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II. Nitrogen Dynamics in Rice Soils

II. Nitrogen Dynamics in Rice Soils

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4



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



aerated. At this time, either the soil is left fallow or upland crops are

grown.

In Asia, lowland rice soils are frequently flooded before plowing and

harrowing for rice production. The process of tillage at soil saturation,

referred to as puddling, destroys soil aggregates, reduces downward water

flow and loss of nutrients by leaching, and restricts gaseous exchange

between the soil and the outer atmosphere (Sharma and De Datta, 1986).

The rice crop is established either by transplanting or by broadcasting

germinated seeds on flooded or saturated soil. In environments with a

reliable irrigation supply and low percolation, puddled soils typically remain continuously saturated until just before rice harvest. In rainfed environments and imgated environments with inadequate or irregular water

supply, the soil can undergo alternate drying and rewetting during rice

growth.

An alternative method of rice crop establishment, common in the United

States, is to sow germinated seeds onto nonpuddled, flooded soil. The rice

fields are irrigated and essentially left flooded throughout rice growth

(Westcott and Mikkelsen, 1988).

Some rainfed lowland rice in the tropics and much of the irrigated rice

outside Asia are sown on aerobic, nonpuddled soil. Rice grows as a

dryland crop until sufficient rainwater accumulates for soil submergence

or until permanent flooding by irrigation.

In aerobic soils, ammonium formed from mineralization of organic N or

from N fertilizer can be nitrified to nitrate, which can accumulate in the soil

or be used by plants. When aerobic soils are flooded, soil oxygen is rapidly

depleted and soil nitrate is prone to loss by denitrification and leaching. In

flooded soils, the conversion of ammonium to nitrate is restricted by the

limited supply of soil oxygen; hence, ammonium is the form of mineral N

that accumulates.

At the end of a flooded rice crop, soil nitrate is normally negligible and

soil ammonium, the dominant form of mineral N , is typically low because

of N uptake by rice (Fig. 1). Subsequent drying of the soil favors conversion of ammonium N formed by mineralization to nitrate N. Soil water

status (Linn and Doran, 1984), tillage (Dowdell et al., 1983), and weed

growth (Buresh et al., 1989) influence the accumulation of soil nitrate.

Intermittent rains can stimulate N mineralization and nitrate formation

(Birch, 1958). In a survey of 28 Philippine lowland soils, nitrate N before flooding for rice ranged from 5 to 39 mg/kg and averaged 13 mg/kg

(Ponnamperuma, 1985). In a greenhouse study, Ventura and Watanabe

(1978) reported nitrate N levels of 19 to 35 mg/kg after a dry-season fallow.

Cropping with rice during the dry season decreased nitrate N to 3 mg/kg

before the subsequent wet-season rice crop.



5



NITROGEN IN RICE-LEGUME CROPPING SYSTEMS

Soil aerotion status

Anaerobic



1



7



Rice



N concentration



,-N



loss



FIG. 1. Inorganic N dynamics in lowland rice soils as affected by soil aeration status.



Buresh et al. (1989) showed that substantial quantities of nitrate can

accumulate during the dry season in a mung bean (Vigna radiata [L.]

Wi1czek)-fallow-lowland rice sequence in the Philippines (Fig. 2). At

maturity of late wet-season rice in January, no nitrate was present in the

top 60-cm soil layer. During the subsequent dry-season mung bean crop,

25 and 18 kg nitrate Nlha accumulated in 1986 and 1987, respectively.

Additional nitrate N accumulated during the fallow following mung bean;

52 and 77 kg nitrate N/ha were present in early June immediately before

flooding by imgation for wet-season rice. The soil nitrate rapidly disappeared after flooding. Other researchers (Strickland, 1969; Bacon et al.,

1986) have similarly reported rapid disappearance of nitrate after soil

flooding.

An incubation study with "N-labeled nitrate incorporated into flooded

soil from the study site for the research shown in Fig. 2 revealed that the

added nitrate completely disappeared after 9 days. Only 5% or less of the

added "N-labeled nitrate N remained in the soil as ammonium N and

organic N , indicating that nitrate assimilation and dissimilatory reduction

to ammonium were negligible (Buresh et al., 1989). Denitrification and

leaching appeared to be the mechanisms for nitrate disappearance.



6



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



FIG.2. Nitrate N in the top 60-cm soil layer during a mung bean-weedy-fallow-lowland

rice sequence in the Philippines. (Adapted from Buresh er al., 1989.)



Buresh et al. (1989) found that soil nitrate N before flooding for wetseason rice correlated inversely with dry matter and N accumulation of

weeds (Fig. 3). Other research in the Philippines has shown significantly

lower soil nitrate levels in weedy fallow than in weed-free fallow before

wet-season rice (unpublished IFDUIRRI collaborative research). Other

studies demonstrated a higher yield of wet-season rice following weedy

fallow than following weed-free fallow (adapted from IRRI, 1986, p. 404):

Grain yield (t/ha)

~



Prerice treatment



No applied N



35 kg N/ha



Weedy fallow

Weed-free fallow



3.2

2.8



3.9

3.3



Lower soil nitrate and higher rice yield following weedy rather than

weed-free fallow suggest that uptake of nitrate N by weeds conserves soil

N from subsequent loss after soil flooding. Nitrate N taken up by weeds is



7



NITROGEN IN RICE-LEGUME CROPPING SYSTEMS

Nitrate N (kg/ha)



*\I



Y =95 -0.56X



r = - 0.80'"



30



Y = 103 -1.89X

r = -0.84* *



0



1



1



I



1



I



I



50



70



90



110



15



20



0



25



.



30



35



Weed N (kg/ha)



Weed dry matter (g/m')



FIG.3. Relationship of nitrate N in the top 60-cm soil layer to weed dry matter and weed

N before soil flooding for wet-season rice in the Philippines. (From Buresh er al., 1989.)



recycled to the soil when weeds are incorporated during land preparation.

The relatively low accumulation of N by weeds growing in tropical ricelands before wet-season rice (Table I) and the appreciable soil nitrate even

at the highest level of weed growth in Fig. 3 suggest that substantial nitrate

N may still accumulate and be lost in traditional weedy fallow-wet-season

rice cropping systems.

The weed biomass incorporated before rice might be an important

source of mineralizable N to flooded rice. Rerkasem and Rerkasem (1984),



Table I



Dry Weight and N Accumulation of Weeds in Unweeded Fallow Plots before Land

Preparation for Wet-Season Rice at Los Baios, Philippines

Aboveground

dry weight

(t/ha)

1.6

1.5



2.1

2.1

1.4



1.7

2.1

2.5



N accumulation

(kdha)

25

21

18

20

I1

12

20

29



C/N

ratio



31



Reference

IRRI (1985, p. 413)

IRRI (1986, p. 403)

IRRI (1986, p. 416)

John (1987)

Alam (1989)

Alam (1989)

Alam (1989)

R. J. Buresh et al. (unpublished)



8



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



for instance, observed in a heavily weed-infested rice field in Thailand that

removal of weeds before the rice crop resulted in a subsequent rice yield of

2.6 t/ha and a strong response of rice to N fertilizer. Incorporation of

weeds increased yield of unfertilized rice by 40%, and the response to N

fertilizer was less pronounced.



Ill. LEGUMES IN RICE-BASED CROPPING SYSTEMS

Legumes are grown in rice-based cropping systems for protein, oil,

fodder, green manure, and fuel production. In irrigated environments of

the tropics and subtropics, legumes can be grown in rotation with one or

more rice crops per year. In subtropical and temperate regions, where the

growing period for rice is restricted by low temperatures, leguminous

green manures can be grown as the winter crop.

Rice growing areas in tropical Asia are typically monsoonal with a

distinct wet and dry season. In rainfed lowlands, which comprise about

40% of the total rice area in South and Southeast Asia, only one rice crop is

normally possible per year. Production of a second rice crop is limited to

regions with a supplemental water supply or a long, reliable rainy season.

Food legumes can be grown in the postmonsoonal period following rice

when soil water is sufficient (Zandstra, 1982).

A. FOODLEGUMES

Food legumes are a rather minor crop in Asia as compared with cereals.

Yet, they are an important component of Asian farming systems, both in

terms of human and animal nutrition and as a source of biological N. The

ability of legumes to fix N2 enables them to grow on soils with low plantavailable N and to produce high-protein seed and N-rich plant residues.

Yields of food legumes are generally low because they are often grown

with low management and inputs under marginal production conditions, in

which cereals perform poorly or cannot grow. At least 18 food legume

species are considered important at various locations in Asia (Byth et

al., 1987).

The major food legumes grown on ricelands include soybean (Glycine

mux [L.] Merr.), mung bean, groundnut (Aruchis hypogaea [L.]), and

cowpea (Vigna unguicutafa [L.] Walp.). Soybean is an important crop on

ricelands in China, Indonesia, Vietnam, Thailand, and India (Carangal,

1986; Carangal et a / . , 1987). Mung bean is an important crop in India,



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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