nitrifikation. Die Ammonifizierung von Harnstoff wurde durch Versalzung nur wenig beeinfluJ3t, der ProzeJ3 war eher chemischer als biologischer Natur. Die Nitritanreicherung korrelierte mit dem Versalzungsgrad posit.iv, d. h., Versalzung er-hohtc die Nitritmenge, welche fur lange Zeit erhalten blieb. Die relative Empfindlichkeit von N'itrobacter gegeniiber Versalzung wird diskutiert.

Salinity, whether inherent in soil or added in irrigation water, adversely affects soil chemical and biological properties. The effects are usually related to total concentration (higher osmotic pressure) or to the presence of specific ions. Nitrification was found, by many investigators, to be greatly affected by salinity. RANKOV (1962), SINDHU and CORNFIELD (1967), and LAURA (1977), found that low salinity stimulated nitrification. The process, however, though having been found by others to be retarded, suppressed or inhibited depended mainly on the salt concentration. Retardation of nitrification with increasing salinity was observed by SINGH et al. (1969), and LAURA (1977). Over about 0.4 % salinity, nitrification was suppressed (severely retarded for long time). At 0.8 % or more the process was inhibited completely (RANKOV 1962, SINDHU and CORNFIELD 1967, LAURA 1977, and MCCORMICK and WOLF 1980). Nitrobacter, in general is relatively more sensitive to different physical and chemical factors than Nitrosomonas organisms. This explains the accumulation of N0 2-N in saline soil. The persistence and quantity of N0 2-N, depended on the level of salinity (JOHNSON and GUENZI 1963, LAURA 1977, MCCORMICK and WOLF 1980). Ammonification was often found to be a chemical and not biological process at relatively high salinity levels (SINGH et al. 1969, and LAURA 1974), whereas MCCORMICK and WOLF (1980), found somewhat different results. In the light of results of mentioned workers, the authors found of interest to reemphasize in this paper, the effect of salinity on ammonification of urea and nitrification of urea and ammonium sulphate fertilizers in soil.

344

R. M. EL·SHAHAWY and A. S. MASHHADY

Materials and Methods A normally productive sandy clay loam soil (0.48 % C, 547 ppm total N, 15 ppm N0 3·N, 25 ppm NH4·N, C: N ratio of 8.3 and pH 7.4) was air-dried and prepared for incubation studies by passing it through a 2 mm sieve, thoroughly mixed and distributed into 1,000 ml capacity glass jars at the rate of 300 g/jar. Jars were supplemented with appropriate amounts of (KH4)2S04 and urea standard solutions to obtain a final concentration of 200 ppm N. Saline soils were pre. pared by adding to soil samples different amounts of a saline solution containing equal equivalent weights of NaCl and CaCl 2 : 58.5: 55.5 of NaCl to CaCl2 on w l v« basis (LAURA 1977). The calculated quantities of the salt mixture together with ammonium sulphate or urea, were dis, solved in a known volume of distilled water. The final concentration of salt mixture used were 0.114, 0.228, 0.456 and 0.912 % in the soil. 36 jars were prepared for each treatment. After thorough mixing, soil moisture was raised to 60 % of WHC using distilled water, and jars were incubated at 30°C. Soil moisture was kept constant throughout the experiment by the addition of distilled water at 2,day intervals to compensate for loss by evaporation. Chemical analysis was carried out at weekly intervals for the determination of the different forms of inorganic nitrogen according to BREMNER (1965). At each sampling date three jars, representing one soil treatment, were taken at random for analysis. Mineral nitrogen was extracted using 2 N KCl solution and ammonium was determined by alkaline distillation in presence of MgO. Combined N0 2 and N0 3 were then determined in a second distillation using Devarda's alloy, Nitrite was determined colorimetrically in another soil portion according to CHARLOT (1964). Sampling was discontinued when N0 2, N0 3,N formed represented 95 % of the total inorganic N of the soil or after 12 weeks.

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Results Assessment of nitrification was achieved by following up the decrease in NH 4,N NOa,N concentrations in soil. However, changes of as well as the increase in N0 2 , 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 was ascertained by following up the periodical changes in total inorganic nitrogen. Using (NH 4)2 804 and urea, in present work, differences of -5 to -7 % were only observed in the high salt treatments (i.e. 0.456 and 0.912 %). Periodical decrease in NH 4,N was governed by the type of fertilizer and the level of salt added to the soil as illustrated in Tables 1 and 2. In non-saline treatments, the decrease in NH 4,N was similar in both ammonium sulphate and urea fertilizers and within 5-6 weeks about 95 % of the added ammonium disappeared. Increasing salinity level to 0.114 and 0.228 %, resulted in delaying nitrification of ammonium sulphate and urea compared with control soil (no salt). The delaying period was 1-6 and 1-5 weeks over control soil for ammonium sulphate and urea, respectively. In the previously mentioned two levels of salinity the effects appeared to be retardation of nitrification process, and after certain delaying periods the process was completed. While higher levels of salt (0.456 and 0.912 %) led to complete suppression of nitrification for different periods. At 0.456 % salt, nitrification stopped for about 5 weeks and thereafter continued but at slower rate up to the end of the 12-weeks period. At high salt concentration (0.912 %), nitrification of both fertilizers, was inhibited completely all over the experimental period. It is of interest to mention that the ammonification of urea (either chemical or biological) was not inhibited by any salt concentration. At high salt concentration retardation was observed. Complete ammonification of all added urea occurred at the end of the week 5 at 0.912 % salt treatment. At lower salinity, complete soil ammonification took place after the first week. Regarding nitrate and nitrite formation, it is evident, that the increase in (NO a, N0 2,N) was closely proportional to the decrease in NH 4,N at 0.0, 0.114 and 0.228 %

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Nitrification of Ammonium Sulphate Table 1. Changes in NH!, N0 3 N form

Salinity

0.114 0.228 0.456 0.912

o 0.114 0.228 0.456 0.912

o 0.114 0.228 0.456 0.912

affected by salinity (ppm N)

Time in weeks

%

o

+ NO z and NOz-N of (NH!hSO! as

345

2

3

4

5

6

7

8

9

10

11

12

107 III 195 225 225

82 92 183 221 220

56 73 171 220 220

33 48 157 219 219

16 33 143 215 217

18 129 210 215

116 200 215

103 195 215

67 192 214

45 190 214

19 190 214

190 214

118 114 30

143 133 42 0 0

169 192 209 152 177 192 54 68 82 000 o 0 0

207 96 2 0

109 7

122 9

158 11

180 16

206 20

25

9

11

11

12

o o o o

o

2 2

12

16 0 0

o o

0 2 2 0 0

5 8

o o

0 0 0 0

o

0 0 0 0

o

5

o

7

o

o

o

o

o

o

o

o

o

Each value is a mean of three replicates.

salt concentrations of ammonium sulphate (Table 1). However, at higher salinity levels i.e. 0.456 and 0.912 %, somewhat misleading result was obtained due to 5-7 % loss in soluble nitrogen. In urea treatments similar trends were observed (Table 2). The increase in (NO aN0 2-N) was not proportional to the increase in NH 4-N. This may be due to the hydrolysis of urea which limited the process of ammonification and thus provided ammonium for nitrification. Consequently, nitrification of urea needs longer time than ammonium sulphate irrespective of the level of salt added.

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Table 2. Changes in NH!, N0 3 N form

Salinity

0.114 0.228 0.456 0.912

o 0.114 0.228 0.456 0.912

o 0.114 0.228 0.456 0.912

affected by salinity (ppm N)

Time in weeks

%

o

+ NO z, and NOz-N of urea as

2

3

4

5

6

7

8

9

10

11

12

127 142 213 184 170

124 126 202 200 182

86 123 189 222 200

67 102 176 222 222

52 83 160 222 220

25 60 142 218 218

18 38 125 215 218

22 110 210 216

99 203 214

75 194 212

42 190 210

20 188 210

98 83 12

101 99 23 0

139 102 36

173 200 207 142 165 187 65 83 100 025 o 0 0

203 115 8 0

126 13

150 19

0

o o

158 123 49 0 0

183 23

205 29 0

o

o

2

o

0

5

5

12 0 0

2 8

0 3

12

15

15

20 0

o o

o o

o o

0 0

Each value is a mean of three replicates.

o o o o

o

o

o

0 0

3

6

0

0

0

o

o

o

346

R.

M. EL-SHAHA wv

and A.

S. MASHHADY

The accumulation of N0 2-N in soil was governed mainly by the level of salinity (Tables 1 and 2). The control treatment contained no N0 2-N all over the experimental period. The addition of 0.114 % salt mixture led to the accumulation of 5 ppm N0 2-N at maximum after 3 weeks period. N0 2-N persisted for one week and then disappeared in both fertilizers. The addition of 0.228 % gave similar results, but N0 2-N was slightly higher, i.e, 12-16 ppm at maximum. Increasing salinity to 0.456 % also led to accumulation of N0 2-N after longer time (at week - 7), and persisted to the end of week - 12 with maximum accumulation of 12 and 20 ppm for (NH4)2S04 and urea fertilizers, respectively. N0 2-N accumulation at 0.912 % salinity was nill all over the experimental period in both fertilizers.

Discussion The suggestion of SINGH et al. (1969) and LAURA (1974, 1975), regarding ammonification of urea or inorganic nitrogen under saline condition, did not clearly indicate whether it is a chemical or a biological process. Consequently, the influence of added salts might be due to their effects on the degree of dissociation of water. This suggestion was ascertained in the present work on urea ammonification. In the laboratory, small scale experiment was conducted to study the effect of sterilization on ammonification of urea nitrogen. The data obtained here indicates that at any level of salinity, ammonification of urea nitrogen took place in sterilized as well as unsterilized soil. The comparison between the effect of different concentrations of added salts showed that increasing salinity retarded (delayed) but did not inhibit ammonification of urea. This agrees with the findings of LAURA (1977) on urea and MCCORMICK and WOLF (1980) on native organic nitrogen. The loss in total mineral nitrogen was the same in low saline and non-saline soil. This indicates that addition of salt mixture had no effect on mineralization or immobilization of added or native nitrogen at low salinity levels. BROADBENT and NAKASHIMA (1971), and LAURA (1977) got similar results. The 5-7 % loss of total mineral nitrogen at high salinity levels may be due to volatilization, denitrification, or immobilization of soluble nitrogen (WESTERMAN and TUCKER 1974, MONIB et al. 1980, and MALEK and AZAM 1980). Nitrification and nitrifying microorganisms are well known to be sensitive to saline conditions. Thus, nitrification may be retarded, suppressed or inhibited depending upon salt concentration. Concentrations of 0.114 and 0.228 % delayed nitrification (SINGH et al. 1969, LAURA 1974, 1975). Whereas concentration of 0.456 % or more suppressed nitrification for long periods. Similar trends were also observed by LAURA (1977), who found that commencement of nitrification after long period might have been occurred by adapted nitrifiers, Also WESTERMAN and TUCKER (1974) obtained similar results on ammonium chloride nitrification under saline condition. 0.912 % salt inhibited nitrification completely and no N0 3 N0 2-N was produced all over the experimental period (WESTERMAN and TUCKER 1974, LAURA 1974, 1977, and MCCORMICK and WOLF 1980). However, the priming effect as described by RAMKOV (1962) and WESTERMAN and TUCKER (1974) and ascertained by LAURA (1977), was not pronounced in the present work. Accumulation of N0 2-N was affected mainly by the level of salinity, i.e. as salinity increases N0 2-N also increases and persists for long time. This phenomenon may be due to the relative sensitivity of Nitrobacter to salinity compared with Nitrosomonas microorganisms. This agrees with GUENZI (1963) and MCCORMICK and WOLF (1980), and does not support the finding of LAURA (1977).

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Nitrification of Ammonium Sulphate

347

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Eingegangen am 5. 11. 1982. Authors' address: Dr. RABEE M. EL-SHAHAWY and Dr. A. S. MASHHADY, Soil Science Department, Faculty of Agriculture, King Saud University, Riyadh, Saudi Arabia.