- Email: [email protected]
Nitrification in acid tea soils and a neutral grassland soil: Effects of nitrification inhibitors and inorganic salts
0038-0717/85$3.00+ 0.00 Pergamon Press Ltd
Soil Biol. Biochem.Vol. 17, No. 2, PP. 249-252, 1985 Printed in Great Britain
NITRIFICATION IN ACID TEA SOILS AND A NEUTRAL GRASSLAND SOIL: EFFECTS OF NITRIFICATION INHIBITORS AND INORGANIC SALTS K. N. WICKRAMASINGIIB,* G. A. RODGERS and D. S. JENKINSON Rothamsted Experimental Station, Harpenden, Herts, AL5 2JQ, U.K. (Accepted 25 September 1984) Summary-Nitrification was much slower in five strongly acid (pH 4.0-4.3) soils from tea plantations in Sri Lanka than in a near-neutral grassland soil from the U.K., suggesting that the nitrifiers in these tea soils are close to the lower limit of their pH range. Nitrapyrin effectively inhibited nitrification in all the soils: dicyandiamide was less effective. Low concentrations of KC1 slowed nitrification in the acid tea soils but not in the near-neutral grassland soil. The concentrations of KC1 used (up to 20 mM) were sufficient to cause measurable decreases in soil pH in both acid and neutral soils. It is proposed that this salt-induced decrease in pH is detrimental to nitrifying organisms operating at the limit of their pH range but not to nitrifiers nearer their pH optimum.
INTRODUCTION
The tea plant (Camellia sinensis) responds to very high levels of fertilizer N, usually given as ammonium sulphate or urea. Urea is being increasingly used in Sri Lanka as a source of N for tea plantations, mainly because it is cheaper per unit of N and acidifies the soil less than (NH,), SO,. Ammonium is nitrified in these soils, despite the low pH (Walker and Wickramasinghe, 1979) and the nitrate thus formed can then be lost by dentrification (Wickramasinghe and Talibudeen, 1981) or leaching. Little is known, however, about the effects of specific commercial nitrification inhibitors such as dicyandiamide (DCD) or nitrapyrin (N-serve) on nit&cation in these acid soils. Similarly, although it has often been observed that inorganic salts can inhibit nitrification in nearneutral soils (Hahn et al., 1942; Pathak and Jain, 1965; Sindhu and Cornfield, 1967; El-Shinnawi, 1975; McCormick and Wolf, 1980; Campino, 1982) there is relatively little information on the effects of salts on nitrification in acid soils. Heilman (1975) examined the effects of salts on mineralization and nitrification in moderately (pH 4.3-5.1) acid forest soils. Golden et al. (198 1) showed that KC1 suppressed nitrification in strongly-acid tea soils. We have compared the effects of two nitrification inhibitors, dicyandiamide and nitrapyrin, with those of the inorganic salts, KCl, NaCl and KN03, on nitrification of urea-N in some acid tea soils from Sri Lanka. A neutral grassland soil from the U.K. was also included so that the results on these acid soils could be related to those obtained on neutral soils.
MATERIALS AND METHODS Air-dry
soils
from
tions (St Coombs,
five Sri Lankan
Passara,
Hantana,
tea plantaRatnapura,
Kottawa) and a grassland soil (Highfield) from Rothamsted Experimental Farm were used. The relevant characteristics of the soils (sieved <2 mm) are given in Table 1. Soil pH was measured on a suspension containing 10 g soil (oven-dry basis) and 20ml water. The effect of KC1 on soil pH was measured on a paste containing 20 g soil and 8 ml of either water, 4mr41 KCl, 8 mM KC1 or 16m~ KCl. The values given in Table 1 for this effect are the slopes of the linear regression lines relating paste pH to KC1 molarity over the range O-16mM. Small deviations from linearity over this concentration range were disregarded. Other measurements in Table 1 were made as described by Brookes et al. (1982). Incubation procedure
The soils were initially wetted to 20% WHC and held at 25°C for 1 week before adding the soil amendments. Moist soil (equivalent to 20 g oven-dry soil) was then treated with an aqueous solution of urea containing 2 mg N, followed by the nitrification inhibitor (nitrapyrin or dicyandiamide) or the salt solution (KCl, NaCl or KN03) and finally the moisture content of each soil was adjusted to 50% WHC. Although the same quantities of nitrification inhibitors or of salts were added to the different soils (Table 2), the soil solution concentrations varied between soils, each being incubated at 50% of its (individual) WHC. The soil was mixed thoroughly on a sheet of polythene after the additions had been made, transferred to a loosely-stoppered polypropylene centrifuge tube and incubated aerobically at 25°C. Forty eight samples of each soil (8 treatments x 3 sampling times x 2 replicates) were set up initially. Duplicate samples from each treatment were removed, either immediately, after 12 or 60 days and stored at - 15°C to await analysis. Extraction of inorganic N
*Present address: Tea Research Institute of Sri Lanka, St Coombs, Talawakele, Sri Lanka.
Exchangeable NH: and extractable NO, in the soil were extracted by shaking the soil in the centri249
250
K. N. WICKRAMASINGHE
et al.
Table 1. Descriution of soils
Soil -____ St Coombs Passara Hantana Ratnapura Kottawa Highfield
Sampling depth (cm) O-15 &15 &15 &I5 O-15 G-20
PH in H,O
Decrease in pH caused by a 1mMincrease in KC1 concentration
4.0 4.0 4.2 4.3 4.1 6.8
0.011 0.014 0.013 0.017 0.015 0.018
Particle size analysis -~____ Sand Silt Clay
Organic carbon
Total N
WHC
0.37 0.29 0.16 0.11 0.12 0.35
78.8 78.4 57.5 64.0 56.6 78.9
Percent (%) 25.0 56.0 49.0 63.0 76.0 12.0
14.0 12.0 15.0 11.0 7.0 57.0
61.0 32.0 36.0 26.0 17.0 31.0
4.9 2.6 1.7 1.5 1.5 4.3
Table 2. Exoerimental treatments Quantity of nitritication inhibitor or of salt added (pg g-’ oven-dry soil) Treatment
Urea N
Control DCD Nitrapyrin KC1 (K,) KC1 (KJ KC1 (KA NaCl KNO,
100 100 100 100 100 100 100 100
DCD 10 -
Nitrapyrin 10
-
fuge tubes with 50ml of 0.5~ K2S04 for 2 h. The NH:-N and NOT-N in the soil extracts were determined by steam distillation with MgO and Devarda’s alloy (Bremner, 1965).
K 50 100 200
Na -
59 -
100
Cl
NO,
45.5 91.0 182.0 91 -
159
St Coombs
60
r
•a o-12 0
days
12-60doys
RESULTSANDDISCUSSION Nitrification was relatively slow in all the acid tea soils compared with the neutral grassland soil, presumably because of the low activity of nitrifying bacteria at pH 4.0-4.3 (Fig. 1 and Table 1). In the tea soils the highest rate of nitrification was observed in Ratnapura soil and the least in Passara. Nit&cation was faster from 12 to 60 days than during the 0-12-day period in the control tea soils. The tea soil that nitrified fastest (Ratnapura) still contained almost 5Opg NH:-N g-’ soil after 60 days, so that nitrification was not substrate-limited in any of the tea soils. In contrast, the NH:-N level in the control Highfield soil had fallen to 10 pg N gg ’ soil by 12 days and to 8 fig by 60 days. It follows that nitrification was effectively substrate-limited in this soil from 12 to 60 days. Nitrapyrin was the most effective nitrification inhibitor; no soil treated with nitrapyrin produced more than 1Opg NOT-N g-’ soil throughout the 60-day incubation except St Coombs and Highfield. This was probably because the St Coombs and Highfield soils contained much more organic matter than the others (Table 1): the efficiency of nitrapyrin is negatively correlated with soil organic matter content (Bundy and Bremner, 1973; Lewis and Stefanson, 1975). Dicyandiamide inhibited nitrification in all the soils, but was less effective than nitrapyrin and after 60 days untreated and DCD-treated Highfield soil contained similar amounts of NOT-N (Fig. 1). Nitrification progressively decreased in the acid tea soils as the concentration of KC1 increased (Fig. 1).
Kottawo
Hqhfield
ll-LnrJn0n .2345678
12345678
Treatments
Fig. I. Increase in NO,-N in soils with added urea during aerobic incubation. Treatments: I, control; 2, DCD; 3,
nitrapyrin; 4, KCl(K,); 5, KCl(K,); 6, KCI(K,); 7, NaC1; 8, KNO,. Details of these treatments are given in Table 2.
Nit&cation
soils suggest that the nitrifying bacteria are close to the lower limit of their pH range. In this situation, a further decrease in pH caused by KC1 will have a much larger effect than if the organisms were nearer their pH optimum. It is noteworthy that the decrease in pH caused by a given molarity of KC1 was greatest in the near-neutral Highfield soil (Table 1) yet KC1 had no effect on nitrification in this soil over the whole range of concentrations tested (Fig. 1). Heilman (1975) showed that AlCl, was a more effective inhibitor of nitrification in acid forest soils than CaCl,, which in turn was more effective than KCl, all applied at the same molarity. All these salts would cause a decrease in pH, the order of effectiveness being AlCl, > CaCl, > KC1 (Schofield and Taylor, 1955; Bathe, 1970). Heilman (1975) found that nitrification in acid soils was particularly sensitive to low concentrations of salts and it is likely that the cause was the same as in our experiments-a salt-induced decrease in pH, operating on organisms at the limit of their pH tolerance. The effects of salts on the mineral N (i.e. NH:-N + NOT-N) content of all the soils (except Hantana) after 60 days incubation were small and inconsistent (Table 4). So too were the effects of the nitrification inhibitors DCD or nitrapyrin (Table 4). Four of the salt treatments caused appreciable increases in the total mineral N content of Hantana soil: why in this soil and in none of the others is not known. The experimental findings of this paper substantiate the suggestion made by Golden et al. (1981), that muriate of potash (KCl), when used with N in mixed fertilizers, may in addition to supplying the K requirement of the tea crop, also delay nitrification of NH:-N. Thus it could reduce losses of NOT-N by leaching and denitrification, hence increasing the quantity of fertilizer N available to the tea plants.
Table 3. Effect of KC1on nitrification in soils. Y = nitrification after 60 days @g NOT-N g-l oven-dry soil); X = KC1 concentration (mM) Soil
Correlation
Regression equation
St Coombs Passara Hantana Ratnapura Kottawa Highfield
Y=-1.29X+45 Y = -0.37 X + 18 Y=-1.24X+48 Y= -1.17x+114 Y=-1.02X+30 Y = -0.04x+ 117
251
in acid tea soils
-0.936. -0.92’ -0.97’ -0.989’ -0.971’ -0.085
*Significant correlation (P = 0.05)
In contrast, even the highest concentration of KC1 tested (13.0 mM) has no inhibitory effect in the neutral Highfield soil. This can also be seen from the regressions in Table 3; in the acid tea soils, nitrification was significantly (and negatively) correlated with KC1 molarity, whereas there was no correlation in the Highfield soil. Potassium chloride, NaCl and KNO,, applied at the same molarity, decreased nitrification in the tea soils by similar extents (Fig. 1), although there was a tendency, particularly clearly seen in the fastnitrifying Ratnapura soil, for NaCl to be the least effective. KCl, NaCl and KN03, when applied singly at 6.5 mM, did not depress nitrification in the Highfield soil (Fig. 1), again a fast-nitrifying soil. Excluding salts with toxic metal cations such as Cu or Ni, inhibition of nitrification by soluble salts is usually associated with high concentrations of chloride ions (Hahn et al., 1942; Pathak and Jain, 1965; Sindhu and Cornfield, 1967; El-Shinnawi, 1975). The highest KC1 concentration we used was 18.1 mM in the Kottawa soil, a concentration well below those usually associated with inhibition of nitrification, but compatible with commercial applications of K fertilizer. Even low salt concentrations decrease soil solution pH (Schofield and Taylor, 1955) and the concentrations we used were quite sufficient to produce measurable pH decreases (Table 1). Thus the pH of a soil paste made from 20 g Ratnapura soil and 8 ml water was 4.1; similar pastes made with 4.0, 8.0 and 16.0 mM KC1 had pHs of 4.0, 3.9 and 3.8 respectively. The paste pH of 8.0m~ NaCl was 4.1; of 8.0m~ KNO, it was 4.0. These molarities are the same as used (see Table 2) in the incubation experiments with Ratnapura soil, although the soil-to-solution ratios could not be the same because satisfactory pH measurements were not feasible at the ratio used in the incubations (20 g soil to 6.4 ml solution). The low nitrification rates in the unamended acid
Acknowledgements-K. N. Wickramasinghe thanks UNESCO for a fellowship which enabled this work to be done. We thank the Dow Chemical Co., Kings Lynn, Norfolk, for providing the nitrapyrin. REFERENCES
Bathe B. W. (1970) Determination of pH, lime potential, and aluminium hydroxide potential of acid soils. Journal of Soil Science 20, 28-37. Bremner J. M. (1965) Inorganic forms of nitrogen. In Methorlr of Soil Analysis Vol. 2, (C. A. Black, Ed.), pp.
Table 4. inorganic nitrogen (NH,+ + NO;)-N (NH: + NO;)-N
in soils after 60 days incubation
in soils (pg N g-l oven-dry soil)
St Coombs
Passara
Hantana
Ratnapura
Kottawa
Nil
299
287
203
177
160
222
DCD Nitrapyrin KC1 FL) KC1 W KC1 K)
294 298 300 293 289’
297* 292* 297* 295’ 289
204 198’ 220* 227* 223’
182’ 180 174 175 173’
163’ 161 161 161 1648
209* 219 217’ 226 219
NaCl KNO,
288. 294
296* 306’
223’ 207
189* 183*
163” 171’
226’ 227’
Treatment
*Significantly different from corresponding NOT-N added in KNO, treatment.
Highfield
nil treatment (P = 0.05). Values for KNO, exclude
252
K. N. WICKRAMASINGHE et al.
1179-1237. American Society of Agronomy, Madison, Wisconsin. Brookes P. C., Powlson D. S. and Jenkinson D. S. (1982) Measurement of microbial biomass phosphorus in soil. Soil Biology & Biochemistry 14, 319-329.
Bundy L. G. and Bremner J. M. (1973) Inhibition of nitrification in soils. Soil Science Society of America Proceedings 37, 396-398.
Campino I. (1982) The effects of superphosphate and potassium fertilizer and salts on the nitrogen mineralization of incubated meadow soil. Fertilizer Research 3,
778-782.
Lewis D. C. and Stefanson R. C. (1975) Effect of “N-Serve” on nitrogen transformations and wheat yield in some Australian soils. Soil Science 119, 273-279. McCormick R. W. and Wolf D. C. (1980) Effect of sodium chloride on CO, evolution, ammonification, and nitrification in a Sassafras sandy loam. Soil Biology & Biochemistry 12, 153-157.
Pathak A. N. and Jain S. L. (1965) Effect of alkali salts on nit&cation. Journal of Soil and Water Conservation, India 13, 30-32.
325-336.
El-Shinnawi M. M. (1975) Salts affecting microorganisms in certain soils. Zentralblatt fir Bakteriologie, Parasitenkunde, Infektionskrankheiten 130, 387-394.
area. Soil Science Society of America Proceedings 39,
und Hygiene,
Abt. II
Golden D. C., Sivasubramaniam S., Sandanam S. and Wijedasa M. A. (1981) Inhibitory effects of commercial potassium chloride on the nitrification rates of added ammonium sulphate in an acid red yellow podzolic soil. Plant and Soil 59, 147-151.
Hahn B. E., Colson F. R. and Roberts J. L. (1942) Influence of potassium chloride on nitrification in Bedford silt loam. Soil Science 54, 113-121. Heilman P. (1975) Effect of added salts on nitrogen release and nitrate levels in forest soils of the Washington coastal
Schofield R. K. and Taylor A. W. (1955) The measurement of soil pH. Soil Science Society of America Proceedings 19, 164-167.
Sindhu M. A. and Cornfield A. H. (1967) Comparative effects of varying levels of chlorides and sulphates of sodium, potassium, calcium and magnesium on ammonification and nitrification during incubation of soil. Plant and Soil 27, 468-472. Walker N. and Wickramasinghe K. N. (1979) Nitrification and autotrophic nitrifying bacteria in acid tea soils. Soil Biology & Biochemistry 11, 231-236. Wickramasinghe K. N. and Talibudeen 0. (1981) Denitrification in a very acid tropical soil. Journal of Soil Science 32, 119-l 3 1.
Soil Biol. Biochem.Vol. 17, No. 2, PP. 249-252, 1985 Printed in Great Britain
NITRIFICATION IN ACID TEA SOILS AND A NEUTRAL GRASSLAND SOIL: EFFECTS OF NITRIFICATION INHIBITORS AND INORGANIC SALTS K. N. WICKRAMASINGIIB,* G. A. RODGERS and D. S. JENKINSON Rothamsted Experimental Station, Harpenden, Herts, AL5 2JQ, U.K. (Accepted 25 September 1984) Summary-Nitrification was much slower in five strongly acid (pH 4.0-4.3) soils from tea plantations in Sri Lanka than in a near-neutral grassland soil from the U.K., suggesting that the nitrifiers in these tea soils are close to the lower limit of their pH range. Nitrapyrin effectively inhibited nitrification in all the soils: dicyandiamide was less effective. Low concentrations of KC1 slowed nitrification in the acid tea soils but not in the near-neutral grassland soil. The concentrations of KC1 used (up to 20 mM) were sufficient to cause measurable decreases in soil pH in both acid and neutral soils. It is proposed that this salt-induced decrease in pH is detrimental to nitrifying organisms operating at the limit of their pH range but not to nitrifiers nearer their pH optimum.
INTRODUCTION
The tea plant (Camellia sinensis) responds to very high levels of fertilizer N, usually given as ammonium sulphate or urea. Urea is being increasingly used in Sri Lanka as a source of N for tea plantations, mainly because it is cheaper per unit of N and acidifies the soil less than (NH,), SO,. Ammonium is nitrified in these soils, despite the low pH (Walker and Wickramasinghe, 1979) and the nitrate thus formed can then be lost by dentrification (Wickramasinghe and Talibudeen, 1981) or leaching. Little is known, however, about the effects of specific commercial nitrification inhibitors such as dicyandiamide (DCD) or nitrapyrin (N-serve) on nit&cation in these acid soils. Similarly, although it has often been observed that inorganic salts can inhibit nitrification in nearneutral soils (Hahn et al., 1942; Pathak and Jain, 1965; Sindhu and Cornfield, 1967; El-Shinnawi, 1975; McCormick and Wolf, 1980; Campino, 1982) there is relatively little information on the effects of salts on nitrification in acid soils. Heilman (1975) examined the effects of salts on mineralization and nitrification in moderately (pH 4.3-5.1) acid forest soils. Golden et al. (198 1) showed that KC1 suppressed nitrification in strongly-acid tea soils. We have compared the effects of two nitrification inhibitors, dicyandiamide and nitrapyrin, with those of the inorganic salts, KCl, NaCl and KN03, on nitrification of urea-N in some acid tea soils from Sri Lanka. A neutral grassland soil from the U.K. was also included so that the results on these acid soils could be related to those obtained on neutral soils.
MATERIALS AND METHODS Air-dry
soils
from
tions (St Coombs,
five Sri Lankan
Passara,
Hantana,
tea plantaRatnapura,
Kottawa) and a grassland soil (Highfield) from Rothamsted Experimental Farm were used. The relevant characteristics of the soils (sieved <2 mm) are given in Table 1. Soil pH was measured on a suspension containing 10 g soil (oven-dry basis) and 20ml water. The effect of KC1 on soil pH was measured on a paste containing 20 g soil and 8 ml of either water, 4mr41 KCl, 8 mM KC1 or 16m~ KCl. The values given in Table 1 for this effect are the slopes of the linear regression lines relating paste pH to KC1 molarity over the range O-16mM. Small deviations from linearity over this concentration range were disregarded. Other measurements in Table 1 were made as described by Brookes et al. (1982). Incubation procedure
The soils were initially wetted to 20% WHC and held at 25°C for 1 week before adding the soil amendments. Moist soil (equivalent to 20 g oven-dry soil) was then treated with an aqueous solution of urea containing 2 mg N, followed by the nitrification inhibitor (nitrapyrin or dicyandiamide) or the salt solution (KCl, NaCl or KN03) and finally the moisture content of each soil was adjusted to 50% WHC. Although the same quantities of nitrification inhibitors or of salts were added to the different soils (Table 2), the soil solution concentrations varied between soils, each being incubated at 50% of its (individual) WHC. The soil was mixed thoroughly on a sheet of polythene after the additions had been made, transferred to a loosely-stoppered polypropylene centrifuge tube and incubated aerobically at 25°C. Forty eight samples of each soil (8 treatments x 3 sampling times x 2 replicates) were set up initially. Duplicate samples from each treatment were removed, either immediately, after 12 or 60 days and stored at - 15°C to await analysis. Extraction of inorganic N
*Present address: Tea Research Institute of Sri Lanka, St Coombs, Talawakele, Sri Lanka.
Exchangeable NH: and extractable NO, in the soil were extracted by shaking the soil in the centri249
250
K. N. WICKRAMASINGHE
et al.
Table 1. Descriution of soils
Soil -____ St Coombs Passara Hantana Ratnapura Kottawa Highfield
Sampling depth (cm) O-15 &15 &15 &I5 O-15 G-20
PH in H,O
Decrease in pH caused by a 1mMincrease in KC1 concentration
4.0 4.0 4.2 4.3 4.1 6.8
0.011 0.014 0.013 0.017 0.015 0.018
Particle size analysis -~____ Sand Silt Clay
Organic carbon
Total N
WHC
0.37 0.29 0.16 0.11 0.12 0.35
78.8 78.4 57.5 64.0 56.6 78.9
Percent (%) 25.0 56.0 49.0 63.0 76.0 12.0
14.0 12.0 15.0 11.0 7.0 57.0
61.0 32.0 36.0 26.0 17.0 31.0
4.9 2.6 1.7 1.5 1.5 4.3
Table 2. Exoerimental treatments Quantity of nitritication inhibitor or of salt added (pg g-’ oven-dry soil) Treatment
Urea N
Control DCD Nitrapyrin KC1 (K,) KC1 (KJ KC1 (KA NaCl KNO,
100 100 100 100 100 100 100 100
DCD 10 -
Nitrapyrin 10
-
fuge tubes with 50ml of 0.5~ K2S04 for 2 h. The NH:-N and NOT-N in the soil extracts were determined by steam distillation with MgO and Devarda’s alloy (Bremner, 1965).
K 50 100 200
Na -
59 -
100
Cl
NO,
45.5 91.0 182.0 91 -
159
St Coombs
60
r
•a o-12 0
days
12-60doys
RESULTSANDDISCUSSION Nitrification was relatively slow in all the acid tea soils compared with the neutral grassland soil, presumably because of the low activity of nitrifying bacteria at pH 4.0-4.3 (Fig. 1 and Table 1). In the tea soils the highest rate of nitrification was observed in Ratnapura soil and the least in Passara. Nit&cation was faster from 12 to 60 days than during the 0-12-day period in the control tea soils. The tea soil that nitrified fastest (Ratnapura) still contained almost 5Opg NH:-N g-’ soil after 60 days, so that nitrification was not substrate-limited in any of the tea soils. In contrast, the NH:-N level in the control Highfield soil had fallen to 10 pg N gg ’ soil by 12 days and to 8 fig by 60 days. It follows that nitrification was effectively substrate-limited in this soil from 12 to 60 days. Nitrapyrin was the most effective nitrification inhibitor; no soil treated with nitrapyrin produced more than 1Opg NOT-N g-’ soil throughout the 60-day incubation except St Coombs and Highfield. This was probably because the St Coombs and Highfield soils contained much more organic matter than the others (Table 1): the efficiency of nitrapyrin is negatively correlated with soil organic matter content (Bundy and Bremner, 1973; Lewis and Stefanson, 1975). Dicyandiamide inhibited nitrification in all the soils, but was less effective than nitrapyrin and after 60 days untreated and DCD-treated Highfield soil contained similar amounts of NOT-N (Fig. 1). Nitrification progressively decreased in the acid tea soils as the concentration of KC1 increased (Fig. 1).
Kottawo
Hqhfield
ll-LnrJn0n .2345678
12345678
Treatments
Fig. I. Increase in NO,-N in soils with added urea during aerobic incubation. Treatments: I, control; 2, DCD; 3,
nitrapyrin; 4, KCl(K,); 5, KCl(K,); 6, KCI(K,); 7, NaC1; 8, KNO,. Details of these treatments are given in Table 2.
Nit&cation
soils suggest that the nitrifying bacteria are close to the lower limit of their pH range. In this situation, a further decrease in pH caused by KC1 will have a much larger effect than if the organisms were nearer their pH optimum. It is noteworthy that the decrease in pH caused by a given molarity of KC1 was greatest in the near-neutral Highfield soil (Table 1) yet KC1 had no effect on nitrification in this soil over the whole range of concentrations tested (Fig. 1). Heilman (1975) showed that AlCl, was a more effective inhibitor of nitrification in acid forest soils than CaCl,, which in turn was more effective than KCl, all applied at the same molarity. All these salts would cause a decrease in pH, the order of effectiveness being AlCl, > CaCl, > KC1 (Schofield and Taylor, 1955; Bathe, 1970). Heilman (1975) found that nitrification in acid soils was particularly sensitive to low concentrations of salts and it is likely that the cause was the same as in our experiments-a salt-induced decrease in pH, operating on organisms at the limit of their pH tolerance. The effects of salts on the mineral N (i.e. NH:-N + NOT-N) content of all the soils (except Hantana) after 60 days incubation were small and inconsistent (Table 4). So too were the effects of the nitrification inhibitors DCD or nitrapyrin (Table 4). Four of the salt treatments caused appreciable increases in the total mineral N content of Hantana soil: why in this soil and in none of the others is not known. The experimental findings of this paper substantiate the suggestion made by Golden et al. (1981), that muriate of potash (KCl), when used with N in mixed fertilizers, may in addition to supplying the K requirement of the tea crop, also delay nitrification of NH:-N. Thus it could reduce losses of NOT-N by leaching and denitrification, hence increasing the quantity of fertilizer N available to the tea plants.
Table 3. Effect of KC1on nitrification in soils. Y = nitrification after 60 days @g NOT-N g-l oven-dry soil); X = KC1 concentration (mM) Soil
Correlation
Regression equation
St Coombs Passara Hantana Ratnapura Kottawa Highfield
Y=-1.29X+45 Y = -0.37 X + 18 Y=-1.24X+48 Y= -1.17x+114 Y=-1.02X+30 Y = -0.04x+ 117
251
in acid tea soils
-0.936. -0.92’ -0.97’ -0.989’ -0.971’ -0.085
*Significant correlation (P = 0.05)
In contrast, even the highest concentration of KC1 tested (13.0 mM) has no inhibitory effect in the neutral Highfield soil. This can also be seen from the regressions in Table 3; in the acid tea soils, nitrification was significantly (and negatively) correlated with KC1 molarity, whereas there was no correlation in the Highfield soil. Potassium chloride, NaCl and KNO,, applied at the same molarity, decreased nitrification in the tea soils by similar extents (Fig. 1), although there was a tendency, particularly clearly seen in the fastnitrifying Ratnapura soil, for NaCl to be the least effective. KCl, NaCl and KN03, when applied singly at 6.5 mM, did not depress nitrification in the Highfield soil (Fig. 1), again a fast-nitrifying soil. Excluding salts with toxic metal cations such as Cu or Ni, inhibition of nitrification by soluble salts is usually associated with high concentrations of chloride ions (Hahn et al., 1942; Pathak and Jain, 1965; Sindhu and Cornfield, 1967; El-Shinnawi, 1975). The highest KC1 concentration we used was 18.1 mM in the Kottawa soil, a concentration well below those usually associated with inhibition of nitrification, but compatible with commercial applications of K fertilizer. Even low salt concentrations decrease soil solution pH (Schofield and Taylor, 1955) and the concentrations we used were quite sufficient to produce measurable pH decreases (Table 1). Thus the pH of a soil paste made from 20 g Ratnapura soil and 8 ml water was 4.1; similar pastes made with 4.0, 8.0 and 16.0 mM KC1 had pHs of 4.0, 3.9 and 3.8 respectively. The paste pH of 8.0m~ NaCl was 4.1; of 8.0m~ KNO, it was 4.0. These molarities are the same as used (see Table 2) in the incubation experiments with Ratnapura soil, although the soil-to-solution ratios could not be the same because satisfactory pH measurements were not feasible at the ratio used in the incubations (20 g soil to 6.4 ml solution). The low nitrification rates in the unamended acid
Acknowledgements-K. N. Wickramasinghe thanks UNESCO for a fellowship which enabled this work to be done. We thank the Dow Chemical Co., Kings Lynn, Norfolk, for providing the nitrapyrin. REFERENCES
Bathe B. W. (1970) Determination of pH, lime potential, and aluminium hydroxide potential of acid soils. Journal of Soil Science 20, 28-37. Bremner J. M. (1965) Inorganic forms of nitrogen. In Methorlr of Soil Analysis Vol. 2, (C. A. Black, Ed.), pp.
Table 4. inorganic nitrogen (NH,+ + NO;)-N (NH: + NO;)-N
in soils after 60 days incubation
in soils (pg N g-l oven-dry soil)
St Coombs
Passara
Hantana
Ratnapura
Kottawa
Nil
299
287
203
177
160
222
DCD Nitrapyrin KC1 FL) KC1 W KC1 K)
294 298 300 293 289’
297* 292* 297* 295’ 289
204 198’ 220* 227* 223’
182’ 180 174 175 173’
163’ 161 161 161 1648
209* 219 217’ 226 219
NaCl KNO,
288. 294
296* 306’
223’ 207
189* 183*
163” 171’
226’ 227’
Treatment
*Significantly different from corresponding NOT-N added in KNO, treatment.
Highfield
nil treatment (P = 0.05). Values for KNO, exclude
252
K. N. WICKRAMASINGHE et al.
1179-1237. American Society of Agronomy, Madison, Wisconsin. Brookes P. C., Powlson D. S. and Jenkinson D. S. (1982) Measurement of microbial biomass phosphorus in soil. Soil Biology & Biochemistry 14, 319-329.
Bundy L. G. and Bremner J. M. (1973) Inhibition of nitrification in soils. Soil Science Society of America Proceedings 37, 396-398.
Campino I. (1982) The effects of superphosphate and potassium fertilizer and salts on the nitrogen mineralization of incubated meadow soil. Fertilizer Research 3,
778-782.
Lewis D. C. and Stefanson R. C. (1975) Effect of “N-Serve” on nitrogen transformations and wheat yield in some Australian soils. Soil Science 119, 273-279. McCormick R. W. and Wolf D. C. (1980) Effect of sodium chloride on CO, evolution, ammonification, and nitrification in a Sassafras sandy loam. Soil Biology & Biochemistry 12, 153-157.
Pathak A. N. and Jain S. L. (1965) Effect of alkali salts on nit&cation. Journal of Soil and Water Conservation, India 13, 30-32.
325-336.
El-Shinnawi M. M. (1975) Salts affecting microorganisms in certain soils. Zentralblatt fir Bakteriologie, Parasitenkunde, Infektionskrankheiten 130, 387-394.
area. Soil Science Society of America Proceedings 39,
und Hygiene,
Abt. II
Golden D. C., Sivasubramaniam S., Sandanam S. and Wijedasa M. A. (1981) Inhibitory effects of commercial potassium chloride on the nitrification rates of added ammonium sulphate in an acid red yellow podzolic soil. Plant and Soil 59, 147-151.
Hahn B. E., Colson F. R. and Roberts J. L. (1942) Influence of potassium chloride on nitrification in Bedford silt loam. Soil Science 54, 113-121. Heilman P. (1975) Effect of added salts on nitrogen release and nitrate levels in forest soils of the Washington coastal
Schofield R. K. and Taylor A. W. (1955) The measurement of soil pH. Soil Science Society of America Proceedings 19, 164-167.
Sindhu M. A. and Cornfield A. H. (1967) Comparative effects of varying levels of chlorides and sulphates of sodium, potassium, calcium and magnesium on ammonification and nitrification during incubation of soil. Plant and Soil 27, 468-472. Walker N. and Wickramasinghe K. N. (1979) Nitrification and autotrophic nitrifying bacteria in acid tea soils. Soil Biology & Biochemistry 11, 231-236. Wickramasinghe K. N. and Talibudeen 0. (1981) Denitrification in a very acid tropical soil. Journal of Soil Science 32, 119-l 3 1.