Effectiveness of Sesbania rostrata and Phaseolus calcaratus as green manure for upland rice grown in acidic soil

Two field experiments on green manuring were conducted under upland acidic soil (pH = 4.35) conditions with the following objectives: (1) to determine the influence of inoculation site, P fertilization, and liming on the biomass production, N content, N accumulation, and N availability of S. rostrata grown in an acidic soil, (2) to compare the effectiveness of S. rostrata, P. calcaratus and urea as N sources for upland rice as affected by liming and N source-sowing time combination, and (3) to assess the effect of liming and N source-sowing time combination on % Ndff (N derived from the fertilizer), % Ndfs (N derived from soil), % FNU (fertilizer N utilization), and FNY or fertilizer N yield (kg N ha^–1) of upland rice grown in acidic soil. At 2 weeks after incorporating S. rostrata (95 days after lime application), liming significantly increased N availability by more than 2-fold suggesting that the decomposition of S. rostrata by soil microflora was stimulated by lime. Liming, phosphorus application, and inoculation site improved significantly the dry biomass production, N content and N accumulation of S. rostrata; thus, enhancing its green manuring potential. Regardless of liming, S. rostrata whether applied at 0 week or 2 weeks before sowing was superior to urea in improving grain and straw yields. P. calcaratus when applied at 2 weeks before sowing also produced higher grain yield than urea. Immediate sowing of upland rice after green manure incorporation did not affect negatively the growth and development of upland rice; hence, farmers could save at least 2 weeks in their cropping calendar. N source-sowing time combination had a highly significant influence on % Ndff, % Ndfs, % FNU, N uptake, and fertilizer N yield of upland rice. However, only N uptake was influenced significantly by liming. The rice plant obtained significantly higher % Ndfs from the soils treated with green manure than those treated with urea regardless of liming. The % FNU and % Ndff from the green manures were 11-37% and 9-25%, respectively. These values are much lower than those obtained under continuously flooded soil conditions possibly because of the differences in the organic matter decomposer populations and N loss mechanisms between sloping upland conditions and continuously flooded conditions.     

Conservation of urea-N by immobilization-remobilization in a rice-Azolla intercrop

Nitrogen losses are notoriously high in flooded rice fertilized with urea. An Azolla intercrop can reduce such losses by immobilizing urea-N during periods of potentially high N-loss. The reduction in N loss linked with the absorption and remobilization of urea-N by Azolla, was studied in two greenhouse experiments conducted in Goettingen (Germany). Grain yield and N recovery were positively influenced by Azolla more than doubling grain yield and N uptake as compared to the split application of 300 mg N pot^–1 alone (Exp. 1). In the second experiment, the yield increase was 78.3% with single applications of 97.5 and 68.4% after a split-application of a total of 195 mg N pot^–1. In both years the effect of urea and Azolla combined exceeded that of the sum of the factors alone, a clear positive synergistic effect on yield and N uptake by rice. Azolla effectively competed with the young rice plants for applied urea, capturing nearly twice the urea-N than the rice plants up to tillering in experiment 1. In the second experiment, 64.6 mg N of the 97.5 mg applied early in the season was immobilized by Azolla within 2 weeks. This represented 63.1% of the total N accumulated in the Azolla. The fraction of Azolla-N derived from urea sank to 36.4 mg within 4 weeks and only 27.2 mg at maximum tillering as a result of Azolla senescence and N-release. Of this 64.6 mg urea N immobilized 28.7% is eventually taken up by the standing rice plant, representing 43.1% of the remineralized, urea-derived Azolla N. Following the second urea application, only 17.9 mg N were immobilized in the Azolla biomass during the 2 weeks, of which 6.9 mg pot^–1 were still retained in the Azolla at maturity. At this stage, rice is the more effective competitor for applied N. As much as 42.1% of this immobilized N finds its way into the rice by maturity. Thus, Azolla contributed to the conservation of N in the system, particularly of the urea applied early in the season. Loss of N from the system amounted to no more than 15%. Although the early-applied N directly recovered by the rice plant was low (20%), 2/3 of the N captured by Azolla following this first urea application was released to the system by the time of rice harvest, over 40% of which was available to the rice plant. Azolla thus appears to act as a slow release fertilizer.     

Crop performance, nitrogen and water use in flooded and aerobic rice

Irrigated aerobic rice is a new system being developed for lowland areas with water shortage and for favorable upland areas with access to supplementary irrigation. It entails the cultivation of nutrient-responsive cultivars in nonsaturated soil with sufficient external inputs to reach yields of 70–80% of high-input flooded rice. To obtain insights into crop performance, water use, and N use of aerobic rice, a field experiment was conducted in the dry seasons of 2002 and 2003 in the Philippines. Cultivar Apo was grown under flooded and aerobic conditions at 0 and at 150 kg fertilizer N ha^–1. The aerobic fields were flush irrigated when the soil water potential at 15-cm depth reached –30 kPa. A ¹⁵N isotope study was carried out in microplots within the 150-N plots to determine the fate of applied N. The yield under aerobic conditions with 150 kg N ha^–1 was 6.3 t ha^–1 in 2002 and 4.2 t ha^–1 in 2003, and the irrigation water input was 778 mm in 2002 and 826 mm in 2003. Compared with flooded conditions, the yield was 15 and 39% lower, and the irrigation water use 36 and 41% lower in aerobic plots in 2002 and 2003, respectively. N content at 150 kg N ha^–1 in leaves and total plant was nearly the same for aerobic and flooded conditions, indicating that crop growth under aerobic conditions was limited by water deficit and not by N deficit. Under aerobic conditions, average fertilizer N recovery was 22% in both the main field and the microplot, whereas under flooded conditions, it was 49% in the main field and 36% in the microplot. Under both flooded and aerobic conditions, the fraction of ¹⁵N that was determined in the soil after the growing season was 23%. Since nitrate contents in leachate water were negligible, we hypothesized that the N unaccounted for were gaseous losses. The N unaccounted for was higher under aerobic conditions than under flooded conditions. For aerobic rice, trials are suggested for optimizing dose and timing of N fertilizer. Also further improvements in water regime should be made to reduce crop water stress.     

Nitrogen contribution of cowpea green manure and residue to upland rice

Cowpea, Vigna unguiculata (L.) Walp., is well adapted to acid upland soil and can be grown for seed, green manure, and fodder production. A 2-yr field experiment was conducted on an Aeric Tropaqualf in the Philippines to determine the effect of cowpea management practice on the response of a subsequent upland rice crop to applied urea. Cowpea was grown to flowering and incorporated as a green manure or grown to maturity with either grain and pods removed or all aboveground vegetation removed before sowing rice. Cowpea green manure accumulated on average 68 kg N ha⁻¹, and aboveground residue after harvest of dry pods contained on average 46 kg N ha⁻¹. Compared with a pre-rice fallow, cowpea green manure and residue increased grain yield of upland rice by 0.7 Mg ha⁻¹ when no urea was applied to rice. Green manure and residue substituted for 66 and 70 kg urea-N ha⁻¹ on upland rice, respectively. In the absence of urea, green manure and residue increased total aboveground N in mature rice by 12 and 14 kg N ha⁻¹, respectively. These increases corresponded to plant recoveries of 13% for applied green manure N and 24% for applied residue N. At 15 d after sowing rice (DAS), 33% of the added green manure N and 16% of the added residue N was recovered as soil (nitrate + ammonium)-N. At 30 DAS, the corresponding recoveries were 20 and 37% for green manure N and residue N, respectively. Cowpea cropping with removal of all aboveground cowpea vegetation slightly increased (p<0.05) soil (nitrate + ammonium)-N at 15 DAS as compared with the pre-rice fallow, but it did not increase rice yield. Cowpea residue remaining after harvest of dry pods can be an effective N source for a subsequent upland rice crop.     

Nitrogen losses in puddled soils as affected by timing of water deficit and nitrogen fertilization

Erratic rainfall in rainfed lowlands and inadequate water supply in irrigated lowlands can results in alternate soil drying and flooding during a rice (Oryza sativa L.) cropping period. Effects of alternate soil drying and flooding on N loss by nitrification-denitrification have been inconsistent in previous field research. To determine the effects of water deficit and urea timing on soil NO₃ and NH₄, floodwater NO₃, and N loss from added ¹⁵N-labeled urea, a field experiment was conducted for 2 yr on an Andaqueptic Haplaquoll in the Philippines. Water regimes were continuously flooded, not irrigated from 15 to 35 d after transplanting (DT), or not irrigated from 41 to 63 DT. The nitrogen treatments in factorial combination with water regimes were no applied N and 80 kg urea-N ha^–1, either applied half basally and half at 37 DT or half at 11 DT and half at 65 DT. Water deficit at 15 to 35 DT and 41 to 63 DT, compared with continuous soil flooding, significantly reduced extractable NH₄ in the top 30-cm soil layer and resulted in significant but small (<1.0 kg N ha^–1) soil NO₃ accumulations. Soil NO₃, which accumulated during the water deficit, rapidly disappeared after reflooding. Water deficit at 15 to 35 DT, unlike that at 41 to 63 DT, increased the gaseous loss of added urea N as determined from unrecovered ¹⁵N in ¹⁵N balances. The results indicate that application of urea to young rice in saturated or flooded soil results in large, rapid losses of N (mean = 35% of applied N), presumably by NH₃ volatilization. Subsequent soil drying and flooding during the vegetative growth phase can result in additional N loss (mean = 14% of applied N), presumably by nitrification-denitrification. This additional N loss due to soil drying and flooding decreases with increasing crop age, apparently because of increased competition by rice with soil microorganisms for NH₄ and NO₃.     

Absorption of manganese by rice under flooded and unflooded conditions

Summary Manganese absorption by rice plants under flooded and unflooded conditions, in an upland and a lowland soil was studied. Both under flooded and unflooded conditions the rate of manganese absorption was high during 3rd to 4th week and again during 9th to 10th week.Rice grown under flooded conditions, in lowland soil absorbed more manganese and showed higher dry matter accumulation than under unflooded conditions. However manganese absorption and dry matter accumulation in rice grown under flooded conditions in upland soil were lower than under unflooded conditions.Rice grown under flooded conditions may give lower yields than rice grown under unflooded conditions due to some factor or combination of factors one of which may possibly be a high soluble iron content which interferes with the absorption of manganese.     

Effect of urea placement on leaching losses of nitrogen from flooded rice soils

In an effort to provide an explanation for the reported variability in fertilizer N efficiency from deepplaced urea on flooded rice, a set of controlled experiments was conducted to evaluate the effect of water percolation on fertilizer loss and plant uptake from¹⁵N labeled urea supergranules. Three soils of different texture (silt loam-clay) were subjected to various percolation rates (0–20 mm/day) while planted to rice which was harvested after approximately 40 days.The results indicate that moderate to high percolation through silt loam soil will lead to significant fertilizer N losses and drastically decrease the fertilizer uptake by plants. The permeability of the clay soil was too low for any leaching to take place. It is therefore concluded that deep placement of urea supergranules not be recommended in soils where percolation rates may exceed 5 mm/day, particularly if the cation exchange capacity of the soil is low. This experiment points to the need of evaluating and reporting the percolation rates in soils where experiments with supergranular urea are conducted.Contribution from the Agro-Economic Division of the International Fertilizer Development Center (IFDC), Muscle Shoals, Alabama 35660.     

The significance of denitrification of applied nitrogen in fallow and cropped rice soils under different flooding regimes I. Greenhouse experimènts

The role of nitrification-denitrification in the loss of nitrogen from urea applied to puddled soils planted to rice and subjected to continuous and intermittent flooding was evaluated in three greenhouse pot studies. The loss of N via denitrification was estimated indirectly using the¹⁵N balance, after either first accounting for NH₃ volatilization or by analyzing the¹⁵N balance immediately before and after the soil was dried and reflooded. When urea was broadcast and incorporated the loss of¹⁵N from the soil-plant systems depended on the soil, being about 20%–25% for the silt loams and only 10%–12% for the clay. Ammonia volatilization accounted for an average 20% of the N applied in the silt loam. Denitrification losses could not account for more than 10% of the applied N in any of the continuously flooded soil-plant systems under study and were most likely less than 5%. Intermittent flooding of soil planted to rice did not increase the loss of N. Denitrification appeared to be an important loss mechanism in continuously flooded fallow soils, accounting for the loss of approximately 40% of the applied¹⁵N. Loss of¹⁵N was not appreciably enhanced in fallow soils undergoing intermittent flooding. Apparently, nitrate formed in oxidized zones in the soil was readily denitrified in the absence of plant roots. Extensive loss (66%) of¹⁵N-labeled nitrate was obtained when 100 mg/pot of nitrate-N was applied to the surface of nonflooded soil prior to reflooding. This result suggests that rice plants may not compete effectively with denitrifiers if large quantities of nitrate were to accumulate during intermittent dry periods.     

Nitrogen uptake and recovery from urea and green manure in lowland rice measured by15N and non-isotope techniques

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-nodulatingSesbania rostrata andAeschynomene 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 ofSesbania rostrata andAeschynomene afraspera green manure and urea in two field experiments using¹⁵N labeled materials.¹⁵N in the soil and rice plant was determined, and¹⁵N 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 NH₃ 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 fromSesbania rostrata andAeschynomene 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 from¹⁵N 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.Contribution from IRRI, Los Baños, Philippines and Justus-Liebig-University, Giessen, GermanyContribution from IRRI, Los Baños, Philippines and Justus-Liebig-University, Giessen, Germany     

The partitioning of fertilizer-N between soil and crop: Comparison of ammonium and nitrate applications

Field experiments were carried out in 1987 on winter wheat crops grown on three types of soil. ¹⁵N-labelled urea, ¹⁵NH₄NO₃ or NH₄ ¹⁵NO₃ (80 kg N ha^-1) was applied at tillering. The soils (chalky soil, hydromorphic loamy soil, sandy clay soil) were chosen to obtain a range of nitrogen dynamics, particularly nitrification. Soil microbial N immobilization and crop N uptake were measured at five dates. Shortly after fertilizer application (0–26 days), the amount of N immobilized in soil were markedly higher with labelled urea or ammonium than that with nitrate in all soils. During the same period, crop ¹⁵N uptake occurred preferentially at the expense of nitrate. Nitrification differed little between soils, the rates were 2.0 to 4.7 kg N ha^-1 day^-1 at 9°C daily mean temperature. The differences in immobilization and uptake had almost disappeared at flowering and harvest. ¹⁵N recovery in soil and crop varied between 50 and 100%. Gaseous losses probably occurred by volatilization in the chalky soil and denitrification in the hydromorphic loamy soil. These losses affected the NH₄ ⁺ and NO₃ ^- pools differently and determined the partitioning of fertilizer-N between immobilization and absorption.     

A15N tracer study to compare nitrogen supply by Azolla and ammonium sulphate to IR8 rice plants grown under flooded conditions

Summary A pot experiment was carried out using a Bangladesh sandy loam paddy soil of pH 6.9 to compare the rates at which nitrogen from Azolla and ammonium sulphate was available to a high yielding rice variety, IR8, grown for 60 days in pots with 4 cm standing flood water.¹⁵N tracer studies confirm that nitrogen from ammonium sulphate was more available to the rice plants than from Azolla. An application of 6, 9 and 18 mg N of Azolla pot^–1 (each pot contained 250 g soil) increased shoot dry matter yields by 13, 29 and 49% for an uptake of 19, 36 and 85% more nitrogen; the corresponding increases on using ammonium sulphate were 33, 54 and 114% for an increased uptake of 57, 90 and 177% more nitrogen, respectively. About 34% of applied¹⁵N of Azolla was taken up by the rice plants in 60 days but 61% of¹⁵N of the ammonium sulphate was absorbed during this period. About 45% of the Azolla-N was released in 60 days, 55% remained in the soils as undecomposed material and 11% was lost as gas. The gaseous loss of¹⁵N from ammonium sulphate was 14%; 25% remained in the soils.     

Lysimeter studies on recovery of15N-labeled urea in wetland rice

Summary Results of a two year study on the fate on¹⁵N-labelled urea (9.95 atoms percent excess¹⁵N) applied @ 180 kg N/ha to flooded rice in monolith lysimeters at the Punjab Agricultural University Farm, Ludhiana are reported. The soil of the experimental field was sandy clay loam in texture (Typic Ustochrept), had pH 7.9, organic carbon 0.36 percent, available N 187 kg/ha and total N 0.08 percent. The results revealed that 18.1 to 53.0 per cent of the fertilizer N was utilized by the rice plant, 25.1 to 41.1 percent was immobilized in the soil and 4.8 to 7.2 percent was lost by denitrification. The losses due to ammonia volatilization and leaching were negligible. The data on vertical distribution of labelled N in the soil profile reflected a higher concentration (38.3 to 39.5 per cent) in the surface (0–30 cm) soil. The content sharply decreased (1.8 to 2.4, percent) in lower soil layers (30–150 cm). A balance sheet of the various pathways of applied N showed that 58.8 to 72.2 and 66.2 to 83.0 percent N was recovered in 1976 and 1977, respectively and 17 to 41.2 per cent of labelled N still remained unaccounted for. Utilization of fertilizer N by rice was increased and losses decreased when N was applied in three equal splits as compared to the single N application at transplanting.Availability of fertilizer N immobilized in the soil was investigated in the succeeding crops of wheat and rice. The results showed that 2.1 tot 3.4 per cent of the N applied to the preceding rice was utilized by the second rice crop grown in succession. This may look small but cannot be neglected on a long term basis. But there is need to initiate long term studies to investigate the, turnover of residual N and to determine the fate of applied N in varying soil and cropping systems by using improved techniques.     

生物炭添加对不同水氮条件下芦苇生长和氮素吸收的影响

生物炭能改良土壤从而促进植物生长和氮素吸收,但其作用效果是否受水氮条件的影响尚不清楚。以湿地植物芦苇为研究对象,在3种氮添加水平（无添加,30 kg hm^-2 a^-1和60 kg hm^-2 a^-1）和两种水分（淹水和非淹水）条件下分别进行生物炭添加和不添加处理,结果表明：（1）生物炭添加能促进芦苇根系生长,在非淹水条件下根系生物量增加了40.5%,在淹水条件下根系生物量增加了20.1%。（2）生物炭添加能促进非淹水条件下芦苇的氮素吸收,能提高淹水条件下芦苇的氮素生产力。（3）生物炭添加加剧了土壤氮素损失,且在非淹水高氮条件下作用最强,可能是由于生物炭促进了芦苇的氮素吸收。芦苇氮素吸收速率与土壤氮损失之间存在显著的正相关关系。因此,在添加生物炭时,需要考虑土壤水分状况和氮素富集程度以及植物的氮素吸收偏好。该研究结果可为生物炭在湿地生态系统中的应用提供参考。     

Influence of urea on biological N2 fixation and N transfer from Azolla intercropped with rice

The N₂ fixed by Azolla before and after urea application during the rice cycle, the mineralisation of Azolla-N as well as its availability to rice was studied in two greenhouse experiments conducted in 1996 and 1997 and in June 1998 in Goettingen (Germany). Dry matter production of the various rice parts of experiment 1 showed a clear positive synergism between treatment with Azolla and urea with a resulting apparent N recovery by rice increasing from 40% (without Azolla) to 57% in the presence of Azolla. Part of this increase may be due to N fixed biologically by Azolla and transferred to the rice. The second experiment shed some light on the role of BNF. Using an iterative method of estimation, the daily rate of N fixation was estimated at 0.6 – 0.7 kg N ha^–1. The rate was not so much affected by the age of the Azolla crop. At this rate, the BNF would amount to up to 100 kg N ha^–1 over a 130-day season. Assuming that BNF may be inhibited for a period of 5 – 10 days following urea application due to high levels of N in the floodwater, this might reduce the BNF by between 6 and 14 kg N ha over the season. Using the mean-pool-abundance concept, it was estimated that around 75 – 80% of the Azolla-N mineralized during the growth period was actually absorbed by the rice plants. Of the N taken up by rice around 28% was derived from the biologically fixed Azolla N, the remainder was urea N cycled through the Azolla. Azolla also seems to help sustain the soil N supply by returning N to the soil in quantities roughly equal to those extracted from the soil by the rice plant.     

Variation in the incidence of H2-oxidizing chemolithotrophic bacteria in rice grown under different cultivation conditions

Summary The incidence of H₂-oxidizing chemolithotrophic bacteria associated with rice grown under continuous wetland, upland, and rainfed wetland conditions was studied by¹⁴C-autoradiographic technique in a neutral soil at IRRI (Maahas) and an acid rainfed wetland soil (Luisiana).In Maahas soil, H₂-oxidizing chemolithotrophic bacteria were not detected in the endorhizosphere, rhizosphere, and nonrhizosphere soil of rice grown under dryland conditions. Under continuously flooded conditions a very large population of these bacteria were found in the endorhizosphere but not in the oxidized and reduced soil.A very low population of these bacteria were found in the endorhizosphere and basal culm of rice grown under rainfed wetland conditions at Luisiana. Bacteria isolated from Maahas wetland rice and inoculated to rice seedling planted in Luisiana soil failed to establish.Both Maahas and Luisiana soils consumed externally supplied H₂ and produced H₂ and CH₄ almost at the same rate when they were amended with rice straw or sucrose. This paper discusses possible causes of variation in the number of these bacteria and their distribution in rice grown under different cultural and soil conditions.     

Short-term effects of urea amended with the urease inhibitor N-(n-butyl) thiophosphoric triamide on perennial ryegrass

The current study investigated the short-term physiological implications of plant nitrogen uptake of urea amended with the urease inhibitor N-(n-butyl) thiophosphoric triamide (nBTPT) under both greenhouse and field conditions. ¹⁵N labelled urea amended with 0.0, 0.01, 0.1 and 0.5% nBTPT (w/w) was surface applied at a rate equivalent to 100 kg N ha^–1 to perennial ryegrass in a greenhouse pot experiment. Root, shoot and soil fractions were destructively harvested 0.75, 1.75, 4, 7 and 10 days after fertilizer application. Urease activity was determined in each fraction together with ¹⁵N recovery and a range of chemical analyses. The effect of nBTPT amended urea on leaf tip scorch was evaluated together with the effect of the inhibitor applied on its own on plant urease activity.nBTPT-amended urea dramatically reduced shoot urease activity for the first few days after application compared to unamended urea. The higher the nBTPT concentration the longer the time required for shoot activity to return to that in the unamended treatment. At the highest inhibitor concentration of 0.5% shoot urease activity had returned to that of unamended urea by 10 days. Root urease activity was unaffected by nBTPT in the presence of urea but was affected by nBTPT in the absence of urea.Transient leaf tip scorch was observed approximately 7–15 days after nBTPT + urea application and was greatest with high concentrations of nBTPT and high urea-N application rates. New developing leaves showed no visual sign of tip necrosis.Urea hydrolysis of unamended urea was rapid with only 1.3% urea-N remaining in the soil after 1.75 days. N uptake and metabolism by ryegrass was rapid with ¹⁵N recovery from unamended urea, in the plant (shoot + root) being 33% after 1.75 days. Most of the ¹⁵N in the soil following the urea+0.5% nBTPT application was still as urea after 1.75 days, yet ¹⁵N plant recovery at this time was 25% (root+shoot). This together with other evidence, suggests that if urea hydrolysis in soil is delayed by nBTPT then urea can be taken up by ryegrass as the intact molecule, albeit at a significantly slower initial rate of uptake than NH₄ ⁺-N. Protein and water soluble carbohydrate content of the plant were not significantly affected by amending urea with nBTPT however, there was a significant effect on the composition of amino acids in the roots and shoots, suggesting a difference in metabolism.Although nBTPT-amended urea affected plant urease activity and caused some leaf-tip scorch the effects were transient and short-lived. The previously reported benefit of nBTPT in reducing NH₃ volatilization of urea would appear to far outweigh any of the observed short-term effects, as dry-matter production of ryegrass is increased.     

Facultative symbiotrophic nitrogen-fixing associations in rice soils of India

The occurrence of facultative symbiotrophic N₂-fixing associations in three rice soils of India is reported. Considerable variation in N₂-fixing efficiency of these associations was noticed among the soil types studied. Associations from rice straw-amended alluvial soil under both flooded and non-flooded conditions exhibited higher N.-fixing efficiency than those from unamended soils of both water regimes. Despite high salinity and acidity an acid sulphate soil harboured N.-fixing symbiotrophic organisms with appreciable efficiency. Application of rice straw to the soil under both flooded and non-flooded conditions stimulated N₂-fixation in alluvial, laterite and acid sulphate saline soils. These observations suggested the significant contribution of these associations to the nitrogen economy of different soil types.     

Fate of urea-N in floodwater

One day after application, urea-N remaining in the floodwater and determined as water-soluble N (urea-N + NH₄ ⁺-N) was used to calculate the potential N loss from lowland rice soils. Actual N loss calculated from ¹⁵N balance measurements using forced air exchange (airflow rate: 20 L min^-1) in greenhouse pots. Conditions for variable potential N loss were created by manipulating the method of urea application and duration of presubmergence or by selecting soils with diverse cation exchange capacities (CEC). Potential N loss tended to be lower than actual N loss; the differences were, however, nonsignificant. The method of urea application that led to the lowest potential N loss from a Guthrie silty clay loam (Typic Fragiaquult) also led to the least ¹⁵N loss and vice-versa (r=0.99^**). Duration of presubmergence did not alter the relationship between potential and actual N loss although it influenced the rate of urea hydrolysis in floodwater. The primary depencence of actual N loss on water-soluble N was maintained in soils differing in CEC (r=0.83^**). The association between potential and actual N loss was closer for high-CEC soils ( 20 cmol [+] kg^-1 soil, r=0.91^**) than for low-CEC soils (<20 cmol [+] kg^-1 soil, r=0.85^**). Ammonia volatilization could be more closely predicted by potential N loss than could apparent denitrification.The results of this study suggest that potential N loss calculated from one-time determination of water-soluble N in floodwater can be a good index of actual N loss from flooded, puddled rice soils. Notable exceptions are to be expected for soils in which water-soluble N gets lost from floodwater either before (soils with fast urea hydrolysis in floodwater) or after (soils with steady leaching) determination of potential N loss.     

Transformation of15N-labelled urea and its modified forms in tropical wetland rice culture

Summary The fate of 100 kg N ha^–1 applied as¹⁵N-urea and its modified forms was followed in 4 successive field-grown wetland rice crops in a vertisol. The first wet season crop recovered about 27 to 36.6% of the applied N depending upon the N source. In subsequent seasons the average uptake was very small and it gradually decreased from 1.4 to 0.5 kg N ha^–1 although about 18 to 20, 12 to 17 and 14 to 18 kg ha^–1 residual fertilizer N was available in the root zone after harvest of first, second and third crops, respectively. The average uptake of the residual fertilizer N was only 7.6% in the second crop and it decreased to 4.5% in the third and to 3.2% in the fourth crop although all these crops were adequately fertilized with unlabelled urea. The basal application of neem coated urea was more effective in controlling the leaching loss of labelled NH₄+NO₃–N than split application of uncoated urea. In the first 3 seasons in which¹⁵N was detectable, the loss of fertilizer N through leaching as NH₄+NO₃–N amounted to 0.5 kg ha^–1 from neem-coated urea, 1.5 kg from split urea and 4.1 kg from coal tar-coated urea. At the end of 4 crops, most of the labelled fertilizer N (about 69% on average) was located in the upper 0–20 cm soil layer showing very little movement beyond this depth. In the profile sampled upto 60 cm depth, totally about 13.8 kg labelled fertilizer N ha^–1 from neem-coated urea, 12.7 kg from coal-tar coated urea, and 11.8 kg from split urea were recovered. The average recovery of labelled urea-N in crops and soil during the entire experimental period ranged between 42 and 51%. After correcting for leaching losses, the remaining 47 to 56% appeared to have been lost through ammonia volatilization and denitrification.