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VII. Transformations of Green Manure Nitrogen in Wetland Rice Soils

VII. Transformations of Green Manure Nitrogen in Wetland Rice Soils

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FIG.3. Effect of Sesbania green manure on exchangeable NH4+ N in a field (a) without

rice crop and (b) with rice crop. (Adapted from Nagarajah, 1988.)

of occurrence of peak of mineral N released from green manure varied

greatly in different studies. It should not be surprising keeping in view the

effect of plant composition and environmental factors on rate of N release.

In some studies conducted in the absence of rice plant, a decline in

NH4+-Nafter a peak has been observed (P. K. Singh et al., 1981; Bhardwaj and Dev, 1985; Khind et al., 1987b; Beri et al., 1989b). It could be due

to losses of N via nitrification-denitrification and volatilization as NH3.

The simplest model representative of the N mineralization kinetics of

organic substrates added to soil is the mathematical formulation for the

first-order kinetics. Frankenberger and Abdelmagid (1985) found first order N mineralization rate constants for the legume residues incubated at

field capacity moisture regime to range from 0.045 to 0.325Iweek. Bouldin

(1988) proposed a simple two-component model to describe the N mineralization pattern of green manures. The organic material is treated as if there

are two distinct components-one decomposes rapidly, the other slowly.

It was proposed that 65% of the added N mineralizes during the first crop,

14% mineralizes during the second crop, and 3.3% mineralizes during each

succeeding crop. Recent work of Y. Singh rt al. (1988) indicated that N

mineralization kinetics of green manure could be described by two simultaneous first-order reactions: an initial fast reaction ( k = 2.12/week)

followed by a slow release of inorganic N ( k = 0.069Iweek). Gale and

Gilmour (1988) studied C and N mineralization from alfalfa as a threephase process. Under anaerobic conditions, the rate constant for slow

phase was near zero, and for rapid and intermediate phases these were

0.118 and 0.024/day, respectively. Gilmour et al. (1985)confirmed that the

amount of net N mineralized was related to net C mineralization.

There have been few studies on decomposition and N mineralization of

green manures under waterlogged conditions. And in some of the studies

inconclusive results have been obtained due to simultaneous loss of miner-



alized N via ammonia volatilization and/or denitrification (Weeraratna,

1979; Y. Singh et al., 1988). Nevertheless, along with an extensive literature on plant litter decomposition in general, a number of references on N

mineralization of green manures under aerobic soil conditions are available. Thus, only a qualitative understanding of the factors regulating N

mineralization kinetics is possible. While listing different regulating factors an attempt has been made to discuss some of these specifically related

to N mineralization of green manures in wetland rice soils.

Green manure characteristics determining decomposition and mineralization kinetics include N content, C/N ratio, and lignin and lipid contents,

which are primarily determined by green manure species and age and plant

part of legume crops. In a study on four Philippines rice soils, it was found

that at the same N equivalent, NH4+-Nrelease from Azolla rnicrophyllu

was slower and lower than that from Sesbania rostrata, despite its lower

C/N ratio (Nagarajah et al., 1989). The difference was attributed to the

much higher lignin content of A . microphylla (20 vs. 9%). Shi et al. (1981)

reported that recovery of N by the first rice crop was 25.4 and 37.6%from

Azolla and milk vetch, respectively. Both these materials had an almost

similar C/N ratio ( 1 1.2 and 1 1 3)but Azolla had a greater amount of lignin

(20.2%) than milk vetch (13.5%). Kundu et al. (1990) found that rate and

magnitude of N release for Gliricidia were higher than that from Sesbania

when incorporated into submerged rice fields at transplanting. Gliricidia

green manure consisted of green twigs and leaves, which had a higher rate

of N mineralization than that of Sesbania green manure, which consisted

of whole plants. However, Beri ef al. (1989a) could not observe significant

differences in the N mineralization pattern of different green manures.

As the plant matures, the amount of N, proteins, and water-soluble

constituents decrease, and lignin and C/N ratios increase. John er al.

(1989~)reported that initially, N mineralization from cowpea green manure (C/N ratio = 15 : 1) was faster than from cowpea residue (C/N ratio = 21 : l ) , but at 30 days after transplanting, mineral N was higher in

residue-incorporated plots. In a pot culture study using alluvial sandy loam

soil (pH 7.4), Bhardwaj and Dev (1985) found that release of mineral N

after 49 days of incubation under flooded conditions was 112, 80, and

76 kg/ha for 45-, 55-, and 65-day-old, respectively, green manure crops of

S . cannabina. Palm et al. (1988) found that the leaves of Sesbania green

manure, which contained 88% of the total N in the aboveground parts

(83 kg/ha), released about 73% of the N during the first 4 days, and 88%

after 14 days of incubation under submerged soil condition. Release of N

from stems and roots with CIN ratios of 107 and 55, respectively, was

marginal compared to that from leaves.

Important edaphic and management factors influencing N mineraliza-



tion of green manure include soil type (pH, texture, clay mineral, etc.),

temperature, water management, cropping history, method of incorporation, and drying of green manure before incorporation. The addition of the

same rate of green manure N to different soils did not release equal

amounts of NH4+-N,indicating that N release from green manure depended on soil type (Nagarajah, 1988). Zhu et ul. (1984) observed that N

release from vetch green manure and rice straw combined was 26 and 16%

at 32 days and 41 and 29% at 72 days of incubation in clay loam and heavy

clayey soil, respectively. In contrast to organic matter decomposition, N

mineralization proceeds more rapidly under anaerobic than under aerobic conditions (Tusneem and Patrick, 1971; Watanabe, 1984; Gale and

Gilmour, 1988). Increased N mineralization in the anaerobic system could

be due to the lower metabolic efficiencies of the anaerobic microbial

populations (Williams et ul., 1968).

Ishikawa ( 1963) reported that decomposition and N release from milk

vetch green manure increased with increasing temperature. At 30"C, N

mineralization reached its maximum (70% of the N contained in green

manure). Thus, relatively cool early season conditions in some rice soils

may retard N availability from green manures (Westcott and Mikkelsen,

1985). Groffman et a / . (1987) and Varco ef a / . (1987) reported that clover

residue incorporated conventionally into the tilled soil decayed nearly

twice as fast as the clover remaining on the surface in the no-till soil.

Soluble hemicelluloses in green manures get converted into less soluble

forms by drying and retarded decomposition ( Joachim, 1931). Gu and Wen

(1981), however, reported that incorporation of fresh or dried milk vetch

into the soil produced similar yield responses of rice.

B. Loss OF N



Under flooded conditions, mineralization of green manure generally

resulted in accumulation of NH4+-N, which is susceptible to ammonia

volatilization loss. Incorporation of green manure may, however, affect

NH3 volatilization through its influence on pH and Pco,. In an incubation

study (Venkatakrishnan, 1980) on a sandy loam soil (pH 8.3), cumulative

NH3 volatilization was greater from the unamended than from the green

manure amended. In a laboratory study using forced-draft chamber technique, Khind et LII. (1989) observed no N H 3 volatilization 1 0 5 s in a sodic

soil (pH 10.0) amended with Sesbuniu green manure (6 t/ha dry biomass;

2.5% N on dry weight basis). At day 16, cumulative NH3 volatilization was

4.5% for urea applied at 200 kg N/ha as basal and 1 .O% for green manure

combined with urea applied at 100 kg N/ha as basal. In a field experiment



on a silty clay loam soil (pH 8.4), Santra et al. (1988) observed that

cumulative loss of N through NH3 volatilization was highest when the

whole quantity of N (90 kg N/ha) was applied basally as urea and was

minimum when urea was applied in combination with Sesbania green

manure (50 : 50). Green manure application caused a reduction in floodwater pH, and thus loss through NH3 volatilization was small. Similar

results have been obtained by Sarvanan et al. (1988) on a clay loam soil

(pH 7.4). In a waterlogged alkali soil (pH 10.6), Rao and Batra (1983)

reported 4.5% N loss via NH3 volatilization from green manure (applied to

supply 60 ppm N) as compared to 28.9% from urea. Although estimation of

ammonium volatilization losses has been made following widely different

approaches, it has been shown that green manuring can effectively reduce

ammonia volatilization losses.

John et al. (1989a) observed that green manuring had no effect on N loss

from urea applied to rice as assessed from I5N balances and pNH3 in flood

water. Incorporation of green manure into soil increases the availability of

organic C as a substrate for denitrifiers. This may enhance the loss of

applied fertilizer N via denitrification. John et al. (1989b) observed that

incorporation of cowpea as green manure at 15 days before transplanting

of rice had no effect on loss of urea N applied to rice at 15 days after

transplanting via denitrification. Low flood water NO3- levels following

urea application suggested that denitrification loss from urea was controlled by the supply of NO3- rather than availability of organic C . Bhagat

et al. (1988) reported that leaching losses of N in urea (90 kg N/ha in 3

splits) and green manure with urea (1 : 1) were small but similar. In an acid

clay loam soil loss of applied nitrogen in treatments of ammonium sulphate, sunn hemp, and combined application of the two averaged out to be

23.2, 20.4, and 16.2%, respectively (Huang et al., 1981). It indicates that

the loss of inorganic nitrogen could be reduced when applied along with

green manure. Furoc and Morris (1989) reported that at green manure N

levels exceeding 100 kg/ha, more than 70% was not recovered by two rice

crops. Further research is therefore needed to determine the fate of this N.





Legume plants have the ability to utilize insoluble phosphates through

the well-developed root system, and when used as green manures, upon

mineralization, release Pin the available forms (Gu and Wen, 1981; Singh,



1984). Green manuring also helps in increased utilization of fertilizer P by

the crops (Venkatarao and Govindarajan, 1960). Krishna Rao el al. (1961)

and Subbiah and Mannikar (1964) have shown that green manure taps

subsoil P and makes it available to the shallow-rooted crops. Upon decomposition of green manure, organically bound P is mineralized and becomes

available to crops. P mineralization is closely related to the analogous

transformation of N (Thompson et al., 1954).Phosphorus release would be

most rapid under soil and climatic conditions favoring ammonification

(Alexander, 1977). Phosphorus content of the added organic matter is

perhaps the most important factor in regulating the release of P (Fuller et

af., 1956; Singh and Jones, 1976).

In waterlogged soils, green manure increases availability of P through

the mechanisms of reduction, chelation, and favorable changes in soil pH

(Hundal et al., 1987). Changes in soil pH due to green manuring can

influence solubility of P (P. K. Singh er af., 1981). The effect of green

manuring on available P content in the soil has been reported to be greater

in acidic and sodic soils than on calcareous soils (Table VII). Ranjan and

Kothandaraman (1986) reported increased availability of P from rock

phosphate applied to rice with green manuring. The decomposition products of green manures have significant chelation capacity, lowering the

activity of polyvalent cations such as Ca, Fe, and A1 that form insoluble

salts with P and thus liberating phosphates from the basic phosphates of

these elements at very low pH values (Agboola, 1974). Hundal et al. (1988)

reported that green manure incorporation significantly reduced P sorption

capacity of waterlogged soils. Anaerobic decomposition of green manure

reduced the bonding energy and P sorption maxima. This effect was

ascribed to release of P during mineralization of green manure, and accumulation of organic acids-complexed metal cations, thereby inducing solubilization of native soil P or reduced fixation of added inorganic P through

the acidifying and chelation mechanisms.

Table VII

Effect of Incorporation of Sesbania Green Manure (GM) on Olsen P (mg/kg) in the Soils at

Different Days after Flooding

P.K. Singh

a / . (1981)

Soil pH 5.8


Y . Singh

e t a / . (1988)

Soil p H 8.5

Swarup (1987)

Soil pH 10.3


20 days

40 days

14 days

28 days

30 days

60 days

- GM













+ GM

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VII. Transformations of Green Manure Nitrogen in Wetland Rice Soils

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