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IV. Effect of Legumes on Soil Nitrogen

IV. Effect of Legumes on Soil Nitrogen

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14



R. J. BURESH AND S. K. DE DATTA



Table I1

Effect of in Situ Growth of Green Manure and Soil Flooding for 25 Days before Green

Manure Incorporation on Yield and N Accumulation of the Following Wet-Season Rice

Crop in the Philippines”

Grain yield @/ha)

Water regime

Nonfloodedd

Flooded



Fallow

1.8

2.4



N accumulation (kglha)



Green manureC



Fallow



Green manureC



3.6

3.4



34

48



I8

14



Adapted from Furoc and Moms (1989) and Morris et al. (1989). All values are the mean

for 2 years.

No chemical N fertilizer or green manure added to rice.

Each value is the mean of 6 Sesbania green manure treatments with a mean N addition of

135 kg Nlha for the nonflooded and 1 1 1 kg N/ha for the flooded water regime. Rice was

transplanted 5 days after incorporation of green manure.

Soil was flooded and puddled 5 days before rice transplanting.



Sesbania presumably utilized the accumulated soil nitrate in the nonflooded water regime, thereby preventing loss by denitrification and effectively cycling soil N through green manure N back to the soil for use by

rice.

Considerable nitrate may remain in soil after a N2-fixing legume crop.

Postharvest levels of soil nitrate are often higher after food legumes than

after nonfixing crops (Herridge, 1986). The increase in soil nitrate after

growth of a Nz-fixing food legume, as compared with a nonfixing crop

(cereal or unnodulated legume), ranged from 22 to 41 kg N/ha in six studies

reviewed by Hemdge (1986). This phenomenon, referred to as nitrate

sparing, has been attributed to less capacity of N2-fixing legumes than of

nonfixing crops to utilize soil nitrate. Nitrate sparing, rather than a net

increase in the soil N pool following growth of a food legume, may account

for the N benefit of food legumes to a following upland crop (Herridge and

Bergersen, 1988). Another possible benefit of legumes to soil N may be N

released from roots and nodules during legume growth (Poth et al., 1986).

Very few measurements of soil nitrate following growth of legumes on

lowland rice fields are available. Buresh et al. (1989) observed 18 and 25 kg

nitrate N/ha in the top 60-cm soil layer at harvest of mung bean grown after

lowland rice in the Philippines. In another study in the Philippines (unpublished IRRI/Niftal/IFDC collaborative research), nearly identical soil nitrate levels were observed at harvest of postrice inoculated soybean,

cowpea, and mung bean (Table 111). Soil nitrate level was similar following

a nonnodulating soybean, but it was lower following a traditional weedy

fallow.



NITROGEN IN RICE-LEGUME CROPPING SYSTEMS



15



Table I11



Soil Nitrate Levels at Harvest of Postrice Legumes

in the Philippineso

Soil nitrate (kg Nlha)

Postrice treatment



0-20 cm



0-80 cm



Soybean

Cowpea

Mung bean

Nonnodulating soybean

Weedy fallow

L S D ~(.05)



32

32

29

31

13

9



55

53

53

48

32

13



a Unpublished IRRI/Niftal/lFDC collaborative

research.

I/ Least significant difference.



All treatments in Table 111, including weedy fallow, were tilled before

sowing legumes. Because tillage can enhance soil nitrate (Herridge, 1986),

it is conceivable that soil nitrate levels would be lower when legumes are

established with less or no tillage. Nonetheless, higher levels of soil nitrate

remained after legumes than after a traditional weedy fallow. In upland

cropping systems, this additional nitrate would be available to the next

crop, but in legume-lowland rice sequences, this nitrate is prone to loss

during soil wetting and flooding before rice.

B. Loss OF SOILN

Whereas nitrate sparing can benefit a following upland crop (Herridge

and Bergersen, 1989, it appears undesirable in legume-lowland rice sequences. Effective use of soil nitrate by legumes growing before rice could

conceivably reduce subsequent losses of soil N by leaching and denitrification and benefit the rice by cycling soil nitrate N through readily

mineralizable, N-rich residue to the succeeding rice crop. Field research is

needed to investigate this hypothesis.

Soil nitrate N derived from N fertilizers applied to legumes could be

more susceptible to N losses in legume-lowland rice sequences than in

legume-upland crop sequences. Application of 30 kg ammonium sulfate

N/ha as a starter dose to mung bean on a lowland rice field in the Philippines slightly increased nitrate N in the top 60-cm soil layer (unpublished

IFDUIRRI collaborative research).



16



R. J. BURESH AND S. K . DE DATTA

~~~~



Soil nitrate (kg N/ha)

Fertilizer N (kg N/ha)



Mung bean harvest

(16 April)



Before soil flooding

(2 June)



~



0



30



25

30



52

64



The nitrate N completely disappeared after soil flooding, presumably by

denitrification and leaching (Buresh et al., 1989). Although the above

differences in nitrate N are not significant at p = .05 (error df = 5 ) , they

raise concern about the fate of residual nitrate N derived from N fertilizer

applied to legumes in lowland rice environments.

Denitrification in soils requires available carbon as an energy source for

denitrifying microorganisms. Consequently, the addition of leguminous

green manure and residues to soils low in available C could conceivably

enhance denitrifying activity in soil (Beauchamp et al., 1989). Singh et al.

(1988b) observed in an incubation experiment with a sandy loam (organic

C = 3 g/kg) that addition of S. aculeara green manure (C/N = 13) increased the rate of soil nitrate disappearance after soil flooding. Nitrate

disappearance presumably was due to denitrification. Addition of rice

straw (C/N = 85) and wheat straw (C/N = 78) also lead to rapid nitrate

disappearance. However, immobilization may be responsible for nitrate

disappearance following addition of residues with high C/N ratio (Yoneyama and Yoshida, 1977).



V. ACCUMULATION OF LEGUME NITROGEN

Nitrogen accumulation by legumes in tropical rice-based cropping systems is influenced by water regime (Alam, 1989), soil fertility (Herrera et

al., 1989), photoperiod (Becker et al., 1990b), inoculation (Ndoye and

Dreyfus, 1988), and legume growth duration (Bhuiyan et al., 1989). With

an adequate water and nutrient supply, fast-growing, flood-tolerant legumes can accumulate more than 100 kg aboveground N/ha in 50 to 60

days (Table IV). Nitrogen accumulation by legumes sensitive to soil waterlogging, such as cowpea and mung bean grown before wet-season rice, is

retarded by soil saturation (Alam, 1989; Morris et al., 1989). Slower growing drought-tolerant legumes, such as pigeonpea (Cajanus cajan [L.]

Millsp.) and Indigofera tinctoria, are better suited for sowing after wetseason rice with subsequent N accumulation through the dry season and

then incorporation immediately before the next wet-season rice (Garrity et

al., 1989).



Table IV

Abovegound N Accumulation by Green Manure (GM) Crops Grown in Lowland Rice Fields



Species and

country

Aeschynomene afraspera

Philippines



Asfrugalus sinicus

Japan

Crotalaria juncea

India



Duration0.'

(d)



Dry herbage

yield (tiha)



N accumulation

(kdha)



CIN

ratio



N



P



K



42

49

56



2.2, 3.9

3.1, 1.8

4.3, 7.2



17,78

155,204

138, 149



13, 16

-



0

0

0



0

0

0



0

0

0



FB



4.3



I38



13



0



0



0



50-55

50-55

50-55



91

120

149

110

85

I20

144



17

16



30

45

60



3.4

4.0

4.8

5.4

3.1

6.0

7.6



24

-



0

I5

15

0

NA

NA

NA



0

0

7

26

NA

NA

NA



0

0

0

0

NA

NA

NA



60



3.8



87



22



0



26



0



4



30

45



1 .o



60



3 .O



35

62

16



-



NA

NA

NA



NA

NA

NA



NA

NA

NA



5

5

5



60

Philippines



Cyamopsis tetragonoloba

India

Lablab purpureus

Philippines



Fertilization of GM'

(kdha)



1.9



15



Reference"



2



(continued)



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