Effect of plant genotype and nitrogen fertilizer on symbiotic nitrogen fixation by soybean cultivars

Summary Isotopic as well as non-isotopic methods were used to assess symbiotic nitrogen fixation within eight soybean [Glycine max (L.) Merr.] cultivars grown at 20 and 100 kg N/ha levels of nitrogen fertilizer under field conditions.The¹⁵N methodology revealed large differences between soybean cultivars in their abilities to support nitrogen fixation. In almost all cases, the application of 100 kg N/ha resulted in lower N₂ fixed in soybean than at 20 kg N/ha in the first year of the study. However, N₂ fixed in one cultivar, Dunadja, was not significantly affected by the higher rate of N fertilizer application. These results were confirmed by measurements of acetylene reduction activity, nodule dry weight and N₂ fixed as measured by the difference method. Further proof of differences in N₂ fixed within soybean cultivars and the ability of Dunadja to fix similar amounts of N₂ at 20 and 100 kg N/ha was obtained during a second year experiment. Dunadja yield was affected by N fertilizer and produced larger yield at 100 kg N/ha than at 20 kg N/ha. This type of cultivar could be particularly useful in situations where soil N levels are high or where there is need to apply high amounts of N fertilizer.The present study reveals the great variability between legume germplasms in the ability to fix N₂ at different inorganic N levels, and also the potential that exists in breeding for nitrogen fixation associative traits. The¹⁵N methodology offers a unique tool to evaluate germplasms directly in the field for their N₂ fixation abilities at different N fertilizer levels.     

Differences between Bradyrhizobium strains NC92 and TAL1000 in their nodulation and nitrogen fixation with peanut in iron deficient soil

The effects of Bradyrhizobium (strains NC92 and TAL1000) and Fe supply on nodulation and nitrogen fixation of two peanut (Arachis hypogaea L.) cultivars (cv. Tainan 9 (Fe inefficient) and cv. 71-234 (Fe efficient)) grown under Fe deficient conditions (imposed by adding 40% CaCO₃ to a ferruginous soil) were examined in a glasshouse experiment. When inoculated with TAL1000 without Fe, both cultivars had low shoot N concentration, very low nodule numbers and weight and no measurable acetylene reduction activity per plant. Inoculation with NC92 without Fe increased all these parameters substantially; addition of Fe with NC92 had no further effect on N concentration but doubled nodule number, weight and acetylene reduction activity per plant. Addition of Fe with TAL1000 increased all parameters to the same level as Fe+NC92, indicating that the poorer nodulation and N₂ fixation of TAL1000 in the absence of Fe, resulted from a poorer ability in getting its Fe supply from the alkaline soil. The nodules from all treatments with measurable activity had the same specific acetylene reduction activity suggesting that Fe deficiency limited nodule development.The results support previous suggestions that Bradyrhizobium strains differ greatly in their ability to obtain Fe from soils and that selection of Fe efficient strains could complement plant breeding in the selection of legume crops for Fe deficient soils.     

Effect of composted textile sludge on growth, nodulation and nitrogen fixation of soybean and cowpea

The effect of composted textile sludge on growth, nodulation and nitrogen fixation of soybean and cowpea was evaluated in a greenhouse experiment. The compost was incorporated into soil at 0, 9.5, 19 and 38 t ha(-1) (bases upon the N requirement of the crops, i.e., 0, 50, 100 and 200 kg available N ha(-1)). Growth, nodulation and shoot accumulation of nitrogen were evaluated 36 and 63 days after plant emergence. Nodule glutamine synthetase (GS) activity and leghemoglobin content were evaluated 63 days after emergence. Composted textile sludge did not show negative effects on nodule number and weight, nodule GS activity and leghemoglobin content. Nitrogen accumulation in shoot dry matter in soybean and cowpea was higher than other treatments with application of 19 t ha(-1) of compost. Composting can be an alternate technology for the management of solid textile mill sludge. This study verifies that the composted textile sludge was not harmful to growth, nodulation and nitrogen fixation of soybean and cowpea.     

Nodulation of Soybeans as Affected by Half-root Infection with Heterodera glycines

A split-root technique was applied to soybean, Glycine max (L.) Merr. cv. Lee 68, to characterize the nature of the nodulation suppression by race 1 of the soybean cyst nematode (SCN), Heterodera glycines. Root-halves of each split-root plant were inoculated with Rhizobium japonicum, and one root-half only was inoculated with various numbers of SCN eggs. Nodulation (indicated by nodule number, nodule weights, and ratio of nodule weight to root weight) and nitrogen-fixing capacity (indicated by rate of acetylene reduction) were systemically and variously suppressed on both root-halves of the split-root plant 5 weeks after half-root inoculation with 12,500 SCN eggs. Inoculation with 500 eggs caused this suppression only on the SCN-infected (+NE) root-half; nodulation on the companion uninfected (-NE) root-half was stimulated slightly. The +NE root-halves inoculated with 5,000 eggs were excised at 2-week intervals; nodulation on the remaining -NE root-halves was not different from that of the noninoculated control when measured 6 weeks after the SCN inoculation. Thus, the systemic suppression of nodulation was reversible upon the removal of the SCN. Similarly, application of various levels of KNO₃ to the -NE root-halves of the split-root plant did not alleviate the suppressed nodulation on the companion +NE root-halves, even though plant growth was much improved at certain levels of nitrogen (125 μg N/g soil). This indicated that the localized suppression of nodulation by SCN was caused by factors in addition to poor plant growth.     

Nodulation,accumulation and redistribution of nitrogen in soybean (Glycine max (L.) Merrill) as influenced by seed inoculation and scheduling of irrigation

Summary A field experiment was conducted on soybean (Glycine max (L.) Merrill) with a view to find out the effect of seed inoculation and scheduling of irrigation on nodulation, accumulation and re-distribution of nitrogen in plant tops and soil. The eight treatment combinations consists of two seed inoculations,viz. uninoculated and inoculated with rhizobium culture, and four irrigation schedules,viz. irrigation water to the cumulative pan evaporation (IW/CPE) ratio of 0.5, 0.7, 0.9 and a control (rainfed). Seed inoculation by, rhizobium culture increased the number, dry-weight and N content of nodules per plant. Inoculation of seeds also increased the N accumulation rate in plant top and it was 2.48 kg/ha/day during the flower-initiation to the pod-initiation stage (30–60 days interval). At harvest, 32.2, 47.8 and 26.2 kg N/ha was re-distributed from the stems, leaves and pods-wall of inoculated plants to the grains, respectively. A total of 186.5 kg N/ha was harvested and 64.7 kg N/ha, was accumulated in soil under the inoculated condition.Scheduling of irrigation at 0.7 IW/CPE proved better, than other irrigation schedules and helped in increasing the nodulation, nitrogen accumulation and grain yield. As compared to control, 8.4, 17.8 and 18.4 kg more of N/ha was redistributed from the stems, leaves and pods-wall respectively when the irrigations were scheduled at 0.7 IW/CPE ratio. Under this irrigation schedule the total N harvest was 200.1 kg/ha while the total N increased by 55.9 kg over that present in soil at the time of sowing.     

Preincubation of B. japonicum cells with genistein reduces the inhibitory effects of mineral nitrogen on soybean nodulation and nitrogen fixation under field conditions

Genistein is the major root produced isoflavonoid inducer of nod genes in the symbiosis between B. japonicum and soybean plants. Reduction in the isoflavonoid content of the host plants has recently been suggested as a possible explanation for the inhibition of mineral nitrogen (N) on the establishment of the symbiosis. In order to determine whether genistein addition could overcome this inhibition, we incubated B. japonicum cells (strain 532C) with genistein. Mineral N (in the form of NH₄NO₃) was applied at 0, 20 and 100 kg ha^-1. The experiments were conducted on both a sandy-loam soil and a clay-loam soil. Preincubation of B. japonicum cells with genistein increased soybean nodule number and nodule weight, especially in the low-N-containing sandy-loam soil and the low N fertilizer treatment. Plant growth and yield were less affected by genistein preincubation treatments than nitrogen assimilation. Total plant nitrogen content was increased by the two genistein preincubation treatments at the early flowering stage. At maturity, shoot and total plant nitrogen contents were increased by the 40 μM genistein preincubation treatment at the sandy-loam soil site. Total nitrogen contents were increased by the 20 μM genistein preincubation treatment only at the 0 and 20 kg ha^-1 nitrate levels in clay-loam soil. Forty μM genistein preincubation treatment increased soybean yield on the sandy-loam soil. There was no difference among treatments for 100-seed weight. The results suggest that preincubation of B. japonicum cells with genistein could improve soybean nodulation and nitrogen fixation, and at least partially overcome the inhibition of mineral nitrogen on soybean nodulation and nitrogen fixation. This revised version was published online in June 2006 with corrections to the Cover Date.     

The effect of cyanobacteria and azolla on the performance of rice under different levels of fertilizer nitrogen

Cyanobacteria and/or azolla were inoculated, with urea at 0, 72 or 144 kg N/ha, in plots in which azolla-free Indica rice var. IR 28 was grown. Productive tillers, yield and nitrogen contents of grain and straw positively responded to inoculation with cyanobacteria or azolla, even with fertilizer-N up to 144 kg N/ha. Inoculation improved colonization by cyanobacteria. Azolla were superior to the asymbiotic cyanobacteria in enhancing rice performance. Urea at a rate of 72 kg N/ha was found to support the best colonizations when applied with cyanobacteria or azolla or, to give maximum rice yields, both inoculants.     

Cinnamate toxicity expression on phenylalanine ammonia-lyase activity,germination and growth of cucumber (Cucumis sativis) seedlings

Low pH (5.2) decreased nodule number and acetylene reduction. Aluminium further depressed those parameters in theRhizobium leguminosarum-Pisum sativum associations examined. In the Al-treated plants nodule formation by strains 128C53 and 128C30 was not affected by 3 or 15 and 30 or 60 μM Al, respectively, as compared with the number of nodules on plants grown at pH 5.2 in the absence of Al. However, improved nodulation rates by those strains did not enhance plant dry weight or reduced nitrogen content. No differences in nitrogenase activity were found among strains of nodulating plants grown at the same aluminium level. These results suggest that Al-ions affected specifically nitrogenase activity and that this effect was primarily responsible for the reduction in plant growth.     

Relationship between Ureide N and N(2) Fixation, Aboveground N Accumulation, Acetylene Reduction, and Nodule Mass in Greenhouse and Field Studies with Glycine max L. (Merr)

The relationship between ureide N and N₂ fixation was evaluated in greenhouse-grown soybean (Glycine max L. Merr.) and lima bean (Phaseolus lunatus L.) and in field studies with soybean. In the greenhouse, plant N accumulation from N₂ fixation in soybean and lima bean correlated with ureide N. In soybean, N₂ fixation, ureide N, acetylene reduction, and nodule mass were correlated when N₂ fixation was inhibited by applying KNO₃ solutions to the plants. The ureide-N concentrations of different plant tissues and of total plant ureide N varied according to the effectiveness of the strain of Bradyrhizobium japonicum used to inoculate plants. The ureide-N concentrations in the different plant tissues correlated with N₂ fixation. Ureide N determinations in field studies with soybean correlated with N₂ fixation, aboveground N accumulation, nodule weight, and acetylene reduction. N₂ fixation was estimated by ¹⁵N isotope dilution with nine and ten soybean genotypes in 1979 and 1980, respectively, at the V9, R2, and R5 growth stages. In 1981, we investigated the relationship between ureide N, aboveground N accumulation, acetylene reduction, and nodule mass using four soybean genotypes harvested at the V4, V6, R2, R4, R5, and R6 growth stages. Ureide N concentrations of young stem tissues or plants or aboveground ureide N content of the four soybean genotypes varied throughout growth correlating with acetylene reduction, nodule mass, and aboveground N accumulation. The ureide-N concentrations of young stem tissues or plants or aboveground ureide-N content in three soybean genotypes varied across inoculation treatments of 14 and 13 strains of Bradyrhizobium japonicum in 1981 and 1982, respectively, and correlated with nodule mass and acetylene reduction. In the greenhouse, results correlating nodule mass with N₂ fixation and ureide N across strains were variable. Acetylene reduction in soybean across host-strain combinations did not correlate with N₂ fixation and ureide N. N₂ fixation, ureide N, acetylene reduction, and nodule mass correlated across inoculation treatments with strains of Bradyrhizobium spp. varying in effectiveness on lima beans. Our data indicate that ureide-N determinations may be used as an additional method to acetylene reduction in studies of the physiology of N₂ fixation in soybean. Ureide-N measurements also may be useful to rank strains of B. japonicum for effectiveness of N₂ fixation.     

Associative effect ofAzospirilium brasilense withRhizobium japonicum on nodulation and yield of soybean (Glycine max)

Summary Azospirillum was associated with nodules of soybean. In general, seed inoculation with a broth culture ofAzospirillum brasilense alone significantly increased nodulation and grain yield of soybean grown in pots in unsterilized soil with different levels of urea ranging from 0 to 80 kg N/ha. This trend was significantly reproducible in a second experiment when a carrier based inoculant of the bacterium was used for seed inoculation.Inoculation withRhizobium japonicum andA. brasilense in combination generally increased grain yield in both the experiments, although the data were not significant.     

Nodulation,nitrogen fixation and nitrogen uptake in pigeonpea (Cajanus cajan (L.) Millsp) of different maturity groups

Summary The seasonal patterns of nodulation, acetylene reduction, nitrogen uptake and nitrogen fixation were studies for 11 pigeonpea cultivars belonging to different maturity groups grown on an Alfisol at ICRISAT Center, Patancheru, India. In all cultivars the nodule number and mass increased to a maximum around 60–80 days after sowing and then declined. The nodule number and mass of medium- and late-maturing cultivars was greater than that of early-maturing cultivars. The nitrogenase activity per plant increased to 60 days after sowing and declined thereafter, with little activity at 100 days when the crop was flowering. At later stages of plant growth nodules formed down to 90 cm below the soil surface but those at greater depth appeared less active than those near the surface. All the 11 cultivars continued to accumulate dry matter until 140 days, with most biomass production by the late-maturing cultivars (up to 11 t ha⁻¹) and least by the early-maturing determinate cultivars (4 t ha⁻¹). Total nitrogen uptake ranged from 69 to 134 kg ha⁻¹. Nitrogen fixation by pigeonpea was estimated as the difference in total nitrogen uptake between pigeonpea and sorghum and could amount to 69 kg N ha⁻¹ per season, or half the total nitrogen uptake. Fixation by pigeonpea increased with crop duration, but there were differences within each maturity group. The limitations of the methods used for estimating N₂ fixation by pigeonpea are discussed. Submitted as J.A. No. 552 by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT).     

Comparison of isotope techniques and non-nodulating isolines to study the effect of ammonium fertilization on dinitrogen fixation in soybean,Glycine max

Summary Two experiments were carried out with two nodulating and non-nodulating soybean isolines, with three different levels of N as (¹⁵NH₄)₂SO₄ at the equivalent of 0, 25 and 50 kg N/ha. In the first experiment three seeds were sown in each pot and the plants harvested at 35, 55 and 75 days. In the second experiment only one seed was sown per pot and harvested at 75 days.Isotope dilution technique and in certain cases natural isotope variation (¹⁵N) was used to determine directly the origin of nitrogen in the plant, whether from soil, fertilizer or biological N₂-fixation. The use of nodulating and non-nodulating isolines enabled comparison with the classical method of estimating N₂-fixation by difference from total plant N. Results at the 75 day harvest were similar for either method, but at the earlier harvests, particularly at 35 days, the total-N method was inadequate. The isotope method appeared more sensitive while the total-N method suffered from greater variability with correspondingly high standard errors and significant differences.It was found that by the 35 and 55 day harvests hardly any N₂-fixation had taken place, plant nitrogen being almost entirely derived from soil or fertilizer N. Plants in competition used up soil fertilizer N more rapidly, thus stimulating symbiotic nitrogen fixation. When only one plant was grown in each pot it had a greater proportion of N derived from soil or fertilizer, and less N derived from fixation. In general the¹⁵N data showed that only about 25% of the applied fertilizer N was absorbed by the plant.The nodulating isoline absorbed more N than the non-nodulating plants. This suggests a possible synergistic effect of N₂-fixation on N derived from other sources, giving an increase in total-N content of nudulated plants. The N derived from N₂-fixation was scarcely detectable in the roots but appeared to be translocated almost entirely to shoots and pods.With 25 kg N/ha the greater proportion of the nitrogen in the pods was derived from N₂-fixation. Even with 50 kg N/ha the nitrogen in the pods derived from fixation remained high, that being derived from fertilizer being less than 15%. About 80% of the nitrogen in the nodules was due to fixation.In the present experiment the application of 25 kg N/ha appeared sufficient to give maximum N absorption by both isolines. At this level symbiotic fixation by Rhizobium remained high in nodulating plants, while the proportion of total N due to fixation was reduced with 50 kg N/ha.UNDP/IAEA Project BRA 78/006.     

Nitrogen fixation (N-15 dilution) with soybeans under Thai field conditions. IV. Effect of N addition and Bradyrhizobium japonicum inoculation in soils with indigenous B. japonicum populations

The effects of Bradyrhizobium japonicum inoculation and pre-plant additions of N fertilizer on soybean ( Glycine max L. Merrill) yields and levels of N₂ fixation were studied under field conditions at two sites in Thailand. Bacterial inoculants were composed of B. japonicum strains selected for high N₂ fixation levels in Thai soils. Nitrogen fertilizer addition rates used were from 0 to 250 kg N/ha in 50 kg N/ha increments. At the Chiang Mai site in northern Thailand, bacterial inoculation increased nodule weights on plants receiving 100 kg N/ha or less. Increases in nodule parameters due to inoculation were evident at 45 d after planting (DAP) but disappeared by 60 DAP. Addition of N fertilizers decreased the incidence of nodulation and sap ureide contents and decreased the contribution of N₂ fixation to the N content of plants at maturity as measured by N-15 isotope dilution methods. At the Kampang Saen site in central Thailand, bacterial inoculation had significant positive effects on nodule numbers and weights, ARA, sap ureide contents and levels of N₂ fixed as measured by N-15 isotope dilution methods. Addition of N fertilizers at this site also reduced the effectiveness of N₂-fixing symbioses. It was concluded that small additions of N fertilizer added before planting did not significantly decrease N₂ fixation levels, but did have a significant positive effect on plant growth. Larger N additions would reduce N₂ fixation levels in excess of the benefits of adding more N in chemical form.     

Total denitrification and the ratio between N2O and N2 during the growth of spring barley

Summary Total denitrification (N₂O+N₂) and nitrous oxide emission were measured on intact soil cores using the acetylene inhibition technique.Total denitrification from the depth 0–8 cm during the growth period from April to August was 7 kg N/ha from plots supplied with 30 kg N/ha and 19 kg N/ha from plots supplied with 120 kg N/ha. The amounts of precipitation, plant growth, and N application were found to affect the denitrification rate. These factors also affected the ratio (N₂O+N₂)/N₂O, which varied from 1.0 to 7.2. Plant growth and precipitation increased the proportion of N₂ produced, whereas a high nitrate content increased the proportion of N₂O.     

Ecological aspects of nitrogen fixation by Purshia tridentata

Summary This study examines several aspects of nitrogen fixation by Purshia tridentata (Pursh) D.C., a rosaceous shrub widespread in the Central Oregon pumice region, especially as an understory species in Pinus ponderosa and Pinus contorta forests. Acetylene reduction was used to assay nodule activity in both field and greenhouse plants. The maximum rates were observed at 20°C, although summer soil temperatures were frequently around 15°C, at which a much lower rate was observed. Acetylene reduction by excised nodules was linear for 5 h and then slowly declined, finally ceasing after 19 h. Nodule activity declined in water stressed plants, essentially ceasing in plants with xylem pressure potentials below –25 bars.Field studies at five sites revealed that nodule activity began in mid-May or early June when soil temperature at 20 cm increased to above 10°C. Activity began later and remained lower until July 20 in plants located under Pinus contorta, probably because of the cooler temperatures at this site. Nodule activity at all sites was maximum in June and July. In late July, nodule activity declined sharply, corresponding with moisture stress readings in the –25 bar range. Acetylene reduction rates declined sharply during the night; this decline was even more severe late in the season.Only 46 per cent of Purshia plants were nodulated. Several possible explanations for this low nodulation are discussed, but the primary reasons appear to be low soil temperature and unfavorable moisture conditions. Previous speculations that Purshia may contribute significant amounts of nitrogen to the ecosystems in which it occurs are disputed using estimates based on seasonal acetylene reduction rates and a determination of nodule biomass at one site. The estimated nitrogen accretion rate was only 0.057 kg N/ha.yr. re]19760310     

Relation between soybean nodulation byBradyrhizobium japonicum and intensity of infestation with the spider miteTetranychus cucurbitacearum (Saved) under different sowing dates and increasing amounts of nitrogenous fertilizer

Inoculation of soybeans withBradyrhizobium japonicum and also the time of sowing of soybeans were studied under N-fertilization with urea at 0, 96 and 192 kg N/ha. Intensity of infestation with the spider miteTetranychus cucurbitacearum (Saved) was followed over six weeks. Inoculation greatly enhanced nodule numbers and weight, dry weight and N content of 65-day-old plant, seed yield, seed N content, 100-seed weight and efficiency of use of N from urea. Counts of the spider mite were increased by inoculation and N fertilization but decreased sharply under late plantation. Fertilizer N proportionally decreased nodulation, N₂-fixation and productivity of chemical fertilizer. A late sowing date gave the least seed production but the maximum percentage increase in seed yield if inoculated. This indicated a lower efficiency of chemical fertilizer N utilization and the necessity for inoculation.     

Nitrogen fixation by Myrica gale root nodules Massachusetts wetland

Summary Nitrogenase activity was measured by acetylene reduction in excised Myrica gale nodules collected throughout the growing season at two sites associated with a small lake in central Massachusetts. One site was in an open, weakly minerotrophic peatland dominated by M. gale and the other was on the lakeshore. Nitrogenase activity appeared in late May when the leaves unfolded, reached a peak in July when the maximum number of leaves was present, and declined until it was no longer measurable in late October several days after all leaves had fallen. Summer activities were substantially higher at the peatland site than the lakeshore. Maximum activities were 19.8±1.9 and 8.1±0.7 mol/h x g dry weight (x±SE; N=20) at the peatland and lakeshore sites respectively.Nitrogenase activities were very low at 5° C and increased linearly from 10 to 30° C, the highest temperature examined. The maximum soil temperature measured was 20° C, and no significant diurnal fluctuation in activity was detected.Annual nitrogen fixation calculated from the seasonal nitrogenase activity curve was 34 kg N/ha x yr at the peatland site with mean dry weight nodule biomass of 104 kg/ha, and 24 kg N/ha x yr at the lakeshore with 111 kg/ha nodule biomass. These rates of nitrogen fixation are equivalent to 4–5x the amount of nitrogen contained in bulk precipitation and are major components in the nitrogen budgets of the M. gale plants and wetlands in which they grow.     

Nitrogen fixation in soybean as influenced by cultivar and Rhizobium strain

Summary The¹⁵N-substratum labeling technique and other indirect methods were used to compare nitrogen (N₂) fixation in soybean varieties grown in the field in Greece and Romania. Significant variation in the amount (Ndfa) and proportion of N derived from fixation (% Ndfa) was found in different varieties. With 20 kg N/ha applied to soil, N₂ fixed ranged from 22 to 236 kg N/ha in Greece and from 17 to 132 kg N/ha in Romania. In general, varieties or treatments with higher dry matter yield supported greater fixation. Also, varieties with high Ndfa had high % Ndfa andvice versa. Breeding N₂-fixing legumes for high yields at low soil N levels therefore appears to be a reasonable strategy for enhancing N₂ fixation. Heavy applications of inorganic N fertilizer severely depressed N₂ fixation in two out of the three varieties used in Romania. One variety, F 74–412, however, derived slightly higher amounts of N₂ from fixation at 100 kg N/ha rate than when fertilized with 20 kg N/ha. In Greece, Chippewa, Williams and Amsoy-71 inoculated with a Nitragin inoculant fixed similar amounts of N₂ at both 20 and 100 kg N/ha fertilizer rates. However, when Chippewa and Williams were inoculated with amother, locally-isolated Rhizobium strain, N₂ fixation was substantially depressed at the higher N rate.     

Effects of N fertilization on N2 fixation and N balances of soybean grown after lowland rice

We report a study in northern Thailand to examine the effects of fertilizer N, applied both to paddy rice and to a subsequent soybean crop on symbiotic and yield characteristics of soybean and on the differences between inputs of fixed N₂ and the removal of N as harvested product. Treatments were a factorial arrangement of 0, 100 and 300 kg N ha^-1 applied to the rice (designated R0, R100 and R300, respectively), and 0,25 and 50 kg N ha^-1, applied as starter fertilizer to the soybean (S0, S25 and S50, respectively).Nitrogen applied to the rice increased rice yields by up to 74% but proportions recovered by the rice were low (45% [R100] and 14% [R300]). The rice N treatments had only marginal effects on soybean nodulation (up to 17% reduction in early growth) and above-ground dry matter (up to 9% increase). Effects on soybean seed yield and total N₂ fixed were insignificant. Starter N, applied to the soybean at sowing, also marginally reduced nodulation and enhanced above-ground dry matter. Total N₂ fixed was unaffected but seed yield was increased by up to 6%. For all treatments, total above-ground N ranged from 145 to 179 kg ha^-1 with 72 to 85% (122 and 140 kg ha^-1) derived from N₂ fixation. When harvested product consisted of seed only, differences between inputs of fixed N₂ and removals of seed N were close to zero (-10 to+9 kg N ha^-1) with little effect of fertilizer N. The N balances were reduced by an average of 18 kg N ha^-1 when straw was included as harvested product. We concluded that N applied to the rice and to the following soybean was inefficiently used by those crops and had only marginal effects of symbiotic activity of the soybean. Furthermore, the benefit of the N₂ fixing soybean in this system was to slow the decline of, rather than enhance, the N fertility of the soil     

Response to inoculation and N fertilization for increased yield and biological nitrogen fixation of common bean (Phaseolus vulgaris L.)

Common bean (Phaseolus vulgaris L.) is able to fix 20–60 kg N ha^–1 under tropical environments in Brazil, but these amounts are inadequate to meet the N requirement for economically attractive seed yields. When the plant is supplemented with N fertilizer, N₂ fixation by Rhizobium can be suppressed even at low rates of N. Using the ¹⁵N enriched method, two field experiments were conducted to compare the effect of foliar and soil applications of N-urea on N₂ fixation traits and seed yield. All treatments received a similar fertilization including 10 kg N ha^–1 at sowing. Increasing rates of N (10, 30 and 50 kg N ha^–1) were applied for both methods. Foliar application significantly enhanced nodulation, N₂ fixation (acetylene reduction activity) and yield at low N level (10 kg N ha^–1). Foliar nitrogen was less suppressive to nodulation, even at higher N levels, than soil N treatments. In the site where established Rhizobium was in low numbers, inoculation contributed substantially to increased N₂ fixation traits and yield. Both foliar and soil methods inhibited nodulation at high N rates and did not significantly increase bean yield, when comparing low (10 kg N ha^–1) and high (50 kg N ha^–1) rates applied after emergence. In both experiments, up to 30 kg N ha^–1 of biologically fixed N₂ were obtained when low rates of N were applied onto the leaves.