Plant and Soil 150: , Erratum

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1 Plant and Soil 150: , Erratum K.H. Diekmann, S.K. De Datta and J.C.G. Ottow, Nitrogen uptake and recovery from urea and green manure in lowland rice measured by 15N and non-isotope techniques, Plant and Soil 148 (1993): Unfortunately, the authors' names had disappeared from their previously published article.

2 Plant and Soil 148: 91-99, Kluwer Academic Publishers. Printed in the Netherlands. PLSO 8742 Nitrogen uptake and recovery from urea and green manure in lowland rice measured by and non-isotope techniques* K.H. DIEKMANN l, S.K. De DATTA 3 and J.C.G. OTTOW 2 ~ Agronomy-Physiology-Agroecology Division, The International Rice Research Institute, Los Ba~os, P.O. Box 933, Manila, Philippines, 2Institute of Microbiology and Agronomy, Justus-Liebig University, DW-6300 Giessen, Germany and 3International Research and Development, Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State University, 1060 Litton Reaves Hall, Blacksburg, VA , USA Received 14 May Accepted in revised form 18 September 1992 Key words: Aeschynomene afraspera, green manure, lowland rice, ~SN balance, Sesbania rostrata Abstract In the recent past considerable attention is paid to minimize dependence on purchased inputs such as inorganic nitrogen fertilizer. Green manure in the form of flood-tolerant, stem-nodulating Sesbania rostrata and Aeschynomene afraspera is an alternative N source for rice, which may also increase N use efficiency. Therefore research was conducted to determine the fate of N applied to lowland rice (Oryza sativa L.) in the form of Sesbania rostrata and Aeschynomene afraspera green manure and urea in two field experiments using 15N labeled materials. 15N in the soil and rice plant was determined, and 15N balances established. Apparent N recoveries were determined by non-tracer method. 15N recoveries averaged 90 and 65% of N applied for green manure and urea treatments, respectively. High partial pressures of NH 3 in the floodwater, and high ph probably resulted from urea application and favoured losses of N from the urea treatment. Results show that green manure N can supply a substantial proportion of the N requirements of lowland rice. Nitrogen released from Sesbania rostrata and Aeschynomene afraspera green manure was in synchrony with the demand of the rice plant. The effect of combined application of green manure and urea on N losses from urea fertilizer were also investigated. Green manure reduced the N losses from 15N labeled urea possibly due to a reduction in ph of the floodwater. Positive added N interactions (ANIs) were observed. At harvest, an average of 45 and 25% of N applied remained in the soil for green manure and urea, respectively. Introduction Nitrogen (N) is the major nutrient limiting yield of semidwarf rice cultivars. Inorganic N fertilizers are considered expensive by resource poor farmers in view of falling real price of rice. Therefore, resource-poor farmers are looking for alternative N sources to bring down cost of *Contribution from IRRI, Los Bafios, Philippines and Justus- -Liebig-University, Giessen, Germany production. It is also important to evaluate alternative sources of N for rice which will not contribute to environmental degradation. Green manure is being evaluated as a potential N source of rice for resource-poor Asian farmers. Green manure is a source of N, which may also enhance soil fertility through its effect on soil physico-chemical properties. Green manure may either supplement or substitute for inorganic N fertilizers for rice depending on a number of factors such as growing seasons, water availabili- 313

3 92 Diekmann et al. ty, availability of labor and opportunity cost to grow another food crop or engage in other employment opportunities. To date, the fate of N in green manure in lowland rice has been studied in only a few instances (Becker et al., 1991; Beri et al., 1989; Morris et al., 1989; Nagarajah et al., 1989). Critical data are missing on decomposition, N mineralization, N transformation processes, and N use efficiency in order to develop effective management practices which will determine the relevance of green manure use in lowland rice production systems. In lowland rice cropping systems, the leguminous Sesbania rostrata and Aeschynomene afraspera may become important sources of green manure in that they are flood tolerant, and can fix, through nodules on both stems and roots, large amounts of N within a short period of time (Becker et al., 1986; Dreyfus et al., 1988; Ghai et al., 1988; Ladha et al., 1989; Rinaudo et al., 1983). These green manure species either alone or in conjunction with inorganic N fertilizers offer greater opportunities to evaluate N use efficiency in lowland rice in the tropics. The objectives of this study were therefore: 1. To determine the effect of S. rostrata and A. afraspera green manure on N uptake and grain yield of lowland rice. 2. To compare the recoveries of N from green manure and inorganic fertilizer sources, using ~5N labeled materials. 3. To determine whether green manure application may increase the N use efficiency of urea. Materials and methods A field experiment was conducted at the former Maligaya Rice Research and Training center (MRRTC) now called Phillippine Rice Research Institute in Mufioz, Nueva Ecija province during the 1988 dry and wet season. Soil was a fine, mixed, isohyperthermic Vertic Tropaquept with ph 5.9 (1:1 water); organic C, 14g kg-~; total N, 1.1 g kg- 1; available P, 6 mg kg- 1 (extracted with 0.5M NaHCO3, ph 8.5); cation exchange capacity, 38.1 cmolc kg-1; clay, 450 g kg-a; sand, 50g kg-1; and bulk density, 0.65 Mg m -3 (surface soil). Field experiments were conducted using a one- factorial randomized complete block design. Treatments varied with the season. In the dry season, there were eight green manure and fertilizer treatments plus a control without green manure or N fertilizer application (Table 1), replicated four times. The treatment number was reduced to 5 during the wet season, consisting of four green manure and fertilizer treatments plus the control treatment (Table 1). Plot size was 29.4m 2. For ~SN studies, two 0.8-x 0.8-m microplots were installed within each main plot (29.4 m2). Each microplot was surrounded by a 0.4-m-high border made of painted, galvanized metal, inserted to a depth of 0.3 m into the soil. A no-fertilizer N control treatment was also included as microplot. Phosphorus (25 kg ha -~) as single superphosphate, potassium (50 kg ha -~) as KC1, and zinc (4.5 kg ha -1) as zinc sulfate heptahydrate were broadcast and incorporated before transplanting the improved indica, lowland semidwarf variety IR64 with 3-4 seedlings/hill at 0.2-x0.2-m spacing. Total N rate was 90 kg ha-1 for the dry season and 60 kg ha-1 for the wet season except for the combined application of labeled urea and nonlabeled green manure (Table 1). The green manure was 15N-labeled by growing it in the greenhouse for 45 days in nutrient solution using modified Hoagland solution (Yoshida et al., 1976) with labeled ammonium sulfate as N source (5 atom % excess). Nonlabeled green Table 1. Treatments, 1988 dry season (wet season only treatments 1 to 5) Treatment N source Organic N Inorganic N No. (kg N ha -1) 1 Control S. rostrata" A. afraspera a Urea 0 60b/30 c 5 Urea 0 60b/30 c 6 S.r. + PU S.r. + PU PU PU 0 30 a green manure basally applied. b Labeled N source. c 30 N topdressed at 3-5 days before panicle initiation. S.r. = S. rostrata green manure basally applied. 314

4 Recovery from urea and green manure in lowland rice 93 manure plants were grown in the field outside the trial area simultaneously with those in the green-house. Plants were harvested 45 days after seeding, chopped to 0.5 cm (greenhouse plants) or to about 5 cm lengths (field-grown plants) and incorporated in microplots (labeled material) or in main plots (nonlabeled material) at their equivalent N rates based on dry matter and N concentration. The properties of the green manure plants are listed in Table 2. Fertilizer materials, except topdressed urea, were applied one day before transplanting, into moist soil without standing water to minimize N losses (De Datta et al., 1987). For two treatments, 2/3 N as urea was basally applied and 1/3 N topdressed at 5-7 days before panicle initiation (DBPI). Topdressed urea was broadcast into floodwater. Floodwater was maintained at 0.05 m level above soil surface up to two weeks before harvest, and thereafter soil was kept water-saturated until crop maturity. Plant and soil samples from the microplots were taken by destructive sampling twice, 5-7 days before panicle initiation (35 days after transplanting = 35 DT), and at crop maturity (95 days after transplanting). Four rice hills from the center of the microplot were sampled for dry matter yield, N concentration, and 15N recovery measured. At crop maturity, grain and straw components were analyzed separately. Soil and root samples were collected inside microplots simultaneously with the plant samples to complete the 15N balance for the soil-plant system. Before sampling soil, floodwater inside the microplot was removed using a vacuum pump, filtered, and subsampled for 15N analysis. Two soil samples, each measuring 0.2mx 0.4m x 0.15m for the m and m soil depths, were collected from the sampled area following the method of Buresh et al. (1982). Roots and weed materials were col- Table 2. Plant material properties (above ground) after 45 days growth in nutrient solution (15N labeled) and in the field (nonlabeled; values in parentheses) Crop ~SN excess C/N ratio N Lignin (%) (%) (%) S. rostrata (12) 4.4 (3.2) 9 (12) A. afraspera (11) 4.9 (3.7) 8 (12) lected, rinsed with distilled water into the layer from where the materials were taken, dried at 70 C to constant weight, ground, and analyzed separately from the soil. Plant samples were also dried at 70 C to constant weight, ground, and analyzed. Soil samples were dried at 40 C, ground to pass a 100-mesh sieve and analyzed for 15N using the semi-micro Kjeldahl procedure. Exchangeable ammonium was extracted by 2N KC1 (Bremner 1965) from fresh soil samples which was subsampled for moisture determination immediately after sampling. A 50-mL beaker attached to a pole was used to sample 25-mL of floodwater from each of nine equally spaced locations in each main plot (Buresh, 1987). Ammonium N in floodwater was determined following the method of Douglas and Bremner (1970). Simultaneously with floodwater sampling, floodwater ph and temperature were measured in situ at two locations in each mainplot using a portable ph meter with a combination electrode with integrated temperature compensation. Floodwater depth was recorded from four stakes positioned in each plot. Partial pressure of ammonia (pnh3) in the floodwater was calculated from ammoniacal N concentration, ph, and temperature using the corrected equation of Denmead et al. (1982, 1983) to estimate N losses from fertilizer. Plant N and total soil N were analyzed by the semi-micro Kjeldahl method digesting at 400 C for 2 h with 10 ml concentrated sulphuric acid (Bremner and Mulvaney 1982). Determination of soil nitrate-n was not included because of the negligible concentrations apparent in flooded rice soils. To determine 15N (nitrogen isotope ratio analysis), the distillates were titrated with 0.01N H2SO4, collected, acidified with a few drops of 1N H2SO4, and evaporated to dryness in glass vials. Plant and Soil 15N were determined by mass spectrometry using a VG Micromass M 622 (VG Isogas Ltd., Cheshire, England) after converting (NHa-N) to molecular N 2 with alkaline lithium hypobromite (LiOBr; Buresh et al., 1982). The subsoil ( m) was not included for the 15N balance because less than 2% of the total 15N applied was present in this layer. In mainplots (outside microplots) fresh soil samples (0-0.20m) were taken weekly up to 4 315

5 94 Diekmann et al. weeks after fertilizer application and fortnightly thereafter to determine for exchangeable NHn-N and thus to monitor N release from applied materials. Floodwater samples were taken daily up to 8 days after basal application of fertilizer to analyze for floodwater ph, floodwater temperature and NH4-N concentration. N recoveries were calculated based on 15N uptake by plants (isotope dilution method) and by the non-isotopic difference method (Harmsen and Moraghan, 1988): calculated by the difference method and by the isotope dilution method. The agronomic efficiency (AE) was calculated as kg additional grain produced per kg fertilizer N applied. Dry matter production and grain yield are expressed as dry weight basis. Floodwater samples analyzed for XSN balances showed negligible 15N content and were, therefore, not included into 15N balances. Difference method NP - NPo ARF - NF Isotope dilution method Yxp NP 15NRF Yxf NF Results and discussion Total ~SN balances (plant + soil + roots) NF = fertilizer N applied ARF = apparent recovery fraction 15NRF = 15N recovery fraction NPo = N uptake in control treatment (no fertilizer N applied) NP = N uptake in fertilized treatment Yxp, Yxf = atom percent excess 15N in the plant and the applied fertilizer, respectively The so-called "priming effect" or added N interaction (ANI) indicated (Harmsen and Moraghan, 1988; Jenkinson et al., 1985) is equivalent to the difference between N recoveries In the 1988 dry season the rice crop recovered 93 and 87% of applied 15N-labeled S. rostrata and A. afraspera green manure, respectively. At the same N rate (90 kg N ha -1) 50% of the labeled basal dose of urea (60 kg N ha -1) and 83% of the topdressed dose (30 kg N ha -l, applied 5-7 DBPI), was recovered, which is equivalent to 61% 15N recovery of both basal and topdressed applications (Table 3). Results were similar in the 1988 wet season (Table 4). A combined application of non-labeled S. rostrata equivalent to 30 kg N ha -1 and labeled urea (30 or 60 kg N ha -1) resulted in significantly Table 3. 15N balance of applied labeled Sesbania rostrata (Sr), Aeschynomene afraspera (Aa), and urea (PU) at harvest in lowland IR64 rice. MRRTC, Nueva Ecija, Philippines, 1988 dry season Treatment" Recovery of 15N-labeled fertilizer (% of N applied) Soil Straw Grain Total Total Unaccounted + roots plant plant+ for soil 90 N Sr b N Aa b b / 30 N PU c /30 b n PU c N Sr + 60 N PU b N Sr+ 30 N PU b N PU b N PU b LSD (0.05) CV (%) a All fertilizers basally applied. b Labeled N source. c 30 N topdressed at 3-5 days before panicle initiation. N = N rate (kg ha-l). 316

6 Recovery from urea and green manure in lowland rice 95 Table 4. 15N balance of applied labeled Sesbania rostrata (Sr), Aeschynornene afraspera (Aa), and urea (PU) at harvest in lowland IR64 rice. MRRTC, Nueva Ecija, Philippines, 1988 wet season Treatment a Recovery of 15N-labeled fertilizer (% of N applied) Soil Straw Grain Total Total Unaccounted + roots plant plant + for soil 60 N Sr ~ NAa b b/20NPU /20bNPU LSD (0.05) CV (%) " All fertilizers basally applied except 20 N PU topdressed at 3-5 days before panicle initiation. b Labeled N source. N = N rate (kg ha-t). (p = 0.05) higher recoveries of N in the soil and p NH5 (Pa) roots (50 or 30%) than that of labeled urea applied alone at the same N rates (36 or 23%; Table 3). 0.25[ Thus, based on total 15N balances, N losses (N unaccounted for) from the soil-plant system in urea treatments (37-54%) were significantly 0.20 higher (p = 0.05) than those in green manure treatments (7-16%). The substantial amount of N unaccounted for from urea is probably due to losses by ammonia volatilization, and denitrification from the flood- 0.l 5 water occurring during the first few days after fertilizer application as reported by Vlek and Byrnes (1986). This suggestion is supported by the results of calculations of the partial pressure O.lO of ammonia in the flood-water (pnh3) which was significantly lower in the green manure and green manure +urea treatment than urea ap plied alone (Fig. 1). Results show that application of green manure with urea may reduce ammonia losses from urea by lowering ph of the floodwater (Fig. 2). The total recoveries of urea 'SN found in the present study support the 0 results of previous experiments with labeled urea (De Datta et al. 1987; John et al., 1989). The large recoveries of 15N derived from labeled Days after fertilizer application green manure reported here are supported by the results of Westcott and Mikkelsen (1985) and Watanabe et al. (1989). These recoveries are Fig. 1. Partial pressure of ammonia (pnh3) in the floodwater at h as affected by Sesbania rostrata (Sr) and presumably due to less N loss resulting from the urea (PU) amendment in lowland transplanted rice. MRRTC, Nueva Ecija, Philippines, 1988 dry season. N = N low ammonium concentrations in the floodwater rate in kg ha 1. B & I/BI=basal incorporation. TD= because of slow decomposition of green manure. topdressing at 5-7 days before panicle initiation (DBPI). 317

7 96 Diekmann et al. Floodwater ph NoN (control) 8.6 _// ",,.,,'"I~ PU N ~/ ]" ~rn30 / 8.2 t~ 8.0! :-BI 'v-t -- V 1, I I I I Days after fertilizer application Fig. 2. Floodwater ph at h as affected by Sesbania rostrata (Sr) and Aeschynomene afraspera (Aa) and urea (PU) amendment in transplanted lowland rice. MRRTC, Nueva Ecija, Philippines, 1988 dry season. N = N rate in kg ha -I. BI = basal incorporation. ~SN recoveries by rice plants At crop maturity, total 15N recoveries of plants were 49, 47, and 42% (basal dose 28%, topdressed dose 71%) for the S. rostrata, A. afraspera and urea treatments, respectively (dry season, Table 3). Slightly lower values were obtained for green manure during the wet season (Table 4). Application of non-labeled S. rostrata equivalent to 60 and 30 kg N ha-1 with labeled urea did not result in increased plant 15N recovery compared to urea applied alone (Table 3). Total N uptake at crop maturity was similar for urea and green manure treatments at the same N rate, but at 35 DT, N uptake for green manure treatments was significantly higher (p = 0.05) than for urea (Table 5). This is probably due to the fact that only 2/3 of N was applied as urea up to this sampling stage (plant sampling done before topdressing). Additionally a substantial part of the basal applied urea presumably was lost by ammonia volatilization. Topdressing of urea (1/3N) at 35 DT (5-7 DBPI) resulted in similar N uptake at crop maturity as in green manure treatments (Table 5). The N uptake by rice plants further increased Table 5. Effect of 15N-labeled green manure or urea on plant N of IR64 lowland rice at 5-7 days before panicle initiation (PI) and at crop maturity (MAT)(microplots). MRRTC, Nueva Ecija, Philippines, 1988 N source" Plant N (kg ha-l) Added N interaction Total Derived from (kg ha- ~) fertilizer- 15N PI MAT PI MAT PI MAT Dry season (90 kg N ha-l) b S. rostrata 85 a 116 a 44 a 44 a 7 a 17 b A. afraspera 81 b 109 b 41 a 43 a 6 a 11 c Urea 52 c 118 a 13 b 38 b 5 b 25 a No fert. N 34 d 55 c.... Wet season (60 kg N ha 1)b S. rostrata 53 a 95 a 25 a 25 a 8 a 17 a A. afraspera 48 b 89 b 23 a 24 a 5 b 12 b Urea 39 c 85 b 10 b 23 a 9 a 9 c No fert. N 20 d 53 c.... "All of Sesbania rostrata and Aeschynomene afraspera green manure basally applied and incorporated; urea applied in 2/3 N (basal) + 1/3N (topdressed at 5-7 days before panicle initiation). In a column, means followed by a common letter are not significantly different at the 5% level by DMRT. No fert. N designates No fertilizer N applied. b N rate applied. 318

8 Recovery from urea and green manure in lowland rice 97 from 35 DT up to maturity (Table 5). S. rostrata gave significantly (p =0.05) higher N uptake than did A. afraspera. Although C/N ratio and lignin content was similar (Table 2), the different N uptake in S. rostrata and A. afraspera treatments could be due to different mineralization rates of stem-compared to leaf materials of the green manure plants. According to Watanabe (pers. commun.) stem material of A. afraspera decomposes faster because of lower C/N ratio and thus could have resulted in different N Exch NH4~-N (rng kcj -1, dry soil) lo0 80 L Dry season, 90kgNha -1 I I A All Sr (BI) ~i [3 All Ao (BI) No N (control) 40 u~,~. ILSDO'05 F + I/3N 20 /P "" -.. I ~BI " l ' ' ' l ~ I I i Wet season, 60kg N ha "I I zo I E +1/3N 0 2O O DGys after fertilizer opplicotion Fig. 3. Kinetics of exchangeable NH~-N in flooded soil in the presence of transplanted 1R64 as affected by Sesbania rostrata (Sr) and Aeschynomene afraspera (Aa) green manures or prilled urea (PU) application. MRRTC, Nueva Ecija, Philippines, BI=basal incorporation. Urea applied in 2/3-1/3 split. losses. Losses (15N not accounted for) were higher for A. afraspera than for S. rostrata treatments. This was supported by lower NHn-N concentrations in the soil (Fig. 3). However stem- and leaf material in this study was not analyzed separately. In contrast to the total N uptake, no significant 15N uptake (p = 0.05) occurred between PI and crop maturity in the green manure treatments, whereas in the urea treatment due to split application of labeled urea the 15N recovery increased significantly (p = 0.05, Table 5). Presumably, the major amount of 15N remaining in the soil was not in plant available form and was immobilized in organic or inorganic form. However, N uptake increased substantially from PI up to harvest, indicative of "pool substitution" with native soil 14N mineralized (Jenkinson et al., 1985). Kai et al. (1973) reported that under optimum conditions for microbial activity and in the presence of an available C source, added ~SN was rapidly immobilized and reached its maximum at incubation periods of as short as three days with a simple substrate (e.g., glucose) to as much as two months for a more complex substrate (e.g., rice straw). Since the S. rostrata and A. afraspera green manures had narrow C/N ratios and low lignin contents, decomposition and net N mineralization proceeded rapidly as shown by the large exchangable NHa-N concentrations in the soil (Fig. 3). The ANI, a result of pool substitution, increased from 35 DT up to maturity during the dry and wet seasons, with the exception of the urea treatment during wet season (Table 5). The results of this study show that after PI, N was mainly taken up from the soil system as ~4N. Similar results were obtained by Watanabe et al. (1989) in a field study comparing 15N-labeled Azolla and urea. Soil N recoveries Results show that considerable amounts of applied 15N-labeled fertilizer remained in the soil after harvest of the dry and wet season rice crops (Tables 3 and 4). The amount of 15N remaining in the soil at crop maturity from basally applied urea was significantly lower (p = 0.05) than that from basally applied green manure (Tables 4 and 319

9 98 Diekmann et al. Table 6. Grain yield, biological yield (dry matter yield), harvest index (HI), and agronomic efficiency (AE) in IR64 lowland rice as affected by green manure or urea amendment (microplots). Nueva Ecija, Philippines, 1988 N source a Grain Dry matter HI AE (g m -2 ) (g m -2) (kg kg -1 N) Dry season (90 kg N ah-1) ~ S. rostrata 727 a 1451 a 0.50 b 29 b A. afraspera 649 b 1274 b 0.51 b 20 c Urea 734 a 1358 b 0.54 a 30 a No fertilizer N 465 c 889 c 0.53 a - Wet season (60 kg N ha ~)b S. rostrata 567 a 1136 a 0.50 a 44 a A. afraspera 495 b 985 b 0.50 a 32 c Urea 498 b 974 b 0.51 a 33 b No fertilizer N 303 c 635 c 0.48 a - aall Sesbania rostrata and Aeschynomene afraspera green manure basally applied; urea applied in 2/3N (basal)+l/3n (topdressed at 5-7 days before panicle initiation). bn rate applied. In a column, means followed by a common letter are not significantly different at the 5% level by DMRT. 5). Presumably this was because some green manure remained undecomposed, more 15N was retained in the microbial biomass, and N losses were lower from green manure. A combined application of nonlabeled S. rostrata equivalent to 30 kg N ha- 1 + labeled urea at rates of 30 or 60 kg N ha -1 as basal dose caused significantly higher (p = 0.05) recoveries in the soil than in the treatments where urea was applied alone without green manure (Table 3). Probably the lower floodwater ph in the combined treatment could reduce N losses from labeled urea. Huang and Broadbent (1989) could also show that organic residues have the potential to increase the N use efficiency of urea. Rice grain yield The rice yield parameters as affected by green manure and urea amendment are listed in Table 6. Dry and wet season results showed that green manure as S. rostrata and A. afraspera was as effective as urea at the same N rate in producing rice grain yield. Similar results were obtained by Morris et al. (1989) although N rates for S. rostrata green manure exceeded those of urea. Conclusions The present study showed that in general, the interpretation of the ANI results with labeled urea and labeled green manure is difficult (Table 5), since organic substrates have to undergo decomposition first prior to release of plant available N. Thus, lsn derived from urea and that from green manure undergo different transformations, causing different rates of pool substitution or immobilization over time. Fertilizer efficiencies calculated by the isotope dilution method will be underestimated for mineral N compared with green manure or other organic fertilizers as shown in this study. Thus, fertilizer efficiencies of urea and green manure cannot be compared directly, unless the mineralizationimmobilization processes are better understood and the loss mechanisms are defined and quantified. However, the non-isotope methods employed in this study could show that N derived from S. rostrata and A. afraspera green manure is readily available and can be used efficiently by a lowland rice crop. Thus, green manure has the potential to substitute or supplement for mineral N. The combined application of green manure and urea can be an alternative fertilizer management method to reduce N losses from urea applied to tropical lowland rice. For future research, long-term effects of green manure amendment should be studied in detail to develop integrated N management practices that include both organic and inorganic sources of N. Furthermore socioeconomic aspects of green manure use (e.g. labor costs, acceptance 320

10 Recovery from urea and green manure in lowland rice 99 by the farmer, opportunity cost) have to be considered. Acknowledgement This study was supported by a grant from the German Agency for Technical Cooperation (GTZ), References Eschborn. Becker M, Alazard D and Ottow J C G 1986 Mineral nitrogen effect on nodulation and nitrogen fixation of the stem-nodulating legume Aeschynomene afraspera. Z. Pflanzenernaehr. Bodenkd. 149, Becker M, Diekmann K H, Lahda J L, De Datta S K, and Ottow J C G 1991 Effect of NPK on growth and nitrogen fixation of Sesbania rostrata as green manure for lowland rice (Oryza sativa L.). Plant and Soil 132, Beri V, Meelu O P and Khind C S 1989 Studies on Sesbanai acculeata Pers. as green manure for N accumulation and substitution of fertilizer N in wetland rice. Trop. Agric. (Trinidad) 66, Bremner J M 1965 Inorganic forms of nitrogen. In Methods of Soil Analysis. Ed. C A Black. pp ). American Society of Agronomy, Madison, WI. Bremner J M and Mulvaney C S 1982 Total nitrogen. In Methods of Soil Analysis. Eds. A L Page et al. Agro. 9(2). pp American Society of Agronomy, Madison, WI. Buresh R J, Austin E R and Craswell E T 1982 Analytical methods in ~SN research. Fertil. Res. 3, Buresh R J 1987 Relative susceptibility of conventional and experimental nitrogen sources to ammonia loss from flooded rice fields. Fertil. Res. 13, De Datta S K and Buresh R J 1989 Integrated nitrogen management in irrigated rice. Adv. Soil Sci. 10, De Datta S K, Fillery I R P, Obcemea W N and Evangelista R C 1987 Floodwater properties, nitrogen utilization, and nitrogen-15 balance in a calcarous lowland rice soil. Soil Sci. Soc. Am. J. 51, Denmead O T, Freney J R and Simpson J R Dynamics of ammonia volatilization during furrow irrigation of maize. Soil Sci. Soc. Am. J. 46, Denmead O T, Freney J R and Simpson J R Dynamics of ammonia volatilization during furrow irrigation of maize. Soil Sci. Soc. Am. J. 47, 618. Douglas L A and Bremner J M Extraction and colorimetric determination of urea in soils. Soil Sci. Soc. Am. Proc. 34, Dreyfus B, Garcia J L and Gillis M 1988 Characterization of Azorhizobium caulinodans gen nov., sp. nov., a stem nodulating, nitrogen fixing bacterium isolated from S. rostrata. Int. J. System. Bacteriol. 38, Ghai S K, Rao D L N, Batra L 1988 Nitrogen contribution to wetland rice by green manuring with Sesbania spp. in an alkaline soil. Biol. Fertil. Soils 6, Harmsen K and Moraghan J T 1988 A comparison of the isotope and differences methods for determining nitrogen fertilizer efficiency. Plant and Soil 105, Huang Zhi-Whu and Broadbent F E 1989 The influences of organic residues on utilization of urea N by rice. Fertil. Res. 18, Jenkinson D S and Fox R H 1985 Interaction between fertilizer nitrogen and soil nitrogen- the so-called "priming effect". J. Soil Sci. 36, John P S, Buresh R J, Pandey R K, Prasad R and Chua T T 1989 Nitrogen-15 balances for urea and neem coated urea applied to lowland rice following two cowpea cropping systems. Plant and Soil 120, Kai H, Ahmad Z and Harada T 1973 Factors affecting immobilization and release of nitrogen in soil and chemical characteristics of the nitrogen newly immobilized. Soil Sci. Plant Nutr. 19, Ladha J K, Miyan S and Garcia M 1989 Sesbania rostrata as a green manure for lowland rice: Growth, N2 fixation, Azorhizobium sp. inoculation and effects on succeeding crop yields and nitrogen balance. Biol. Fertil. Soils 7, Morris R A, Furoc R E, Rajbhandari N K, Marqueses E P and Dizon M A 1989 Rice response to waterlog-tolerant green manures. Agron. J. 81, Nagarajah S, Neue H U and Alberto M C R 1989 Effect of Sesbania, Azolla and rice straw incorporation on the kinetics of NH4, K, Fe, Mn, Zn and P in some flooded rice soils. Plant and Soil 116, Rinaudo G, Dreyfus B and Dommergues Y 1983 Sesbania rostrata green manure and the nitrogen content of rice crop and soil. Soil Biol. Bicohem. 15, Vlek P L G and Byrnes B H 1986 The efficacy and loss of fertilizer N in lowland rice. Fertil. Res. 9, Watanabe I, Ventura W, Mascarina G and Eskew D L 1989 Fate of Azolla spp. and urea nitrogen applied to wetland rice (Oryza sativa L.). Biol. Fertil. Soils 8, Westcott M P and Mikkelsen D S 1985 Comparative effects of an organic and inorganic nitrogen source in flooded soils. Soil Sci. Soc. Am. J. 49, Yoshida S, Forno D A, Cock J H and Gomez K A 1976 Laboratory Manual for Physiological Studies of Rice. 3rd ed. International Rice Research Institute, P.O. Box 933, Manila, Philippines. Section editor: H Lambers 321