Sesbania rostrata as a green manure for lowland rice: Growth,N2 fixation,Azorhizobium sp. inoculation,and effects on succeeding crop yields and nitrogen balance

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.     

N2 fixation in two Sesbania species and its transfer to rice (Oryza sativa L.) as revealed by 15N technology

Summary We used ¹⁵N technology to investigate N₂ fixation by Sesbania speciosa and Sesbania rostrata and its transfer to a lowland rice crop after incorporation of the Sesbania spp. into soil as green manure. During the first 50 days after establishment in November–December 1989, S. speciosa and S. rostrata produced 1126 and 923 kg dry matter ha^-1 respectively. They gathered 31 and 23 kg N ha^-1 respectively, of which 62%±5% and 55%±3% respectively, came from N₂ fixation. Both these species produced a greater biomass during September–October 1989, with S. rostrata producing more than S. speciosa. These results reflected differential responses by the plants to different day lengths at different times of the year. Furthermore, the dry matter yield and %N of ¹⁵N-labelled S. speciosa were smaller than those of the unlabelled plants, possibly due to inhibition of N₂ fixation in root nodules by the chemical N fertilizers added during labelling. These differences were not so pronounced in the stem-nodulated S. rostrata. The increased grain yield of rice fertilized with N in the form of chemical fertilizer or green manure was a result of an increased number of panicles per hill. The rice crop manured with S. speciosa produced a lower grain yield, with a lower grain weight than that manured with S. rostrata. This was due to a low uptake of soil N by rice manured with S. speciosa. Recovery of N from the green manure in rice straw with S. speciosa was significantly higher than from rice manured with S. rostrata, because of the higher applied N uptake by rice manured with the former.     

Estimating N2 fixation by Sesbania rostrata and S. cannabina (syn. S. aculeata) in lowland rice soil by the 15N dilution method

Summary A field experiment in concrete-based plots was conducted to estimate the contribution of N derived from air (Ndfa) or biological N₂ fixation in Sesbania rostrata and S. cannabina (syn. S. aculeata), using various references, by the ¹⁵N dilution method. The two Sesbania species as N₂-fixing reference plants and four aquatic weed species as non-N₂-fixing references were grown for 65 days after sowing in two consecutive crops, in the dry and the wet seasons, under flooded conditions. Soil previously labeled with ¹⁵N at 0.26 atom % ¹⁵N excess in mineralizable N was further labeled by ammonium sulfate with 3 and 6 atom % ¹⁵N excess. The results showed that ¹⁵N enrichment of soil NH ₄ ⁺ -N dropped exponentially in the first crop to half the original level in 50 days while in the second crop, it declined gradually to half the level in 130 days. The decline in ¹⁵N enrichment, in both N₂-fixing and non-fixing species, was also steeper in the first crop than in the second crop. Variations in ¹⁵N enrichment among non-fixing species were smaller in the second crop. The ratio of the uptake of soil N to that of fertilizer N in N₂-fixing and non-fixing species was estimated by the technique of varying the ¹⁵N level. In the second crop, this ratio in non-fixing species was higher than that in N₂-fixing species. Comparable estimates of % Ndfa were obtained by using ¹⁵N enrichment of various non-fixing species. There was also good agreement between the estimates obtained by using ¹⁵N enrichment of non-fixing species and those by using soil NH ₄ ⁺ -N, particularly in the second crop. By 25 days after sowing, the first crop of both Sesbania spp. had obtained 50% of total N from the atmosphere and the second crop had obtained 75%. The contribution from air increased with the age of the plant and ranged from 70% to 95% in 45–55 days. S. rostrata fixed substantially higher amounts of N₂ due to its higher biomass production compared with S. cannabina. Mathematical considerations in applying the ¹⁵N dilution method are discussed with reference to these results.     

Nitrogen fixation,biomass production,and nutrient uptake by annual Sesbania species in an alkaline soil

Summary In three field trials conducted during the summer season of 1986, 1987 and 1989 in an alkaline soil, 17 accessions of annual Sesbania spp. were evaluated for nodulation, N₂ fixation (acetylene reduction assay), dry weight of roots and shoots, woody biomass production, and nutrient uptake. At 50 days after sowing all the accessions were effectively nodulated (average 36.4 root nodules plant^-1) with a high nodule score (3.4). There was a lot of variation in nodule volume and mass and in acetylene reduction activity but not in N content (5.2%). N uptake in shoots, roots and nodules averaged 639, 31, and 13 mg plant^-1, respectively, and much of the fixed N remained in shoots. Accessions of S. cannabina complex performed better than others. S. rostrata had poor root nodulation but exhibited excellent stem nodulation (300 nodules plant^-1) even though not inoculated with Azorhizobium sp. Average concentrations of N, P, K, S, Ca, and Mg in the shoots were high, at 3.2, 0.28, 1.5, 0.28, 1.5, and 0.4% respectively, and Na was low (0.15%), reflecting the usefulness of Sesbania spp. as an integrated biofertilizer source. Green matter production was 26.0 Mg ha^-1 (5.9 Mg dry matter) and N uptake was 158 kg ha^-1, 54 days after sowing. Average woody biomass of six accessions at maturity, 200 days after sowing, was high (19.9 Mg ha^-1), showing its potential for shortterm firewood production. Total nutrient uptake for production of woody biomass (200 days of growth) was no more demanding than growing the plant to the green-manuring stage of 50–60 days' growth.     

Influence of phosphorus or phosphorus-potassium fertilization on biomass and dinitrogen fixation of the stem-nodulating green-manure legume Sesbania rostrata in different marginally productive wetland rice soils

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.     

Yield and nitrogen response of lowland rice (Oryza sativa L.) to Sesbania rostrata and Aeschynomene afraspera green manure in different marginally productive soils in the Philippines

Organic-N fertilizers in the form of flood-tolerant, leguminous, stem-nodulating Sesbania rostrata and Aeschynomene afraspera may be useful alternatives to resource-poor rice farmers if applied as green manure. Therefore, the accumulation of N by these green manure species and their effect on the performance and yield of wetland rice (IR 64) was examined at four different sites in Luzon, Philippines. Soils deficient in N, P, and K were selected and compared with the fertile Maahas clay of the International Rice Research Institute (IRRI) at Los Baños. The green manure plants were grown under flooded conditions for 49 days in the wet season of 1987, chopped, and then ploughed in before transplanting rice seedlings. In a second experiment, the effect of S. rostrata green manure was studied under rainfed conditions. All green manure treatments were compared to an urea treatment (60 kg N ha^–1) and an untreated control. Both legumes developed well, even on the marginally productive soils. S. rostrata accumulated up to 190 kg N ha^–1 and A. afraspera even accumulated 196 kg N ha^–1 in the shoots. In all treatments, green manure increased grain yield significantly (P=0.05) over the untreated control, by 1.3–1.7 Mg ha^–1. The yields were comparable to those obtained with 60 kg N ha^–1 of urea fertilizer. S. rostrata caused the highest grain yield, of 6.5 Mg ha^–1 on the Maahas clay soil of IRRI. The apparent release of exchangeable NH ₄ ⁺ -N in the soils after green manuring and the rice grain yield response showed that both green manure species may provide sufficient available N throughout the development of IR 64 in the wet season. In the rainfed marginal soil site, green manure with S. rostrata produced even higher rice grain yields than urea. Green manure therefore seems particularly attractive for poor farmers on marginally productive soils, at least as a temporary strategy to improve yield and yield sustainability.     

Field evaluation of N2 fixation and N partitioning in climbing bean (Phaseolus vulgaris L.) using 15N

Summary The common bean (Phaseolus vulgaris L.) is generally regarded as a poor N₂ fixer. This study assessed the sources of N (fertilizer, soil, and fixed N), N partitioning and mobilization, and soil N balance under field conditions in an indeterminate-type climbing bean (P. vulgaris L. cv. Cipro) at the vegetative, early pod-filling, and physiological maturity stages, using the A-value approach. This involved the application of 10 and 100 kg N ha^-1 of ¹⁵N-labelled ammonium sulphate to the climbing bean and a reference crop, maize (Zea mays L.). At the late pod-filling stage (75 days after planting) the climbing bean had accumulated 119 kg N ha^-1, 84% being derived from fixation, 16% from soil, and only 0.2% from the ¹⁵N fertilizer. N₂ fixation was generally high at all stages of plant growth, but the maximum fixation (74% of the total N₂ fixed) occurred during the interval between early (55 days after planting) and late podfilling. The N₂ fixed between 55 and 75 days after planting bas a major source (88%) of the N demand of the developing pod, and only about 11% was contributed from the soil. There was essentially no mobilization of N from the shoots or roots for pod development. The cultivation of common bean cultivars that maintain a high N₂-fixing capacity especially during pod filling, satisfying almost all the N needs of the developing pod and thus requiring little or no mobilization of N from the shoots for pod development, may lead to a net positive soil N balance.     

Nitrogen fixation and beneficial effects of some grain legumes and green-manure crops on rice

Studies were conducted on paddy soils to ascertain N₂ fixation, growth, and N supplying ability of some green-manure crops and grain legumes. In a 60-day pot trial, sunhemp (Crotalaria juncia) produced a significantly higher dry matter content and N yield than Sesbania sesban, S. rostrata, cowpeas (Vigna unguiculata), and blackgram (V. mungo), deriving 91% of its N content from the atmosphere. Dry matter production and N yield by the legumes were significantly correlated with the quantity of N₂ fixed. In a lowland field study involving sunhemp, blackgram, cowpeas, and mungbean, the former produced the highest stover yield and the stover N content, accumulating 160–250 kg N ha^-1 in 60 days, and showed great promise as a biofertilizer for rice. The grain legumes showed good adaptability to rice-based cropping systems and produced a seed yield of 1125–2080 kg ha^-1, depending on the location, species, and cultivar. Significant inter- and intraspecific differences in the stover N content were evident among the grain legumes, with blackgram having the highest N (104–155 kg N ha^-1). In a trial on sequential cropping, the groundnut (Arachis hypogaea) showed a significantly higher N₂ fixation and residual N effect on the succeeding rice crop than cowpeas, blackgram, mungbeans (V. radiata), and pigeonpeas (Cajanus cajan). The growth and N yield of the rice crop were positively correlated with the quantity of N₂ fixed by the preceding legume crop.     

Nitrogen fixation of Sesbania rostrata: Contribution of stem nodules to nitrogen acquisition

Abstract

Nitrogen contents, nodule numbers, and nodule dry weights of 6-week-oId Sesbania rostrata plants grown in sand culture with only root nodules, only stem nodules or with both were compared and the root nodules were found to contribute to nitrogen acquisition more significantly than the stem nodules. Similar findings were obtained in ¹⁵N₂-fixing experiments. An 8-week-old plant with both stem and root nodules fixed 1.50 mg nitrogen in a 12 h light period, while the fixation decreased to 1.15 mg nitrogen after the removal of the stem nodules, suggesting that root nodules played major role in nitrogen fixation. However, acetylene-reducing activities per nodule dry weight were higher in the stem nodules. Under flooding conditions, the aerenchyma tissues contributed to about 40% of N₂ transport to root nodules, and 60% was supplied through stem.     

Field measurements of nitrogen fixation in leguminous trees used in agroforestry systems: Influence of 15N-labeling approaches and reference trees

Appropriate ¹⁵N-labeling methods are crucial for estimating N₂-fixation in trees used in agroforestry systems. A 4-year field experiment was conducted on an Alfisol in Southwestern Nigeria to compare the estimates of N₂ fixed in Leucaena leucocephala, using two non-N₂-fixing leguminous trees, Senna siamea and S. spectabilis, as reference plants and three different methods of introducing ¹⁵N into soil. The atom % ¹⁵N uptake pattern (as reflected in the leaves) was identical in both N₂- and non-N₂-fixing tree species irrespective of the ¹⁵N-application method. There was a significant decline in atom % ¹⁵N excess in the leaves of L. leucocephala (from 0.266 to 0.039), S. siamea (0.625 to 0.121), and S. spectabilis (from 0.683 to 0.118) from the first sampling 12 months after planting and the second sampling 18 months after sampling. From the second harvest in 1991 until the end of the experiment (fifth) harvest in 1993, however, the atom ¹⁵N % excess decline in leaves of the three species was less pronounced and depended on the method of ¹⁵N application. In those plants to which the tracer was applied once at planting, the ¹⁵N decline was steady between the second and the last prunings. In the split-application treatment, the atom ¹⁵N % excess increased slightly at the third pruning and decreased during the subsequent two prunings. The reference tree and the method of ¹⁵N application influenced the estimated proportion of N derived from atmospheric N₂ by L. leucocephala, calculated as 73 and 64%, corresponding to 119 and 98 kg N ha^-1 of N₂ fixed per 6 months, when S. spectabilis and S. siamea were used as reference trees, respectively. The approach by which ¹⁵N-labeled fertilizer was applied to the soil in three splits gave slightly higher estimates of N derived from the atmosphere but this was of little agronomic significance because total N₂ fixed was similar for all methods.     

Effect of nitrogen application on the A N value of soil

Pot experiments were conducted with two soils, from Rottenhaus and Seibersdorf in Austria, to ascertain whether the rate of fertilizer N application and the test crop would influence the amount of N available in the soil as assessed by the A-value method. ¹⁵N-labelled fertilizer was applied at rates of 10, 25, 40, 60, and 100 mg N kg^-1 soil, corresponding approximately to 20, 50, 80, 120 and 200 kg N ha^-1 respectively, and two crop species, barley (Hordeum vulgareL.) and non-nodulating soybean (Glycine max L.) were used to determine the soil A _N value under the various fertilizer regimes. The results showed that the Rottenhaus soil had a higher A _N value than the Seibersdorf soil, suggesting that the former was more fertile than the latter. The A _N values of both soils were significantly affected by the level of N application. When grown in the same soil, the two test crops showed significantly different fertilizer use efficiency and per cent N derived from fertilizer when the rate of N application exceeded 20 kg ha^-1. Thus, the A _N value as determined by the two test crops differed significantly for the same soil when the rate of N application was greater than 20 kg/ha. The difference was greater when the soil fertility level was high. The dependence of the A _N value on the level of N application and the species of crop seriously compromises the suitability of this method for determining plant-associated N₂ fixation. Hence, considerable caution is required when using this method to estimate plant-associated N₂ fixation.     

Phosphorus requirements and nitrogen accumulation by N2-fixing and non-N2-fixing leguminous trees growing in low P soils

The variation in P uptake and use efficiency and N accumulation by Gliricidia sepium (N₂-fixing tree), Senna siamea and S. spectabilis (leguminous non-N₂-fixing trees) were examined in the field at Fashola (savanna zone), southwestern Nigeria, using four P rates, 0, 20, 40 and 80 kg P ha^-1. Growth of G. sepium and S. spectabilis responded to P application at 24 weeks after planting (WAP) and average yield increases of 58% and 145% were observed by the application of 40 kg P ha^-1 for the two species, respectively. Such a P response was not found in S. siamea at 24 WAP and for any of the species at 48 WAP. G. sepium accumulated more P (on average 162%) than S. siamea and S. spectabilis at 24 WAP and had greater root length and a higher percentage of mycorrhizal infection. However, at 48 WAP S. siamea had 2.5 times more P than G. sepium. Differences in the physiological P use efficiency (PPUE) between G. sepium and the non-N₂-fixing trees were significant at the 0 P level, being higher for S. siamea (average, 0.61 g shoot mg^-1 P) than for G. sepium (0.27 g shoot mg^-1 P). G. sepium had a consistently lower atom % ¹⁵N than S. spectabilis, while that of S. siamea for most of the time did not differ from that of G. sepium. The reference plant affected N₂ fixation extimates, with negative values and a higher variability (CV 60%) associated with S. siamea than with S. spectabilis (CV<20%). Consequently, S. spectabilis was selected as a better reference plant for measuring N₂ fixation in G. sepium. G. sepium fixed on average 35% and 54% of its N at 24 and 48 WAP, respectively. Except at the lowest P rate, percentage and amount of N fixed were not generally enhanced by P application.     

Nitrogen use in maize-grain legume cropping systems in semi-arid Kenya

Locally suitable cultivars of maize, beans, and cowpeas were grown in field experiments for four seasons in semi-arid Kenya. For three seasons, the dry matter production and grain yield of maize and beans were not increased by N fertilizer additions up to 120 kg N ha^-1. Fertilizer recoveries measured by ¹⁵N isotope dilution techniques were low, less than 20%. Inoculated and uninoculated beans failed to fix N₂. By contrast the cowpea derived 50% of its N from fixation, equivalent to 197 kg N ha^-1. The N content of the grain generally exceeded 40 kg N ha^-1, and the N content of the seeds from the grain legumes were greater than those from the cereals. Large inputs of N fertilizer or N by fixation are required if maize-grain legume cropping system in semiarid Kenya are to be sustained in the long term.     

A comparison of nitrogen fixation in genotypes of groundnut (Arachis hypogaea L.) using 15N-isotope dilution

Summary Nitrogen fixation in seven groundnut genotypes was measured by ¹⁵N-isotope dilution using a non-nodulating cultivar of groundnut as the nonfixing reference plant. Nitrogen fixation varied between 100 kg N ha^–1 in genotype J-11 and 153 kg N ha^–1 in Robut 33-1. The amount of plant-available soil N was small, so that 86%–92% of plant nitrogen was derived from N₂-fixation. Thus differences in N₂-fixation between genotypes closely reflected differences in their total N accumulation.ICRISAT Journal Article no. 600     

Effect of successive cuttings on uptake and partitioning of 15N among plant parts of Leucaena leucocephala

Summary We studied the effect of three successive cuttings on N uptake and fixation and N distribution in Leucaena leucocephala. Two isolines, uninoculated or inoculated with three different Rhizobium strains, were grown for 36 weeks and cut every 12 weeks. The soil was labelled with 50 ppm KNO₃ enriched with 10 atom % ¹⁵N excess soon after the first cutting. Except for the atom % ¹⁵N excess in branches of K28 at the second cutting, both the L. leucocephala isolines showed similar patterns of total N, fixed N₂, and N from fertilizer distribution in different parts of the plant at each cutting. The Rhizobium strain did not influence the partitioning of ¹⁵N among the different plant parts. Significant differences in ¹⁵N enrichment occurred in different parts. Live nodules of both isolines showed the lowest atom % ¹⁵N excess values (0.087), followed by leaves (0.492), branches (0.552), stems (0.591), and roots (0.857). The roots contained about 60% of the total plant N and about 70% of the total N derived from fertilizer over the successive cuttings. The total N₂ fixed in the roots was about 60% of that fixed in the whole plant, while the shoots contained only 20% of the fixed N₂. We conclude that N reserves in roots and nodules constitute another N source that must be taken into account when estimating fixed N₂ or the N balance after pruning or cutting plants. ¹⁵N enrichment declined up to about fivefold in the reference and the N₂-fixing plants over 24 weeks following the ¹⁵N application. The proportion and the amounts of N derived from fertilizer decreased, while the amount derived from N₂ fixation increased with time although its proportion remained constant.     

Variation in nodulation and N2 fixation by the Gliricidia sepium/Rhizobium spp. symbiosis in a calcareous soil

Summary Variation in nodulation and N₂ fixation by the Gliricidia sepium/Rhizobium spp. symbiosis was studied in two greenhouse experiments. The first included 25 provenances of G. sepium inoculated with a mixture of three strains of Rhizobium spp. N₂ fixation was measured using the ¹⁵N isotope dilution method 12 weeks after planting. On average, G. sepium derived 45% of its total N from atmospheric N₂. Significant differences in fixation were observed between provenances. The percentage of N derived from atmospheric N₂ ranged from 26 to 68% (equivalent to 18–62 mg N plant^-1) and was correlated with total N in the plant (r=0.70; P=0.05). The second experiment included six strains of Rhizobium spp. and two methods of inoculation and the plants were harvested 14,35 and 53 weeks after planting. In the first harvest significant differences were found between the number of nodules and the percentage and amount of N₂ fixed. There was also a significant correlation between the number of nodules and the amount of N₂ fixed (r=0.92; P=0.05). In the final harvest no correlation was observed, although there were significant differences between the number of nodules and the percentage of N derived from the atmosphere. The amount of N₂ fixed increased with time (from an average of 27% at the first harvest to 58% at the final harvest) and was influenced by the Rhizobium spp. strain and the method of inoculation. It ranged from 36% for Rhizobium sp. strain SP 14 to 71% for Rhizobium SP 44 at the last harvest. Values for the percentage of atmosphere derived N₂ obtained by soil inoculation were slightly higher than those obtained by seed inoculation.     

Isotope 15N enrichment of soil-ammonium N as a reference to estimate N2 fixation by stem and root-inoculated S. rostrata

Summary A pot experiment in the greenhouse was conducted to compare the contribution of N derived from the atmosphere or from biological N₂ fixation by Sesbania rostrata inoculated with Azorhizobium caulinodans, applied either to roots or to roots and stems (single or multiple stem inoculation). Two subsequent crops were grown for 50 days under flooded conditions. N derived from air was estimated by ¹⁵N dilution using ¹⁵N enrichment of soil NH _inf4 ^sup+ -N and of Echinochloa crusgalli as the non-N₂-fixing reference datum and compared with estimates obtained by the N-difference method. The first crop was grown to stabilize the ¹⁵N into the soil organic N fraction. The ¹⁵N enrichment of soil NH _inf4 ^sup+ -N in the second crop declined slowly. The extractability ratio (¹⁵N enrichment of extractable soil N to ¹⁵N enrichment of total soil N) decreased from 4.8 to 4.1 50 days after planting. The enrichment of soil NH _inf4 ^sup+ -N was comparable to that of E. crus-galli, resulting in similar estimates of N derived from air when either soil NH _inf4 ^sup+ -N or enrichment of E. crus-galli was used as a non-fixing reference. The N-difference method did not always provide reliable estimates of N derived from air; percentages ranged from 75 to more than 80 by 50 days after planting in both crops and did not differ among treatments. The study demonstrates the potential of using ¹⁵N enrichment of soil NH _inf4 ^sup+ -N as a non-N₂-fixing reference for reliable BNF estimates of crops in lowland puddled soil.     

Nitrogen and phosphorus use in maize sole cropping and maize/cowpea mixed cropping systems on an Alfisol in the northern Guinea Savanna of Ghana

Summary The use of N and P by mixed and by sole cropping (crop rotation) of maize and cowpeas were compared in a field experiment on an Alfisol at the Nyankpala Agricultural Experiment Station in the northern Guinea Savanna of Ghana, using two levels of N (0 and 80 kg N ha^-1 year^-1 as urea) and P application (0 and 60 kg P ha^-1 year^-1 as Volta phosphate rock). Maize grain yields were significantly reduced in the mixed cropping system. This yield difference became smaller with the application of N and P fertilizer. The N and P concentrations in maize ear leaves at silking indicated that a deficiency in N and P contributed to the maize yield depression in mixed cropping. Competition for soil and fertilizer N between maize and cowpeas was suggested by: (1) A similarity in total N uptake between the two cropping systems; (2) efficient use of soil nitrate by the cowpeas; and (3) low N₂ fixation by the cowpeas, calculated with the aid of an extended-difference method. In general, N₂ fixation was low, with the highest values in the sole cropping (53 kg ha^-1) and a substantial reduction in the mixed cropping system. The application of N fertilizer further reduced N₂ fixation. This was substantiated by nodule counts. The lower N₂ fixation in the mixed cropping system was only partly explained by the lower density of cowpeas in this system. In addition, dry spells during the cropping season and shading by the maize component could have reduced the nodulation efficiency. No N transfer from the legume/rhizobium to the non-legume crop was observed. Impaired P nutrition in the mixed compared with the sole-cropped maize might have been due to less P mobility in the soil. This was indicated by lower soil moisture contents in the topsoil under mixed cropping, especially during the dry year of 1986. The results show that mixed cropping of maize and cowpeas did not lead to improved use of soil and fertilizer N and P or to an enhanced N₂ fixation. On the contrary, an annual rotation of maize and cowpeas was clearly superior.     

Free-living nitrogen-fixing bacteria in temperate cropping systems: Influence of nitrogen source

Sustainable cropping systems rely on a minimum of external inputs. In these systems N is largely acquired in animal manures and leguminous green manures. Little is known of how these organic forms of N fertilizer influence the presence and activity of free-living N₂-fixing bacteria. High concentrations of inorganic N in soil inhibit N₂-fixation in cyanobacteria and Azotobacter spp. It is likely that manure and fertilizer applications would result in concentrations of inorganic N capable of inhibiting N₂ fixation and, ultimately, the presence of these organisms. We investigated the effect of synthetic and organic N fertilizer sources on the populations and N₂-fixation potential of free-living N₂-fixing bacteria in the Farming Systems Trial at the Rodale Research Institute. Field plots received the following N treatments prior to corn (Zea mays L.) production: (1) Legume rotations and green manures supplying about 165 kg N ha^-1; (2) beef cattle manure applied at a rate of 220 kg N ha^-1 (plus 60 kg N ha^-1 from 1994 hay plow-down); or (3) fertilizer N (urea and NH₄NO₃) applied at a rate of 145 kg N ha^-1. Soil samples were collected at two depths from corn plots four times during the growing season, and analyzed for soil moisture, soil pH, numbers of N₂-fixing cyanobacteria and Azotobacter spp., extractable NH _inf4 ^sup+ and NO _inf3 ^sup- , and potentially mineralizable N. Soil samples collected in mid-July were analyzed for nitrogenase activity (by C₂H₂ reduction) and total C and N. Populations of Azotobacter spp. and cyanobacteria were influenced only slightly by treatment; however, cyanobacteria species composition was notably influenced by treatment. Nitrogenase activity in surface soils was greatest in legume-N plots and in subsurface plots levels were greatest in fertilizer-N plots. Populations and activity of free-living N-fixing bacteria appeared to be somewhat reduced in all plots as a result of low soil pH levels and high concentrations of inorganic N across all treatments. Annual applications of N to all plots resulted in high levels of potentially mineralizable N that in turn may have reduced non-symbiotic N₂-fixation in all plots.     

Einfluß von NPK auf die Biomasseproduktion und Stickstoffbindung der stengelknöllchenbildenden Gründüngungsleguminosen Sesbania rostrata und Aeschynomene afraspera im Naßreisanbau

Influence of NPK on performance of the stem-nodulating legumes Sesbania rostrata and Aeschynomene afraspera in lowland rice The stem-nodulating tropical legumes S. rostrata and A. afraspera are promising green manure species for the low-input rice farming systems of lowland areas. Nutrient imbalances and soils low in available nutrients can considerably affect the use of biofertilizers. Use of mineral N, P, and K fertilizers in the growth of S. rostrata and A. afraspera as biofertilizers for lowland rice in the Philippines was evaluated. Applied P and K both stimulated growth, nodulation, and N₂ fixation. N accumulation in PK fertilized S. rostrata was about 40% higher than in nonfertilized green manure. Mineral N application (urea) depressed nodulation and N₂ fixation (ARA) in roots and resulted in an increased ARA in stem nodules. The legumes produced more N gains in the presence of small amounts of N. Use of S. rostrata and A. afraspera green manure increased in all cases significantly rice grain yield. Thus integration of mineral N, P, and K fertilizers in a green manurebased rice farming system can considerably improve biofertilizer production and increase rice grain yield.