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Chapter 5. Nitrification Inhibitors for Agriculture, Health, and the Environment

Chapter 5. Nitrification Inhibitors for Agriculture, Health, and the Environment

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234



RAJENDRA PRASAD AND J. F. POWER



I. INTRODUCTION

Nitrification inhibitors (NIs) emerged as a group of agrichemicals with the development of N-Serve [2-chloro-6(trichloromethyl)pyridine](Dow Elanco trade

name for nitrapyrin) by Goring ( 1962ab), although inhibition of nitrification by a

number of herbicides, insecticides, nematicides, and fungicides were known long

before. Except for a few field experiments, research on nitrification inhibitors during the 1960s was mostly restricted to laboratory studies (Prasad el al., 1971).

Intensive field investigations were carried out in the late 1960s and 197Os, and the

American Society of Agronomy, the Crop Science Society of America, and the

Soil Science Society of America jointly sponsored a symposium on December 6,

1978, at Chicago, Illinois, the proceedings of which were published in 1980

(Meisinger et al., 1980). Three years later a technical workshop on the nitrification

inhibitor dicyandiamide (DCD) was held on December 4-5, 1981, in Muscle

Shoals, Alabama; this workshop was jointly sponsored by the National Fertilizer

Development Center, the Tennessee Valley Authority at Muscle Shoals, Alabama;

the International Fertilizer Development Center, Muscle Shoals, Alabama; and

SKW Trostberg AG, West Germany (Hauck and Behnke, 1981). A second workshop on DCD was held on December 4-5, 1987, at Atlanta, Georgia, and the

proceedings were published as a special issue in Communications in Soil Science

and Plant Analysis (Vol. 20, Nos 18 and 19, 1989) (Hauck et al., 1989).

In addition to specific chemicals such as nitrapyrin or DCD, natural products

like those from neem (Azadirachta indica Juss) are reported to have nitrificationinhibiting properties (Reddy and Prasad, 1975; Sahrawat and Parmar, 1975) and

have been widely evaluated in India. Prasad et al. (1993) addressed the N use

efficiency aspects of urea coated with neem cake and other neem products at the

Neem World Conference held at Bangalore, India (February 24-28, 1993).

An ideal nitrification inhibitor should be mobile, persistent, and, above all, economic in use (Hauck, 1972). It should also be nontoxic to other soil organisms,

animals, and humans and should move with the fertilizer or nutrient solution.

Compounds with high vapor pressure may move fast and compounds easily absorbed may not be very effective. An ideal NI should stay effective in soil for an

adequate time period; at least for the growth period of a crop. Above all, the real

testing ground is in the economics of use; most studies indicate that about a 0.3 to

0.5 mg ha - ' yield increase will pay for the cost. This one factor alone has stopped

many nitrification inhibitors from reaching the farm level. The major goal in using

a NI is to increase the efficiency of fertilizer N applied to agricultural/horticultural

crops by reducing nitrate leaching losses as well as nitrification losses as N,O or

N,. Thus ideal situations where NIs are likely to be the most effective are those

where such losses predominate, such as rice paddies, areas receiving heavy precipitation, irrigated areas (especially furrow) because of leaching, and crops receiving high rates of N fertilization or manures.



NITRIFICATION INHIBITORS



235



During the 1980s there was considerable effort by ecologists, environmentalists, and some agriculturists to reduce fertilizer N use on the farm, mainly due

to its likely role in increasing nitrate concentrations in groundwater and because

N fertilizers are manufactured from a nonrenewable natural resource (natural gas).

However, on a global scale this will neither be possible nor desirable if we are

to feed the increasing world population. The available estimates indicate that

2422 Tg of cereals will be required in 2000 AD (Prasad, 1986) compared to the

1991 estimated production of 1884 Tg of cereals (FAO, 1991). Thus an additional

28.5% of cereals will have to be produced in the next decade, most of it in the

developing countries.

While this increase in cereal production can be achieved in most African and

South American countries by bringing more land under cultivation, Asia has done

it by increasing productivity per unit land per unit time. This calls for a sizable

increase in the consumption of fertilizer, especially nitrogen. It is estimated by

2000 AD that 145.4 Tg of fertilizer N will be consumed annually (UNIDO, 1978),

which is nearly double the 1990-1991 consumption of 77 Tg of fertilizer N

(FAO, 1991). Furthermore, a large number of the developing countries, especially

those in south and southeast Asia, grow rice as a principal crop, the crop for which

fertilizer N losses are greatest (Prasad and De Datta, 1979; Fillery and Vlek, 1986;

Reddy and Patrick, 1986; De Datta, 1986). In addition to the large amounts of

fertilizer N needed in the developing countries, high costs involved in their production or purchase also need to be considered. Also the sustainability of synthetic

fertilizer production from natural gas at some time in the future is a concern. Thus,

efficient use of fertilizer N is necessary, suggesting that nitrification inhibitors

have a role to play. This chapter provides an overview of the literature available

on the use of nitrification inhibitors in relation to production and quality of agricultural and horticultural crops, human and animal health, and the environment.



11. NITRIFICATION INHIBITORS

A fairly large number of chemicals have been reported as nitrification inhibitors: Nitrapyrin (abbreviated as NP in this chapter) or N-Serve [2-chloro-6-(trichloromethyl)pyridine] (Goring 1962ab); AM (2-amino-4-chloro-6-methylpyrimidine) (Toyo Koatsu Industries, 1965); DCD (Amberger and Guster, 1978);

terrazole or Dwell or etridiazole (5-ethoxy-3-trichloromethyl- 1,2,bthiadiazole)

(Olin Corp., I976ab); DCS [N-(2,5-dichlorophenyl)succinamicacid] (Namioka

and Komaki, 1975ab); KN? (potassium azide) (Hughes and Welch, 1970); ATC

(4-amino- 1,2,4-triazole) (Guthrie and Bomke, 1980); TU (thiourea); MBT (2mercaptobenzothiazole); 2-ethynyl pyridine (McCarty and Bremner, 1986); MPC

(3-methyl-pyrazole-I-carboxamide) (McCarty and Bremner, 1990); ST (2-sulfanil-amido thiazole) (Mitsui Toatsu, 1968); CS, (Ashworth et ul., 1977); 2-mer-



236



RAJENDRA PRASAD AND J. F. POWER



capto- 1,2,4-triazole, sodium diethylthiocarbamate; 2,5-dichloroaniline; 4-amino1,2,4-triazole(Bundy and Bremner, 1973);C2H,(acetylene) (Hynes and Knowles,

1981; Berg et al., 1982); gaseous hydrocarbons such as C,H, (ethane), C,H,

(ethylene), and CH, (methane) (McCarty and Bremner, 1991); ammonium thiosulfate (Goos, 1985); and thiophosphoryl triamide (Radel er al., 1992). Of these,

only eight (NP, AM, DCD, ST, TU, Dwell, MBT, and C2H,) have been widely

tested.

In addition to specific chemicals, allelochemicals also have nitrification-inhibiting properties. For example, Rice ( 1984) postulated that because inhibition of

nitrification results in conservation of both energy and nitrogen, vegetation in late

succession or climax ecosystems contains plants that release allelochemicals that

inhibit nitrification in soil. However, a critical appraisal of the available information does not lend support to such a hypothesis (Bremner and McCarty, 1993). As

an example, terpenoids thought to be released by a ponderosa pine (Pinusponderosu Dougl.) and supposed to inhibit nitrification in soil had no such effects

(Bremner and McCarty, 1988). However, some natural products are reported as

nitrification inhibitors. These include “neem” (A. indica Juss.) cake or an acetone/alcohol extract of seed (Reddy and Prasad, 1975; Sahrawat and Parmar,

1975) and “karanj” (Pongarnia glabra Vent.) seed, bark, and leaves (Sahrawat

et a!., 1974).



A.



RELATIVE EFFECTIVENESS

OF M S



Rajale and Prasad (1970) found AM as effective as NP, while Bundy and Bremner (1973, 1974) found that AM was less effective than NP and DCD. Sommer

(1970) compared a number of NIs and ranked them in the following order:

Terrazole > NP > DCS > guanylthiourea > AM > MAST (2-amino-4-methyl6-trichloromethyltraizine) > ST. McCarty and Bremner (1989) compared 12

compounds and found 6 of them to be effective NIs: 2-ethynylpyridine > Dwell >

NP > MPC > ATC > DCD (Table I).

In a number of U.S. studies NP and DCD were found to be equally effective. In

their studies in Illinois, Malzer et al. (1989) at Urbana, Monmouth (Typic Haplaquolls), and Dekalb (Aquic Arguidoll), showed that the disappearance of ammonium was similar between DCD ( 5 % DCD-N) and NP (0.5 kg ha ’). At Brownston (Mollic Albaqualf), however, ammonium disappearance was slower with

DCD than with NP. Bronson et al. (1989) from Alabama reported that DCD in

Norfolk loamy sand (Typic Paleudult) was equal to NP for up to 42 days, but was

less effective than NP in Decatur silt loam (Rhodic Paleudult).

Etridiazole and NP are equally effective in reducing nitrification of ammonium

N in soils up to 160 days after application on silty loam soils (Typic Ochraqualfs

and Aquic Hapludalfs) in Illinois (Shyilon et al., 1984).

Blending of urea with neem cake inhibited nitrification by 70, 40, and 5% at

~



237



NITRIFICATION INHIBITORS

Table I

Effects of 5 mg kg-' Soil with Different Compounds on Nitrification of

Ammonium in Soils"

Soil

Compound



Harps



2-Ethynylpyridine

Etridiazole (Dwell)

Nitrapyrin (N-Serve)

3-Methylpyrazole- I -carboxamide

4-Amino- I ,2,4-triazole

Dicyandiamide

Potassium azide

N-(2,5-Dichlorophenyl)succinamide

Sodium thiocarbonate

Thiourea

2-Mercaptobenzothiazole

Ammonium thiosulfate



79

61

45

43

41

8

0

0

0

0

0

0



Webster



Storden



g%, inhibition



of nitrification

80

I00

70

97

56

94

53

93

52

92

41

20

3

5

2

5

0

0

0

0

0

0

0

0



"Samples of soil (20 g) were incubated at 25°C for 25 days after treatment

with 6 ml water containing 4 mg N as ammonium sulfate and 0 or 100 y g of

the compound specified. Adapted from McCarty and Bremner (1989).



the end of 1,2, and 3 weeks of incubation, respectively; the corresponding figures

for NP at 1% of N were 85,93, and 90% (Reddy and Prasad, 1975). Thomas and

Prasad (1982) evaluated neem cake-coated urea on a number of soils (Entisols,

Vertisols, Ultisols) and found it to be 50% as effective as NP. The active compounds in neem responsible for retardation of nitrification are thought to be meliacins (epinimbin, nimbin, desacetyl nimbin, salanin, desacetylsalanin, and azadirachtin) (Devkumar, 1986). Nitrification retardation after 2 weeks was 73.6,44.6,

and 12.5% for NP, epinimbin, and desacetylnimbin, respectively (Devkumar and

Goswami, 1992).

Neem cake and DCD were evaluated for their efficiency in inhibiting nitrification of prilled urea-derived NH,+-N in a wheat field (Joseph and Prasad,

1993a,b). Prilled urea was blended with 10 and 20% DCD-N or with 10 and 20%

neem cake and incorporated into the soil just before the wheat was sown. Both

DCD and neem cake partially inhibited the nitrification of prilled urea-derived

NH,; DCD was better than neem cake. The nitrification-inhibiting effects of DCD

lasted for 45 days, while that of neem cake lasted for only 30 days.

Most NIs inhibit nitrification by retarding the oxidation of NH,+-N to

NO,--N by Nitrosomonas sp. Research with different strains of Nirrosomonas



RAJENDRA PRASAD AND J. F. POWER



238

40



30



-



1



-



I



0

0



3



6



9



1



2



0



3



6

9

1

D A Y S



2



0



3



6



9



1



2



Figure 1. Effect of dicyandiamide (DCD),nitrapyrin (NP), and thiourea (TU)on the activity of

from Zacheri and Ainberger ( 1990).



Nitrosomonas eurupoeo in pure culture. Adapted



sp. showed remarkable differences in sensitivity to nitrapyrin (Belser and

Schmidt, 1981), and it was concluded that NP does not retard the activity of the

entire population of Nitrosomonas sp. Results of a study done by Zacheri and

Amberger (1990) on the effect of three NIs are shown in Fig. 1. Growth of a pure

culture N . europaea was completely suppressed by 10 ppm NP or 0.5 ppm TU;

inhibition by 300 ppm DCD was 83%. Ammonium oxidation and respiration of

Nitrosomonas cell suspensions were reduced by 93% with 10 ppm NP, 95% with

0.5 ppm TU, and 73% with 300 ppm DCD. When used at 1000 ppm, DCD had

bacteriostatic effects. Enzymatic investigations revealed that hydroxylamine oxidoreductase was not affected by high concentrations of inhibitors (200 ppm DCD,

100 ppm TU). Cytochrome oxidase activity was increased 10% with 200 ppm

DCD, was not affected by 100 ppm TU, and was inhibited by 52% with 100 ppm

NP. These results suggest that different NIs probably have different modes of

action.



B. SOILFACTORS

AFFECTING

EFFECTIVENESS

OF N I S

A number of studies have investigated the effect of different soil factors on the

effectiveness of NIs, and this subject has been well reviewed by Slangen and Kerkhoff (1984). The main findings are summarized below.



1. Organic Matter

Hendrickson and Keeney ( 1979b) found complete inhibition of nitrification

with NP at 0.5 mg kg - I in a soil with 1% organic matter and none in the same

soil when organic matter was raised to 5% by adding active carbon. Similar results

were obtained by McClung and Wolf (1980) with NP and terrazole when they



239



NITRIFICATION INHIBITORS



added compost to the soil. The influence of organic matter is probably due to its

effect on sorption and rate of decomposition of the chemical.



2. Temperature

Most reports suggest that nitrification inhibitors are more effective at relatively

low temperatures, i.e., below 20°C (Goring, 1962a; Bundy and Bremner, 1973).

This is mainly due to the effect of temperature on degradation of a NI and the

consequent persistence. Herlihy and Quirke (1975) found that the half-life of NP

was 43 to 77 days at 10"C and 9 to 16 days at 20°C. Touchton et al. (1979) found

the half-life of NP to be 22 days at 4" C and less than 13 days at 2 1" C for a loamy

soil with pH 6.8 and an organic matter content of 2%. In a soil with pH 5.5 and

an organic matter content of 5%,the half-life of NP was 92,44, and 22 days at 4,

13, and 2 I " C, respectively. Touchton ef a/. ( 1979) reported that the half-life of

NP in a Cisne silt loam was 7 days at 2 1"C and 22 days at 8" C.

DCD is highly sensitive to temperature. Vilsmeier (1980) reported that after

60 days, 0.67 mg DCD-N 100 g I soil degraded to 0.60 mg at 8" C, 0.4 at 14" C,

and 0.1 mg at 20" C in a sandy silt loam soil of Germany with a pH of 6.2. Bronson

et a/. ( 1989) found that the half-life of DCD decreased from 52.2 days at 8" C to

22 days at 22°C in Norfolk loamy sand and from 25.8 days at 8°C to 7.4 days at

22" C in Decatur soils. Data of McCarty and Bremner (1989) for Iowa soils

showed that in 28 days inhibition of nitrification decreased from 72% at 15"C to

19% at 30°C in Harps silty clay soil when DCD was added at 10 mg kg - I soil

(Table 11). At 30°C the inhibitory effect at 10 mg kg - ' of soil with etridiazole

exceeded that at 100 mg kg soil with DCD and the inhibitory effect of 10 mg

~



~



Table I1

Influence of Soil Temperature on Effectiveness of Dicyandiamide (DCD) for

Inhibition of Nitrification of Ammonium in Soils"



Soil



Amount of

DCD added

(mg kg ' soil)

~



Soil temperature ("C)

15



20



25



30



% inhibition of nitrification



Harps



10



50

Webster



10



50

Storden



10



50



72

83

78

85

90

97



60

82

65

84

75

94



48

72

51

13

53

89



19

49

25

62

23

81



"Samples of soil (20 g) were incubated at 15, 20, or 30°C for 28 days after treatment

with 6 ml water containing 4 mg N as ammonium sulfate and 0.0.2, or I .O mg of DCD.

Adapted from McCarty and Bremner (1989).



2 40



RAJENDRA PRASAD AND J. F. POWER



k g - ' soil with NP exceeded that at 50 mg kg - I soil with DCD (McCarty and

Bremner, 1989). In Illinois, DCD and NP were equally effective on Drummer silty

clay loam at 7.2"C. but DCD was more effective than NP at 15.5"C (Sawyer,

1985).



3. pH

The influence of soil pH on the persistence of NP is reported to be minimal

(Hendrickson and Keeney, 1979a). This can be expected since a number of genes

of nitrifying organisms are involved in nitrification, each with different pH optima

(Bhuija and Walker, 1977).



4. Soil Water

Hydrolysis of NP is enhanced in water-saturated soils (Hendrickson and Keeney, 1979a) as compared to aerobic conditions in soils at field capacity (0.01 to

0.033 M Pa). Volatilization of NP is more pronounced in wet than in dry soils

(McCall and Swann, 1978).

In addition to these factors, method and time of fertilizer application and source

of N used can affect the effectiveness of NIs under field conditions (Singh and

Prasad, 1985; Sudhakara and Prasad, 1986a; Thomas and Prasad, 1987).



c. N



S AND NITROGENLOSSES

AND IMMOBILIZATION



1. Urea Hydrolysis

Most of the NIs, such as NP, AM, ST, ATC, KN3, CS,, and DCD, have little

effect on urea hydrolysis. However, TU, ammonium, and potassium ethylxanthate

and thiosulfate retard urea hydrolysis (Mahli and Nyborg, 1979; Goos, 1985; Ashworth ef al., 1980).



2. Ammonia Volatilization

Since NIs retard nitrification, ammonium-N can accumulate and result in a

higher soil pH (Bundy and Bremner, 1974), which is conducive to NH3 volatilization. Enhanced NH3 volatilization losses due to application of NI have been reported (Bundy and Bremner, 1974; Smith and Chalk, 1978; Prakasa Rao and Puttanna, 1987). While Bundy and Bremner (1974) reported a 28-34% N loss from

volatilization of added urea N with a NI (NP, ATC, CL- 1850) and 9% without,

Smith and Chalk (1978) found NH3-N losses of 86 and 92 mg kg I soil without

and with NP. High NH3-N losses reported by Bundy and Bremner (1974) could

~



NITRIFICATION INHIBITORS



241



be due to high rates of N applied (400 mg N kg - I soil). Volatilization losses of

NH, with or without NI can be reduced by incorporation of the fertilizer N. Clay

et al. (1990) reported that NH, volatilization from bare soil was lower with urea

and DCD than with untreated urea. However, when the soil surface was covered

with residue, NH, volatilization was similar with or without DCD. Sudhakara and

Prasad (1 986a) reported that when 120 kg N ha ' was applied 20 days after sowing rice, the NH, volatilization loss was 8.37% of the applied N from urea compared to 3.89% with neem cake-coated urea. Thus at rates of N generally applied

in field crops, an increase in NH, volatilization due to NIs can be considerably

reduced by the incorporation of fertilizer N and NIs in soil. Another possibility

for reducing NH, volatilization is the use of dual purpose (NIIurease inhibitor)

compounds such as thiophosphoryl triamide (Radel et ai., 1992).

-



3. Denitrification

By retarding nitrification, NIs slow down and reduce the potential for N loss by

denitrification as N,O or NZ;this, however, should not be confused with the reduction of denitrification per se. For example, some workers reported that NIs

(NP, DCD, NaN,, Dwell, KN3, ST, PM, ATC) directly retard denitrification

(Mitsui et ul., 1964; Henninger and Bollag, 1976; McElhannon and Mills, 1981),

especially when added at the rate of 50 or 100 mg kg ~I soil (Bremner and Yeomans, 1986). Such rates are too high for general applications to field crops. Bremner and Yeomans (1986) evaluated the effect of 28 NIs and found that only KN,

and 2,4-diamino-6-trichloromethyl-Striazine, when added at the rate of 50 mg

kg ~I soil, inhibited denitrification. The other NIs had no appreciable effect on

denitrification.



4. Nitrogen Losses from Plants

Plants also lose some amount of N from the foliage (Wetselaar and Farquhar,

1980; Patron et al.. 1988; Francis et al., 1993). A high loss of N was observed by

Daiger e t a / . (1976) from winter wheat at different locations in western Nebraska,

following different rates of N application. In general, dry matter and N content of

tops and roots reached a maximum at anthesis. Thereafter, dry matter declined by

about lo%, while losses of N from the tops plus roots ranged from about 20 to

over 60% depending on the fertilizer N rate. Tanaka and Navasero (1964) reported

a loss of 47 kg ha I in the N content of rice tops in the 3 weeks before flowering

and maturity at high N rates. Patron et ai. (1988) reported a NH, loss of 60120 ng N m - > sec - I from spring wheat plants during the presenescence time

period (before milk stage) and 200 to 300 ng m - ? sec ~I during final plant senescence. They found that NH, loss rates on a leaf area basis were similar for the low

and high N plants despite significantly higher N concentrations in high N plants.

-



2 42



RAJENDRA PRASAD AND J. F. POWER



Twice the leaf area was attained by the high N plants, resulting in similar NH,

volatilization rates per plant which translates into nearly twice as high on a plant

N basis for the low N plants. Farquhar et al. (1979) reported an evolution of

0.6 nmol m -,sec - I (36 g N ha I day I at LA1 5) from senescing leaves of corn.

In a study at Lincoln, Nebraska, postanthesis fertilizer N losses as NH, from the

aboveground biomass of corn plants ranged from 10 to 25% of the fertilizer applied (Francis et al., 1993); the apparent total N losses from the aboveground plant

material ranged from 49 to 81 kg N ha-'. Francis er al. (1993) observed that

postanthesis N losses from aboveground plant biomass in corn accounted for 52

to 73% of the total unaccounted for fertilizer N and suggested that failure to include such losses can lead to overestimation of N losses from soil by denitrification and leaching.

Mosier er al. (1990a,b) reported a N, + N,O gas flux of 270 g N ha - I

day ~I 15 days after transplanting rice where plants were included in the measuring chamber as compared to only 240 g N ha - I day - I when the plants were not

included in the chamber. They concluded that young rice plants facilitated the

efflux of N, and N,O from the soil to the atmosphere. Effects of NI on such losses

of N from the plants have so far not been reported.

~



~



5. Immobilization

Immobilization of fertilizer N by soil microorganisms is significantly enhanced

in the presence of a nitrification inhibitor (Osiname et al., 1983; Juma and Paul,

1983). This has been attributed to the NIs maintaining more of the applied fertilizer N as NH,+ for a longer period of time (Prasad et al., 1983; Shyilon et al.,

1984; Norman and Wells, 1989) and preferential utilization of NH,+-N by heterotrophic microorganisms (Broadbent and Tyler, 1962; Alexander, 1977). Bjarnason

(1987) reported that not only is NH,' preferentially immobilized, but its remineralization is at a slower rate.

Norman and Wells (1 989) found that immobilization of fertilizer N by soil microorganisms in a Crowley silt loam (Typic Albaquelf) in Arkansas was approximately the same in urea and urea DCD-amended soils during the 4-week period

when the soils were not flooded (Fig. 2 ) . Immobilization appeared to level off after

2 weeks and stayed relatively constant for the remaining 2 weeks. After flooding,

immobilization of fertilizer N was much greater in the urea + DCD-amended soil

than in the urea-amended soil, and by the end of 8 weeks soil with urea DCD

had nearly 1.5 times more NH,+ than that treated with urea only. Osiname et al.

(1983) reported more N immobilization with NP than with DCD. Preferential immobilization of NH,+-N rather than NO,- -N has been suggested by a number of

researchers (Wickramsingha et al., 1985; Rice and Tiedje, 1989). This would support higher immobilization of fertilizer N with NIs.

Budot and Chone (1985) suggested an interesting pathway of nitrite incorpo-



+



+



243



NITRIFICATION INHIBITORS

FLOODED



NONFLOODED



LSD 0.05=4.83mglkg



0



1



2



3



4



5



6



7



8



9



1011



INCUBATION TIME ( w e e k s )



Figure 2. Immobilization of fertilizer N under nonflooded and flooded conditions. Adapted from

Urea; (A)urea + DCD.

Norman and Wells (1989). (0)



ration into the organic N fraction via nitrite self decomposition and fixation on

organic matter in a humic-rich acid forest soil (pH 4.5; organic matter 46%).

Azhar et al. (1986a,b,c) also supported this pathway. NP not only reduced the loss

of nitrite via chemodinitrification, but Nelson ( 1982) also discussed the incorporation of nitrite into the organic N fraction.

Thus it appears that NIs increase immobilization of N by increasing the persistence of ammonium-N. Also, NIs retard nitrite accumulation in soils and thus

reduce fixation of nitrites into organic matter.



III. NIs, NH,+/NO,- RATIOS, AND PLANT GROWTH

Because NIs maintain a higher concentration of NH,' in the soil/solution for a

longer time by retarding nitrification, these chemicals have a role in determining

amounts of NH,+ and NO,- and their ratios to crop plants during different stages

of crop growth.

Ammonium can be more efficiently metabolized than NO, -N because it does

not need to be reduced when incorporated into amino acids or other organic materials. However, NH,', or rather NH;', is toxic to all plants at certain concentrations (Magalhaes and Wilcox, 1984) and this toxicity is related to the pH of the

growing media (Pill and Lambeth, 1977). Magalhaes and Huber (1989) reported

that NH,' toxicity was more severe at lower (3.5) than at higher pH (5.7). There

is also a difference between crops with respect to tolerance to NH,'. Prasad et al.

~



244



RAJENDRA PRASAD AND J. F. POWER



I



1



MAIZE



2



3



1



2



3



b



WEEKS AFTER FERTILIZER APPLICATION



Figure 3. Ammonium-N and nitrate-N concentration in maize and rice soils. 0,without nitranitrate-N. Adapted from Prasad ef al. (1983).

pyrin; 0,with nitrapyrin; -, ammonium-N; -.-,



(1983) suggested the term "ammoniphilic plants" for species growing better with

NH,'. They maintained high concentrations (40-60 mg kg I NH,+-N soil) using NP (Fig. 3) and found that while maize plants suffered in growth, rice plants

did not (Table 111). Rice absorbed more N with NH4+,while maize absorbed less

N in the presence of higher concentrations of NH,'. They identified rice as an

ammoniphilic plant. Other species of ammoniphilic plants are known (Gigon and

~



Table 111

Plant Height and Dry Matter Accumulation in Rice and Maize Plants

Affected by N-Serve (NP) Treatment"

~~~



~



Plant height (cm)



Dry matter

(g per plant)



Treatment



Rice



Maize



Rice



Maize



Without N-serve

With N-serve

LSD ( P= 0.05)



17.1

18.2

0.66



32.9

18.6

3.47



0.24

0.26

0.023



1.45

0.90

0.13



"Adapted from Prasad e t a / . (1983).



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