ALBERT

Notes: Summary Root and stem nodulation, nitrogen fixation (acetylene-reducing activity), growth and N accumulation bySesbania rostrata as affected by season and inoculation were studied in a pot experiment. The effects ofS. rostrata as a green manure on succeeding wet-season and dry-season rice yields and total N balance were also studied.S. rostrata grown during the wet season showed better growth, nodulation, and greater acetylene-reducing activity than that grown during the dry season. Inoculation withAzorhizobium caulinodans ORS 571 StrSpc® (resistant to streptomycin and spectinomycin) on the stem alone or on both root and stem significantly increased N2 fixation by the plants. Soil and seed inoculation yielded active root nodules under flooded conditions. Plants that were not inoculated on the stem did not develop stem nodules. The nitrogenase activity of the root nodules was greater than that of the stem nodules in about 50-day-oldS. rostrata. S. rostrata incorporation, irrespective of inoculation, significantly increased the grain yield and N uptake of the succeeding wet season and dry season rice crops. The inoculated treatments produced a significantly greater N gain (873 mg N pot−1) than the noinoculation (712 mg N pot−1) treatment. About 80% of the N gained was transferred to the succeeding rice crops and about 20% remained in the soil. The soil N in the flooded fallow-rice treatment significantly declined (−140 mg N pot−1) but significantly increased in bothS. rostrata-rice treatments (159 and 151 mg N pot−1 in uninoculated and inoculated treatments respectively). The N-balance data gave extrapolated values of N2 fixed per hectare at about 303 kg N ha−1 per two crops forS. rostrata (uninoculated)-rice and 383 forS. rostrata (inoculated)-rice.

Notes: Abstract The performance of Sesbania rostrata varies widely from site to site. This makes it difficult to predict the N yield and biomass of this plant in marginally productive soils, and to arouse the interest of farmers in green manure technology. Three consecutive pot experiments were conducted in a greenhouse at the International Rice Research Institute (IRRI) to evaluate growth, nodulation, N2 fixation (C2H2 reduction assay and 15N dilution method), and N yield of 6-week-old S. rostrata on 13 physicochemically different wetland rice soils of the Philippines and on three artificial substrates. The performance of S. rostrata on the unfertilized controls was compared with two fertilizer treatments containing either P (100 mg P kg-1 dry soil) or P+K (100 mg P kg-1 and 200 mg K kg-1 dry soil). In the control soils and substrates, the N yield of S. rostrata varied between 20 and 470 mg N per pot, with the N rate from N2 fixation ranging between 0 and 95%. In three of the nutritionally poor soils even Mn toxicity symptoms apparently occurred with S. rostrata. P application alleviated these symptoms and increased the overall N yield considerably, mainly through increased biological N2 fixation. An additional increase in N yield was obtained by the PK treatment. Multiple regression analysis between soil characteristics and the N yield of S. rostrata showed that the original level of P (Olsen-extracted) and Mn in the soil accounted for 73% of the variance in biomass production by S. rostrata among the unfertilized soils and substrates.

Notes: Abstract Poor adoption of sustainable pre-rice green manure technology by lowland farmers is frequently associated with unreliable legume performance under adverse environmental conditions such as marginal soils, short photoperiod, and unfavorable hydrology. A series of field and microplot experiments were conducted at the International Rice Research Institute (IRRI) in 1991 and 1992 to screen and evaluate 12 promising flood-tolerant legumes for adaptation (N accumulation and biological N2 fixation) to a range of environmental stresses, frequently encountered in rice lowlands. Legumes belonging to the genera Sesbania and Aeschynomene were grown for 8 weeks at 10×10 cm spacing: (1) in a fertile control soil and in four marginally productive irrigated lowland rice soils (sandy Entisol, P-deficient Inceptisol, acid Ultisol, and saline Mollisol); (2) during short- (11.7 h) and long-day (12.3 h) seasons in a favorable irrigated lowland soil; and (3) in an aerobic soil (drought-prone rain-fed lowland) and a deep-flood-prone lowland soil (1 week seedling submergence). A large variability in N accumulation was observed among legume species and across different environments, ranging from less than 1 to over 70 mg N plant–1. The nitrogen derived from the atmosphere (Ndfa) accounted on average for 82% of total N accumulation. Sesbania virgata was least affected by unfavorable soil conditions but its Ndfa was the lowest among the tested species (less than 60%). Stem nodule formation did not convey a significant advantage to legumes grown under adverse soil conditions. However, flooding reduced N2 fixation less in stem-nodulating than in solely root-nodulating species. Most species drastically reduced N accumulation under short-day conditions. Aeschynomene afraspera and S. speciosa were least affected by photoperiod. The considerable genetic variability in the germplasm screened allows the selection of potentially appropriate legumes to most conditions studied, thus increasing N accumulation in green manures.

Notes: Abstract Poor adoption of sustainable pre-rice green manure technology by lowland farmers is frequently associated with unreliable legume performance under adverse environmental conditions such as marginal soils, short photoperiod, and unfavorable hydrology. A series of field and microplot experiments were conducted at the International Rice Research Institute (IRRI) in 1991 and 1992 to screen and evaluate 12 promising flood-tolerant legumes for adaptation (N accumulation and biological N2 fixation) to a range of environmental stresses, frequently encountered in rice lowlands. Legumes belonging to the genera Sesbania and Aeschynomene were grown for 8 weeks at 10×10 cm spacing: (1) in a fertile control soil and in four marginally productive irrigated lowland rice soils (sandy Entisol, P-deficient Inceptisol, acid Ultisol, and saline Mollisol); (2) during short- (11.7h) and long-day (12.3 h) seasons in a favorable irrigated lowland soil; and (3) in an aerobic soil (drought-prone rain-fed lowland) and a deep-flood-prone lowland soil (1 week seedling submergence). A large variability in N accumulation was obsersed among legume species and across different environments, ranging from less than 1 to over 70 mg N plant-1. The nitrogen derived from the atmosphere (Ndfa) accounted on average for 82% of total N accumulation. Sesbania virgata was least affected by unfavorable soil conditions but its Ndfa was the lowest among the tested species (less than 60%). Stem nodule formation did not convey a significant advantage to legumes grown under adverse soil conditions. However, flooding reduced N2 fixation less in stem-nodulating than in solely root-nodulating species. Most species drastically reduced N accumulation under short-day conditions. Aeschynomene afraspera and S. speciosa were least affected by photoperiod. The considerable genetic variability in the germplasm screened allows the selection of potentially appropriate legumes to most conditions studied, thus increasing N accumulation in green manures.

Notes: Summary Ratooning and stem cutting were compared with seeding in order to reduce the amount of seeds of Sesbania rostrata for green-manure growth. Both methods increased the biofertilizer yield highly significantly within a 6-week growth period.

Notes: Abstract An F2 population, consisting of 231 individuals derived from a cross between rice cultivars with a similar growing duration, Palawan and IR42, was utilized to investigate the genetic nature of rice varietal ability to stimulate N2 fixation in the rice rhizosphere. To assess rhizospheric N2 fixation, an isotope-enriched 15N dilution technique was employed, using 15N-stabilized soil in pots. IR42, an indica variety, had 23% higher N derived from fixation (Ndfa) than Palawan, a javanica genotype. Normal segregation of atom% 15N excess was obtained in the F2 population, with an average of 0.218 with 8% of plants below IR42 (0.188) and 10% of plants above Palawan (0.248). One-hundred-and-four RFLP markers mapped on 12 chromosomes were tested for linkage to the putative QTLs. Significant (P〈0.01) associations between markers and segregation of atom% 15N excess were observed for seven marker loci located on chromosomes 1, 3, 6 and 11. Four QTLs defined by the detected marker loci were identified by interval-mapping analysis. Additive gene action was found to be predominant, but for at least one locus, dominance and partial dominance effects were observed. Significant (P〈0.01) epistatic effects were also identified. Individual marker loci detected between 8 and 16% of the total phenotypic variation. All four putative QTLs showed recessive gene action, and no phenotypic effects associated with heterozygosity of marker loci were observed. The results of this study suggest that rice genetic factors can be identified which affect levels of atom% 15N excess in the soil by interacting with diazotrophs in the rice rhizosphere.

Notes: Abstract This paper 1) reviews improvements and new approaches in methodologies for estimating biological N2 fixation (BNF) in wetland soils, 2) summarizes earlier quantitative estimates and recent data, and 3) discusses the contribution of BNF to N balance in wetland-rice culture. Measuring acetylene reducing activity (ARA) is still the most popular method for assessing BNF in rice fields. Recent studies confirm that ARA measurements present a number of problems that may render quantitative extrapolations questionable. On the other hand, few comparative measures show ARA's potential as a quantitative estimate. Methods for measuring photodependent and associative ARA in field studies have been standardized, and major progress has been made in sampling procedures. Standardized ARA measurements have shown significant differences in associative N2 fixation among rice varieties. The 15N dilution method is suitable for measuring the percentage of N derived from the atmosphere (% Ndfa) in legumes and rice. In particular, the 15N dilution technique, using available soil N as control, appears to be a promising method for screening rice varieties for ability to utilize biologically fixed N. Attempts to adapt the 15N dilution method to aquatic N2 fixers (Azolla and blue-green algae [BGA]) encountered difficulties due to the rapid change in 15N enrichment of the water. Differences in natural 15N abundance have been used to show differences among plant organs and species or varieties in rice and Azolla, and to estimate Ndfa by Azolla, but the method appears to be semi-quantitative. Recent pot experiments using stabilized 15N-labelled soil or balances in pots covered with black cloth indicate a contribution of 10–30 kg N ha-1 crop-1 by heterotrophic BNF in flooded planted soil with no or little N fertilizer used. Associative BNF extrapolated from ARA and 15N incorporation range from 1 to 7 kg N ha-1 crop-1. Straw application increases heterotrophic and photodependent BNF. Pot experiments show N gains of 2–4 mg N g-1 straw added at 10 tons ha-1. N2 fixation by BGA has been almost exclusively estimated by ARA and biomass measurements. Estimates by ARA range from a few to 80 kg N ha-1 crop-1 (average 27 kg). Recent extensive measurements show extrapolated values of about 20 kg N ha-1 crop-1 in no-N plots, 8 kg in plots with broadcast urea, and 12 kg in plots with deep-placed urea. Most information on N2 fixed by Azolla and legume green manure comes from N accumulation measurements and determination of % Ndfa. Recent trials in an international network show standing crops of Azolla averaging 30–40 kg N ha-1 and the accumulation of 50–90 kg N ha-1 for two crops of Azolla grown before and after transplanting rice. Estimates of % Ndfa in Azolla by 15N dilution and delta 15N methods range from 51 to 99%. Assuming 50–80% Ndfa in legume green manures, one crop can provide 50–100 kg N ha-1 in 50 days. Few balance studies in microplots or pots report extrapolated N gains of 150–250 kg N ha-1 crop-1. N balances in long-term fertility experiments range from 19 to 98 kg N ha-1 crop-1 (average 50 kg N) in fields with no N fertilizer applied. The problems encountered with ARA and 15N methods have revived interest in N balance studies in pots. Balances are usually highest in flooded planted pots exposed to light and receiving no N fertilizer; extrapolated values range from 16 to 70 kg N ha-1 crop-1 (average 38 kg N). A compilation of balance experiments with rice soil shows an average balance of about 30 kg N ha-1 crop-1 in soils where no inorganic fertilizer N was applied. Biological N2 fixation by individual systems can be estimated more or less accurately, but total BNF in a rice field has not yet been estimated by measuring simultaneously the activities of the various components in situ. As a result, it is not clear if the activities of the different N2-fixing systems are independent or related. A method to estimate in situ the contribution of N2 fixed to rice nutrition is still not available. Dynamics of BNF during the crop cycle is known for indigenous agents but the pattern of fixed N availability to rice is known only for a few green manure crops.