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V. Nitrogen Economy of the System

V. Nitrogen Economy of the System

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73



CEREAL-LEGUME INTERCROPPING SYSTEMS



mixture, the type of management, and the competitive abilities of the component crops.

Legumes of indeterminate growth are more efficient, in terms of N, fixation, than determinate types. Eaglesham et af. (1982) found that in a growing

season, soybean fixed more nitrogen than cowpea, but soybean used a greater

amount of the N2 fixed to produce seed. While cowpea fixed less N, however,

because it had a lower seed N harvest index it thus accrued more N to soil

from residues. They also found that this applied to determinate (det) and indeterminate (ind) cowpea types, but there was a difference in the amount accrued:

kg N/ha

Cowpea

cultivar

ER-1 (det)

TVu 1190 (ind)



N

N

uptake

fixed

~

32

50

33

101



Seed

N



48

49



Residue

N balance

N

34

+ 2

85

+ 52



The literature contains only semiquantitative data on N, fixation of intercrop legume. Graham and Rosas (1978) found that N2 (C,H,) fixation by

climbing bean (cv. P590)was essentially unaffected by intercropping with

maize. N2 fixation reached a maximum of 20.6 moles/plant/hr in the sole

bean at 68 days and was reduced by only 10% when intercropped with

maize.

Shading by the cereal reduces both the seed yield and the N, fixation

potential of the companion legume (Wahua and Miller, 1978a,b). In a

sorghum-soybean intercrop system, a tall variety of sorghum reduced soybean yield by 75% and N, fixation at the early pod-fill stage by 99%

(Wahua and Miller, 1978b).



Soybean intercropped with

Tall sorghum

Short sorghum



N fixation

(mole C,H,/plant/hr)

0.012

3.17



Seed yield

(kdha)



688

2275



With the short sorghum, soybean received more than 90% of the incoming

radiation, compared to less than 50% with the tall sorghum. In a

sorghum-groundnut intercrop system, partial defoliation of sorghum increased the amount of light for the associated legume and enhanced

N,(C,H,) fixation (Nambiar et af., 1983).

In the absence of applied N, N,(C,H,) fixation of groundnut intercropped

with maize was not influenced by shading, although light reaching the

legume was reduced by 33% (Nambiar et af., 1983). When 50% kg/ha of N



74



FRANCIS OFORI AND W. R. STERN



was applied, fixation was reduced by 55% and light reaching the groundnut

canopy was 54% of daylight. Heavy applications of N fertilizer (100 and 150 kg

N/ha drastically reduced fvtation: 7.0 moles/plant/hr at 100 kg N/ha and 3.5

moles/plant/hr at 150 kg N/ha.

Using the "N fertilizer dilution method, estimates of N derived from atmospheric N, by cowpea intercropped with maize were about 60%, corresponding to fixation of 81 kg N/ha, and in the sole cowpea, 56%, corresponding to 83 kg N/ha (Eaglesham et al., 1981).

Ofori et al., (1987) evaluated the N economy of a maize-cowpea intercrop system using both "N natural abundance and "N-labeled fertilizer

methods. They found that cowpea maintained its ability to fix atmospheric

N, when intercropped with maize, but that N, fixation was reduced by N

fertilizer applications (Table X). The comparable P values (percentage of N

derived from atmospheric N,) of the intercropped cowpea with or without

applied N was attributed to greater N uptake by the associated maize, which

induced the companion cowpea to be more symbiotic.



Methods of Estimating N, Fixation

Both direct and indirect methods are used to estimate N, fixation by

legumes in the field. The indirect methods are the ureide and acetylene

reduction assays; these provide qualitative and semiquantitative information only on N, fixation (LaRue and Patterson, 1981). The ureide method is

limited to legumes that produce and translocate N as ureides, e.g., soybean

and cowpea (Atkins, 1982), and will not be discussed further. The three

principal ways of measuring N2fixation in cereal-legume intercropping are

1. the acetylene reduction method

2. the total N-difference method

3. the I5N dilution methods

a. Acetylene Reduction Method. This method evolved from the observations that the enzyme nitrogenase reduced acetylene (C,H,) to ethylene (C,€&)

(Dilworth, 1966, Schollhorn and Bums, 1966). Techniques for field assay include excavation and incubation of freshly excised nodulated roots in a

chamber with 1-209'0 C,H, for 30 to 120 min, followed by analysis of the

ethylene produced with gas chromatography. The ratio of acetylene reduced to

N, fixed is assumed to be 3:1, because the reduction of N, to ammonia uses six

electrons while the production of ethylene requires two (Hardy et al., 1973).

The advantages of the method are sensitivity, speed, and economy.

It has been used to evaluate N, fixation in cereal-legume intercropping

(Graham and Rosas, 1978; Nambiar et al., 1983). It gives an instantaneous,

semiquantitative measure of fixation rather than permitting the measurement of fixation over a growing season by a crop (LaRue and Patterson,

1981), something that most agronomists would be interested in.



75



CEREAL-LEGUME INTERCROPPING SYSTEMS



Table X

Proportional Dependence on N, Fixation (P) and N, Fixed by Sole and

Intercrop Cowpea Estimated by "N Natural Abundance (NA) and

'"-Labeled Fertilizer (NL) Methods under Field Conditionsa

Harvest



Cropping system



P



(070)



N, fixed



No added fertilizer N (NA experiment)

kg N/ha

150 days



(straw)

150 days



(seed)

150 days



(seed



+ straw)



Sole crop

Intercrop



72.0

61.3



32.4

18.0



Sole crop

Intercrop



68.0

67.8



54.7

40.8



Sole crop

Intercrop



69.4

65.7



87.1

58.8



Fertilizer N added (25 kg N/ha) (NL experiment)

kg N/ha

150 days



(straw)

150 days

(seed)

150 days



(seed



+ straw)



Sole crop

Intercrop

Sole crop

Intercrop



47.6

64.9



26.2

25.9



57.1

69.3



55.7

47.1



Sole crop

Intercrop



53.4

67.7



81.9

73.0



aFrom Ofori el al. (1987).



b. Total N-Difference Method. This estimates N, fixed by a legume as

the difference in total N uptake by legume and a nonfixing control plant

such as grass or cereal, expressed as:



N2 fixed by legume = Total NIegume

- Total Nnonfixing plant

The total N of the nonfixing plant is derived solely from soil, and the difference in values of N uptake between the legume and nonlegume is assumed to be the quantity derived by fixation (Williams et al., 1977). The

basic assumption of the method is that both the legume and the nonlegume

remove soil N from the same N pool and in the same proportion as the

amount of N available (LaRue and Patterson, 1981). In addition to

nonlegumes, nonnodulating and uninoculated legume genotypes have been

used (Nutman, 1976).

Bezdicek et al. (1978) compared inoculated and uninoculated soybeans

under field conditions in a 2-year study. They found that uninoculated



76



FRANCIS OFORl AND W. R. STERN



soybeans accumulated an average of 90 kg N/ha, while nodulated soybeans

yielded 378 kg N/ha. By difference, the net N, fixation was calculated to be

288 kg N/ha.

c. I5NDilution Methods. The use of the stable isotope "N for estimating

N2fixation is a direct and definitive method extensively used for checking the

validity of other methods (Burris, 1974). The technique is based on the fact

that 15Noccurs naturally in an almost constant ratio with I4N. The ratio of

I4N: ''N is approximately 272:1, so atmospheric Nz contains 0.3663 at.%

"N or 3663 ppm "N. The only exceptions to this general rule are the slight

variations in Is N enrichment of natural nitrogenous substances such as soils,

rocks, and coal (Hauck and Bremner, 1976). The changes in isotopic composition of samples relative to the atmosphere permit calculation of enrichment of "N in samples for the study of fixation, transfer, and transformations of N in systems. The main factors limiting the widespread use of "N

techniques have been the high cost of N compounds and the need for mass

spectrometers to analyze for "N abundance (Hauck and Bremner, 1976).

With l5 N methods, there are two approaches, namely, "N-labeled fertilizer and "N natural abundance. Both methods are based on the assumptions that the N,-fixing and nonfixing plants have similar rooting patterns

and obtain equal proportions of N from the soil, and that there is no

transfer of N during the measuring period (Chalk, 1985).

Eaglesham et al. (1981) used the "N-labeled fertilizer method to

demonstrate N transfer in a maize-cowpea intercrop system, and Ofori et

af. (1987) used both I5N natural abundance and "N-labeled fertlizer to

quantify N2 fixation and evaluate the N economy of a similar intercrop

system (Table X).

B. NITROGEN

TRANSFER

Evidence in the literature suggests that the N, fixed by the intercrop

legume may be available to the associated cereal in the current growing

season (Agboola and Fayemi, 1972; Remison, 1978; Eaglesham et al.,

1981; Pandey and Pendleton, 1986) or as a residual N for the benefit of a

succeeding cereal crop (Nair et al., 1979; Searle et af., 1981; Singh, 1983).

Both forms of N transfer are considered to be important and could improve

the N economy of various legume-based intercrop systems. This has led to

the suggestion that both current and residual N benefits should be evaluated

in intercrop systems in which legumes are a component (Willey, 1979).

Roots and nodules of legumes are thought to be the important sources of

N transfer because of their high N contents (Butler and Bathurst, 1956). In

cowpea, Minchin et al. (1978) found N from these sources to be only 6% of

the total plant N; this may be inadequate to produce any substantial N



CEREAL-LEGUME INTERCROPPING SYSTEMS



77



benefit for a subsequent crop. From pot studies, Peoples et al. (1983) report

that N from roots and nodules of cowpea are 13% of the total plant N.

The degree to which N from intercrop legume may benefit a cereal crop

depends on the quantity and concentration of the legume N, microbial

degradation (mineralization) of the legume residues, utilization of these

residues, and the amount of N2 fixed by the legume (Henzell and Vallis,

1977; Herridge, 1982). The N in legume residues may be tied up in the soil

organic N pool and may not be readily available to the cereal crop (Ladd et

al., 1983). The rate of mineralization of organic N, determined by microbial

activity, is primarily influenced by the prevailing moisture and temperature

regimes (Ladd et al., 1984). Henzell and Vallis (1977) estimated that under

tropical conditions, 30% of the N in legume residues could be mineralized

and taken up by grass after 24 weeks.



1. Current N Transfer



The idea of possible N tranfer in the current season from legumes to intercropped cereals originated from early studies on legume-nonlegume mixtures in pots under greenhouse conditions (Nicol, 1935; Virtanen et al.,

1937). In a greenhouse experiment, Agboola and Fayemi (1972) observed

that early-maturing legumes could possibly improve yields and N nutrition

of associated maize in the current season, while late-maturing types could

have this effect in the following season. They found that greengram fixed

224 kg/ha of N2in 49 days and improved intercropped maize yield by 72%.

Although cowpea fixed 450 kg/ha of N, in 98 days and calopo fixed 354

kg/ha of N, in 84 days, the N2 fixed by these legumes did not influence the

yields of the associated maize.

Eaglesham et al. (1981) presented evidence from the field of transfer of N

from legume to an intercrop cereal, using the "N-labeled fertilizer method.

It was evident that at the low N rates, i.e., No and Nzs, N concentrations

(percentages) of the intercrop maize were higher in the presence of cowpea

than in the sole maize. The transfer of N was confirmed by the significant

dilution of IsN in the intercrop maize compared to sole maize at NZs.This

was because fixed N would have an enrichment close to natural abundance

and its transfer would result in the dilution of "N enrichment in the intercrop maize derived from the fertilizer.

Using replacement series designs, Patra et al. (1986) have reported

substantial transfer of N from the legume component to the associated

cereal in wheat-gram and maize-cowpea intercrop systems in both

greenhouse and field studies. However, they did not provide "N data to

demonstrate transfer of legume N that would have obviously resulted in



78



FRANCIS OFORI AND W. R. STERN



lower 15Nenrichment of the intercrop cereal compared to sole crop.

In a maize-soybean intercrop system, Pandey and Pendleton (1986)

reported that soybean green manure provided 28 kg/ha of N to maize when

no N fertilizer was applied.

The transfer of N from cowpea to the associated maize was not evident

from either the field or the greenhouse pot studies by Ofori et al. (1987),

because similar I5Nenrichments were obtained in the sole and intercropped

maize. They concluded that cowpea and associated maize were competing

for applied N and that the N, fixed by cowpea ended up in the seed and was

harvested from the system. These findings are consistent with those

reported by Danso et al. (1987)using fava beans and barley.

In a maize-cowpea combination planted without N fertilizer, Remison

(1978)attributed a 72% increase in intercrop maize grain yield over that of

sole maize to the transfer of N from cowpea to the associated maize. Unfortunately, no crop N analysis data were provided to justify the conclusion of

current N transfer claimed by the author.

Based on yield and N contents, Waghmare et al. (1982) found that

sorghum benefited from greengram, groundnut, soybean, and fodder and

grain cowpea intercrops. Intercrop sorghum grain yield increases ranged

from 3 to 16%, grain protein from 10 to 33V0, and N uptake from 16 to

49%.

2. Residual N Transfer

The intercrop legume may accrue to N the soil and this may not become

available until after the growing season, improving soil fertility to benefit a

subsequent cereal crop. Table XI shows results from various studies that

used wheat to measure residual fertility of various cereal-legume intercrop

systems.

Nair et al. (1979)found a mean wheat yield increase of about 30% after a

maize-soybean intercrop, and after maize-cowpea the yield increase was

34% when compared to wheat after sole maize. De (1980)evaluated the

residual N value of various legume-based intercrop systems and found that

blackgram intercropped with either maize or sorghum improved succeeding

wheat yield in all N treatments.

Searle et al. (1981)found N uptake of wheat following maize-groundnut

and maize-soybean intercrop systems to be higher than after maize alone.

At 60 days, N uptake by wheat was about 18 kg/ha when preceded by either

intercrop systems, and this was similar to N uptake of wheat supplied with

100 kg/ha.

Singh (1983) estimated the N benefits to wheat of various preceding

legume intercrops. When comparing wheat after sole sorghum with wheat



79



CEREAL-LEGUME INTERCROPPING SYSTEMS



Table XI

Effects of Preceding Cereal-Legume Intercrop System and Applied Nitrogen on Grain Yield

and Nitrogen Uptake of Succeeding Wheat

N applied (kg N/ha)

Parameter and treatment



0



50



100



Reference



Grain yield (kg/ha)

After sole maize

After maize-soybean

After maize-cowpea

After sole maize

After maize-blackgram

After sole sorghum

After sorghum-blackgram



940

1240

1260

2520

2900

1610

2280



920

1150

1250

4270

5080

41 10

4540



900



Nair et al. (1979)"



1120

1190

5450

5690

5250

5910



Nitrogen uptake (kg N/ha)

After sole maize

After maize-soybean

After maize-groundnut



11.6

17.5

17.8



15.6

21.8

21.2



17.5

21.8

21.2



De (1980)b



Searle et al. (1981)"



"Derived data; N was applied only to the preceding treatment.

bN applied to the wheat crop.



after intercrops, he obtained N fertilizer equivalents of 3 kg/ha with soybean, 31 kg/ha with greengram, 46 kg/ha with grain cowpea and with

groundnut, and 54 kg/ha with fodder cowpea.

It seems that indeterminate legumes with lower seed yield potentials

benefit the associated cereals in terms of N in the current season or as

residual N for succeeding crops.



C. NITROGENLOSSES

Nitrogen is lost from cropping systems through harvested crop products,

gaseous N, and leaching of nitrates from soil beyond the root zone

(Greenland, 1977).

The complexity of the processes involved in gaseous N losses, namely,

denitrification and volatilization, and the paucity of direct measurements

that vary widely with environmental conditions, especially temperature and

water content, has generally limited an accurate assessment of the

magnitude of N losses by these means under field conditions (Vlek et al.,

1981). Efforts to quantify N gaseous losses are a recent development (Simpson and Steele, 1983), and earlier estimates were made by inference from



80



FRANCIS OFORI AND W. R. STERN



measurements in nitrogen balance studies (Hauck, 1971). The losses of N in

cereal-legume intercropping through denitrification and volatilization have

not been studied and the examples cited in this section are mainly from

other cropping systems.

1. N Harvested in Crop Products



Nitrogen harvested from crops as seed is the largest source of N loss from

any cropping system. Assuming N concentration of 1-3% in cereal grain

and 3-6% in legume seed, a cereal yield of 3000 kg/ha of grain removes

30-90 kg/ha of N from the soil, and 800 kg/ha of legume seed removes

24-48 kg/ha of N.

2. Gaseous Losses



The important pathways of gaseous N losses from cropping systems are

through denitrification, the reduction of NO, to N 2 0 and N, by

microorganisms, and volatilization of NH,.

a. Denitrifeation. Denitrification results in an array of gaseous N products that renders it difficult to quantify satisfactorily the N loss via this

pathway. Hauck (1971) reviewed the literature pertaining to N balances in

plant-soil systems and concluded that N losses via denitrification could be

of the order of 10-30070 of the N applied, and that this commonly occurs in

soils wet for prolonged periods, with low CO, concentrations (Freney et al.,

1978).



b. NH, Volatilization. The review by Chalk and Smith (1983) shows that

N losses through NH, volatilization are usually small and that these are

generally less than 2% of the total N applied. However, on a calcareous

soil, Smith and Chalk (1980) measured significant losses of N, (10% of applied NH, N) and N,O (6% of applied NH, N). Simpson (1968) showed up

to 60% loss of urea N applied as topdressings between 67 and 112 kg N/ha

and considered NH, volatilization to be important.

NH, losses from applied N depend on the ammonium source of the fertilizer and on the soil pH (Zamyatina et al., 1968). The rate loss from urea is

more than from either ammonium sulfate or ammonium nitrate. Vlek et

al. (1981) presented data from various studies in which different forms of N

had been applied ranging between 100 and 500 kg N/ha. Assuming mean N

applied to be 300 kg N/ha, the N loss via volatilization was 16.7% of N applied from urea, 1 1.4% from ammonium sulfate, and 5% from ammonium

nitrate.

A review on NH, volatilization by Freney et al. (1983) has shown that N

losses increase with rising pH. This effect was demonstrated by Jewitt



81



CEREAL-LEGUME INTERCROPPING SYSTEMS



(1942) who found NH, losses equivalent to 0, 13, and 87% of N when ammonium sulfate was applied to soils of pH 7.0, 8.6, and 10.5, respectively.



3. Leaching

Wetselaar (1962) demonstrated that rainfall is the most important factor

affecting leaching of nitrates in soils. He estimated that for each millimeter

of rain, the mean movement of nitrates down free-drained soils was 1.075

mm. Although severe leaching of soils of the humid tropics is regarded as a

major constraint to their productivity, actual quantitative data on removal

of nitrogen from soils remain sparse (Greenland, 1977).

The only documentation we could find regarding N loss in cereal-legume

intercrop systems was concerned with leaching of soil nitrates. Singh et af.

(1978) found in a 180-cm soil profile that maize intercropped with mungbean reduced NO, N loss by 60% and intercropped with blackgram, by 41%

when compared with sole maize (Table XII). The total N uptake of the

maize-mungbean and maize-blackgram intercrop systems were 36.8 and

23.3 kg N/ha, respectively, over and above the N uptake by sole maize.

In an 83-day maize and 155-day pigeonpea intercrop system, Yadav

(1982) found that the soil profile (0-105 cm) contained larger amounts of

NO, N in either the sole pigeonpea or the sole maize systems than in the

mixture. The NO, N content after the mixture was 240 kg/ha, compared to

619 kg/ha after the sole maize and 419 kg/ha after the sole pigeonpea. The

higher soil nitrate N in sole crops may be due to insufficient early spread of

roots to absorb nitrates from all the interrow spaces, resulting in nitrate N

leaching down and so enriching the lower soil profiles.

From what has been said so far, it is apparent that a lack of quantitative

data does not permit a satisfactory partitioning of N losses via the three main

pathways, viz., denitrification, leaching, and volatilization. It can be surmised that N losses by each of these pathways could limit the productivity and



Table XI1

Proportion of Applied NO, N in 1804111Soil Profiles and Total N Uptake

of Maize, Maize-Mungbean, and Maize-Blackgram Intercrop System@

NO, N loss as @lo of

quantity applied



Total N uptake

*g/ha)



N recovery



Treatment

Sole maize

Maize-mungbean

Maize-blackgram



56.8

22.7

33.6



61.9

98.7

85.2



48.3



“Derived from Singh et at. (1978).



(@lo)



78.2

67.5



82



FRANCIS OFORI AND W. R. STERN



efficiency of cereal-legume intercrop systems. However, it appears that

leaching losses of nitrates in intercropping systems may be less than in sole

crops.



D. NITROGEN

BUDGETING

There are three main sources of nitrogen in cereal-legume intercrop

systems: these are N fixed by the legume component from the atmosphere,

from fertilizer, and from soil. The only published data that offer some scope

for illustrating N budgeting are those of Eaglesham et al. (1981) in Western

Nigeria and Ofori et al. (1987) in Western Australia, both studies with maize

and cowpea. Using equations suggested by Rennie et al. (1982) to calculate N

from fixation, from fertilizer, and from soil, a N balance sheet was constructed for such a system with the data of Eaglesham el al. (1981) (Table

XIII).The densities of component crops as sole crops were 60,OOOplantdha

of maize and 110,OOO plantslha of cowpea; the intercrop density was half of

each sole crop density.



Table XIII

N Balances (kg N/ha) of Sole Crops and Intercrops of Maize and Cowpea Determined by

"N-Labeled Fertilizer Method with N Applied at 25 kg/hau

Cropping system

Sole crop

Components

Sources of N

Soil (available)

Seed at sowing

Fertilizer

Atmosphere

Total

Crop N

Harvested seed

Residues

Total

N lossesb

Soil N balance

after return

of residues'



Maize



Intercrop



Cowpea



23.9



60.4



0.5



2.0



4.0

28.4



4.0

83.2

149.6



25.6

2.8

28.4



53.6

96.0

149.6



-



21.0

-21.1



Maize

28.7

0.3

1.6



Cowpea

24.6

1.O



Total

53.3

1.3

3.3



-



1.7

40.7



40.7



30.6



68.0



98.6



28.0

2.6

30.6



24.5

43.5

68.0



52.4

46.1

98.6



21 .o



4.7



4.6



9.3



+ 35.9



- 26.1



+ 18.9



-1.2



'Derived from data of Eaglesham el at. (1981).

bLosses of applied N through unknown mechanisms.

1v in residues minus soil N available to crop.



CEREAL-LEGUME INTERCROPPING SYSTEMS



83



The N contributed by seeds of maize and cowpea at sowing was less than

2 kg/ha, fixed N, by intercrop cowpea was about 41 kg/ha, and N from fertilizer N was 3 kg/ha. The total N in the crops was about 99 kg/ha, consisting of N from seeds, fertilizer, N, fixation, and 53 kg/ha from the soil.

Assuming a seed N harvest index of 36% for cowpea and 90% for maize, the

quantity of N removed in the intercrop system was about 52 kg/ha, 28



kg/ha from maize and 24 kg/ha from cowpea. The N remaining in residues

was 46 kg/ha.

If the N losses in the study were due to denitrification, leaching, and

volatilization, these accounted for 74% of the N applied. NH, volatilization

could account for a large proportion of the N loss in the maize-cowpea

study if the N fertilizer was not well incorporated in the soil. Denitrification

and leaching could also be important; however, the use of ammonium fertilizer suggests that loss through leaching may be low because of the possible

sorption of ammonium ions on clay minerals and organic matter (Freney et

al., 1983).

The resultant net change in soil N after the grain harvests and the return

of residues may be calculated as

N = N (residues) - N (uptake from soil)



The maize-cowpea intercrop would result in a loss of 14 kg N/ha to the soil,

compared to a loss of 21 kg N/ha after sole cropping of maize and a gain of

36 kg N/ha after sole cowpea.

The data of Eaglesham et al. (1981) indicate that growing maize and

cowpea together does not deplete N excessively from the soil. Sole cropping

of cowpea may enhance soil nitrogen fertility and could be beneficial in

cereal and legume rotations if stover of high N concentrations are returned

to the soil, whereas sole cropping of maize depletes soil nitrogen. The findings of Ofori et al. (1987) are consistent with these conclusions.



VI.



SUMMARY AND CONCLUSIONS

A. AGRONOMIC

CONSIDERATIONS



It is apparent that light and nitrogen are the main factors influencing the

production efficiency of cereal-legume intercropping systems, as determined by the land equivalent ratio (LER). Various studies conducted at

ICRISAT, India, and evidence from the cassava-soybean intercrop study

by Tsay (1985) show that the taller component suppresses the companion

legume through shading, and this is accentuated by application of nitrogen

fertilizer.



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