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IV. Effect of Legumes on Soil Nitrogen
R. J. BURESH AND S. K. DE DATTA
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)
N accumulation (kglha)
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
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 In another study in the Philippines (unpublished IRRI/Niftal/IFDC collaborative research), nearly identical soil nitrate levels were observed at harvest of postrice inoculated soybean,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
NITROGEN IN RICE-LEGUME CROPPING SYSTEMS
Soil Nitrate Levels at Harvest of Postrice Legumes
in the Philippineso
Soil nitrate (kg Nlha)
L S D ~(.05)
a Unpublished IRRI/Niftal/lFDC collaborative
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).
R. J. BURESH AND S. K . DE DATTA
Soil nitrate (kg N/ha)
Fertilizer N (kg N/ha)
Mung bean harvest
Before soil flooding
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
Abovegound N Accumulation by Green Manure (GM) Crops Grown in Lowland Rice Fields
Fertilization of GM'