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III. Legumes in Rice-Based Cropping Systems

III. Legumes in Rice-Based Cropping Systems

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NITROGEN IN RICE-LEGUME CROPPING SYSTEMS



9



Thailand, Burma, and Indonesia (Singh, 1988). Cowpea is an important

food legume in Sri Lanka (Singh, 1988). It performs better than other food

legumes on highly acid soil (Pandey and Ngarm, 1985).

In northern India, the production of irrigated mung bean as a third crop

between wheat and rice is increasing (Singh, 1988). However, throughout

tropical Asia most food legume production on lowland rice fields is under

rainfed conditions immediately after an irrigated or rainfed rice crop

(Chandra, 1988; Brotonegoro et af.,1988).

When grown after wet-season lowland rice, legumes can encounter

excess water during the vegetative phase and water deficit during the

reproductive phase (Fig. 4). Postrice legumes depend primarily on residual

soil water, and their roots may follow a receding water table (Timsina,

1989). Mung bean is more sensitive to reproductive-phase and full-season

water deficit than are cowpea and soybean (Pandey et af., 1984; Senthong

and Pandey, 1989). Water deficit reportedly reduces N2 fixation more than

it reduces plant growth and N uptake (Kirda et af., 1989).

When grown immediately before wet-season rice, legumes can encounter water deficit during the vegetative phase and excess water during the

reproductive phase (Timsina, 1989). Soil saturation and temporary waterlogging adversely affect growth, N accumulation, and N2 fixation of food

legumes (Wien et al., 1979; Lawn and Williams, 1987). Soybean is more

tolerant of excess water than is cowpea (Wien et al., 1979; Hulugalle and

Lal, 1986), and cowpea is more tolerant of temporary soil waterlogging

than is mung bean (Minchin and Summerfield, 1976; IRRI, 1985, p. 410).



A



-



-



-



Po st rice



Pre rice



Saturated orflooded soil



FIG.4. Rainfed lowland rice cropping patterns in tropical Asia



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



B. GREENMANURE

LEGUMES

Green manuring with effective N2-fixinglegumes can increase the soil N

pool while also improving soil physical and chemical properties through

the addition of organic matter ( Jiao, 1983). Green manures can be grown in

rice fields before rice and then incorporated during land preparation for

rice. Alternatively, the green manure crop can be grown elsewhere, such

as border areas, nearby upland fields, or levees, and then transported as

cut green matter to the rice field for incorporation. This practice is called

green leaf manuring.

In temperate regions, where temperature restricts the period suitable for

rice, leguminous green manures have historically been grown as a winter

crop in rotation with rice. In China, winter green manures, of which milk

vetch (Astragalus sinicus L.) is the most important, continue to occupy

large areas (Wen, 1989). Milk vetch tolerates cold temperature and shading, but it is sensitive to soil submergence. Milk vetch seeds are normally

broadcast into the field before late rice is harvested in mid-November. In

the following April, part of the vetch is typically removed for forage or

compost and the remainder is directly incorporated as a green manure

(Chen, 1988; Liu, 1988). Alternatively, green manure can be basally applied to rice after composting under waterlogged conditions (Wen, 1989).

Wen (1989) indicated that waterlogged compost can eliminate possible

adverse effects of toxins initially formed during anaerobic decomposition

and provide a steady, long-lasting release of N. However, production and

use of waterlogged compost is labor intensive and N can be lost during

composting.

The use of green manures in rice-based cropping systems has declined

worldwide. In Japan, where milk vetch was formerly an important green

manure, green manures are now of minor importance (Ishikawa, 1988). In

the United States, green manure crops, including vetches and clovers,

have been grown in rotation with rice, but use of green manure crops has

declined to less than 5% of the planted rice area (Westcott and Mikkelsen,

1988). Berseem clover (Trifolium alexandrinum L.) is used as a winter

green manure in Egypt (Hamissa and Mahrous, 1989).

In the tropics, Sesbania species, especially dhaicha IS.cannabina, syn:

S . aculeata), are used as green manure in rice cropping systems. Sesbania

species are well adapted for use as a green manure before rice because of

their ability to withstand soil waterlogging and flooding, to grow on finetextured soils, and to tolerate soil salinity (Evans and Rotar, 1987). Sesbania cannabina and Crotalariajuncea L. (sunn hemp) are common green

manures in India (Abrol and Palaniappan, 1988; Garrity and Flinn, 1988).

The inclusion of a green manure legume between wheat and rice in a



NITROGEN IN RICE-LEGUME CROPPING SYSTEMS



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rice-wheat rotation in northern India requires irrigation (Singh et

al., 1981). Therefore, in determining the cost effectiveness of green manures in this rotation one must consider the irrigation and fertilization,

particularly phosphorus, required for the green manure.

In a region of the Philippines, indigo (Indigofera tinctoria L.) is grown as

a green manure after wet-season rice in rainfed environments and after a

second rice crop in partially irrigated environments. Normally, it is intercropped with upland food or cash crops. Initial growth of indigo is rather

slow. After the intercrop is removed, the indigo continues growing

throughout the dry season. The indigo is incorporated during land preparation for wet-season rice, and rice is transplanted immediately after biomass incorporation (Bantilan et al., 1989; Garrity et al., 1989).

Woody legume species, particularly Gliricidia sepium ( Jacq.) Steud.,

Leucaena leucocephala (Lam.) de Wit, and Sesbania bispinosa ( Jacq.)

W. F. Wight (syn: S. aculeara, S. cannabina), are used as green leaf

manures in rice-based cropping systems (Brewbaker and Glover, 1988).

When grown near rice fields, these legumes can provide leaf matter for

green manuring as well as for fodder and fuel. Green leaf manure incorporated before transplanting can significantly increase rice yield ( Jeyaraman

and Purushothaman, 1988; Zoysa et al., 1990).

Worldwide, the use of leguminous green manures in rice cropping systems is currently found primarily in irrigated environments. Rainfed rice

environments prone to soil waterlogging appear to have the greatest potential for future green manure cultivation (Garrity and Flinn, 1988). The

recent identification of flood-tolerant, stem-nodulating legumes has increased research interest in green manures for environments prone to

waterlogging (Rinaudo et al., 1988). Sesbania rostrata (Rinaudo et al.,

1983) and Aeschynomene afraspera (Alazard and Becker, 1987) have been

examined in great detail for their potential as green manures.

Production of seed and scarification frequently are constraints in the use

of leguminous green manures, such as S. rostrata. An alternative is to

grow S. rostrata by vegetative propagation (Becker et al., 1988, 1989).

This method requires additional labor to make and plant cuttings, but it

requires less seed, land preparation, and water management.



C. DUAL-PURPOSE

LEGUMES

The quantity of legume N available as a N source for a succeeding rice

crop depends upon N accumulation by the legume and whether it is used

for sole green manuring, seed production, or fodder. Rice farmers are

often reluctant to devote land and resources to growth of legumes solely



12



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



for green manure because it provides no immediate income or food, yet

requires human labor. Food legumes, in contrast to green manures, offer

the attractive dual benefits of seed production for income or food and

production of residue, which can be used for animal feed or a N source on

the following rice crop (Kulkarni and Pandey, 1988).

Alam (1989) compared cowpea, mung bean, and Sesbania rosfrufuas

prerice crops during the dry-to wet-season transition period in the Philippines. Each crop was sown on two dates and at three sites differing in

internal soil drainage and water table depth. Aboveground biomass remaining after harvest of cowpea and mung bean grain was incorporated.

Grain yields of legumes were adversely affected by soil waterlogging and

heavy rains. Grain yields ranged from 0 to 0.73 t/ha for mung bean and 0 to

0.66 t/ha for cowpea. Mung bean biomass after removal of grain ranged

from 0.2 to 2.5 t/ha and contained 3 to 30 kg N/ha. Cowpea biomass after

removal of grain ranged from 0.5 to 3.9 t/ha and contained 7 to 79 kg N/ha.

Nitrogen accumulation was consistently less for mung bean and cowpea

residue than for S. rosfrata green manure. Rice yields were slightly increased by legume residues and S. rostrata green manure. Whenever soil

drainge and water regime did not prevent production of legume grain, the

economic benefit was greater for mung bean and cowpea than for S.

rostrata because of the high market value for legume grain. Garrity and

Flinn (1988) in a survey of green manure management systems in South,

Southeast, and East Asia concluded that green manures considered only in

terms of N fertilizer savings are currently not economical for rice farmers

in many parts of Asia.

Irrigated mung bean in northern India, grown with recommended management practices for grain production and incorporation of residue, reportedly reduces the industrial N fertilizer requirements on the following

rice crop by 20 to 30 kg N/ha (Chandra, 1988). The benefits of legume

residue are attributed both to direct N effects and improvement of soil

physical properties.

In some regions, legume residues may serve as animal feed. Ruminant

animals are an important source of draft power in many rice-based

cropping systems, but feed for these animals is frequently insufficient,

especially during the dry season. Considerable opportunity still exists for

increasing fodder and forage legume production in tropical rainfed rice

environments (Blair et al., 1986). Recognizing that farmers are often reluctant to grow crops solely for animal feed, Carangal ef al. (1988) proposed postrice intercropping of food legumes or cereals with forage legumes to provide food, fodder, and residue for the next wet-season rice

crop.



NITROGEN IN RICE-LEGUME CROPPING SYSTEMS



13



IV. EFFECT OF LEGUMES ON SOIL NITROGEN

Legumes utilize both soil N and atmospheric N2 in meeting their N

requirements. In general, the proportion of legume N derived from soil

rather than from N2 fixation increases with increased availability of soil N

(George et al., 1988; Herridge and Brockwell, 1988). Herridge et al. (1984)

showed that nitrate in the top 120-cm layer of a high-nitrate soil was

reduced during growth of irrigated soybean in Australia. Initial nitrate at 15

days after soybean sowing was 30 mg N/kg soil in the top 30-cm layer and

267 kg N/ha in the top 120-cm layer. Uninoculated soybean accumulated

164 kg N/ha after 126 days, and soil nitrate in both inoculated and uninoculated treatments was reduced to less than 8 mg N/kg soil in all soil

layers to 120-cm depth at soybean maturity after 154 days. By comparison,

nitrate in bare fallow fluctuated little between 15 and 154 days, except for

redistribution down the soil profile, and was 292 kg N/ha after the 154-day

soybean crop.

Sharma et al. (1985), on the other hand, reported an increase in soil

nitrate during growth of a mung bean crop between wheat and rice on a

sandy loam in Punjab, India. During mung bean cropping, nitrate in the top

120-cm soil layer increased from 73 kg N/ha in April to 223 kg N/ha in

June. Nitrate accumulation can also differ among legumes. Wetselaar et

al. (1973) observed that soil nitrate, after three consecutive years of legume cropping in tropical Australia, was greater under cowpea and

groundnut than under Townsville stylo (Stylosanthes humilis Kunth) and

clusterbean (Cyarnopsis tetragonoloba [L.] Taub.).



OF SOILN

A. CONSERVATION



Singh (1984) speculated that legumes grown in rotation with lowland rice

can scavenge soil mineral N , which might otherwise be lost by denitrification or leaching after the soil is flooded for rice production. This

hypothesis is consistent with observations of Furoc and Morris (1989) and

Morris et al. (1989) (Table 11). Without green manure before wet-season

rice, soil flooding for 25 days before land preparation increased rice yield

and N accumulation as compared with leaving the field nonflooded. They

speculated that nitrate accumulated in the nonflooded but not the flooded

fallow. Accumulated soil nitrate in the nonflooded fallow would have been

lost after soil flooding for rice. With Sesbania green manure, however, the

prerice water regime had no effect on rice yield and N accumulation.



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.



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