Effect of nitrogen fertilization and cropping system of the reference crop on estimation of N2 fixation by vetch using15N methodology

This study was conducted to examine the effects of varying N rates and cropping systems (mixedversus pure stand) on the suitability of oats (Avena sativa L.) for estimating N₂ fixed in sequentially harvested vetch (Vicia sativa L.) over two growing seasons (1984–85 and 1985–86). The N rates were, 20 and 100 kg N ha^–1 in 1984–85 and 15 and 60 kg N ha^–1 in 1985–86. In the 1984–85 season, vetch at maturity derived 76 and 63% N from fixation at the high and low N rates respectively. The corresponding values for the second season were 66 and 42%. Except in the 1985–86 season when some significantly higher values of % N₂ fixed were estimated by using the reference crop grown at the higher (A-value approach) than at the lower N rate (isotope-dilution approach), both approaches resulted in similar measurements of N₂ fixed. In the 1984–85 season, similar values of N₂ fixed were obtained using either the pure or mixed stand oats reference crops. Although in the 1985–86 season, the mixed reference crop occasionally estimated lower % N₂ fixed than pure oats, total N₂ fixed estimates were always similar (P<0.05). Thus, in general, N fertilization and cropping system of the reference crop did not significantly influence estimates of N₂ fixation.     

Dinitrogen fixation in white clover grown in pure stand and mixture with ryegrass estimated by the immobilized 15N isotope dilution method

Dinitrogen fixation in white clover (Trifolium repens L.) grown in pure stand and mixture with perennial ryegrass (Lolium perenne L.) was determined in the field using ¹⁵N isotope dilution and harvest of the shoots. The apparent transfer of clover N to perennial ryegrass was simultaneously assessed. The soil was labelled either by immobilizing ¹⁵N in organic matter prior to establishment of the sward or by using the conventional labelling procedure in which ¹⁵N fertilizer is added after sward establishment. Immobilization of ¹⁵N in the soil organic matter has not previously been used in studies of N₂ fixation in grass/clover pastures. However, this approach was a successful means of labelling, since the ¹⁵N enrichment only declined at a very slow rate during the experiment. After the second production year only 10–16% of the applied ¹⁵N was recovered in the harvested herbage. The two labelling methods gave, nonetheless, a similar estimate of the percentage of clover N derived from N₂ fixation. In pure stand clover, 75–94% of the N was derived from N₂ fixation and in the mixture 85–97%. The dry matter yield of the clover in mixture as percentage of total dry matter yield was relatively high and increased from 59% in the first to 65% in the second production year. The average daily N₂ fixation rate in the mixture-grown clover varied from less than 0.5 kg N ha⁻¹ day⁻¹ in autumn to more than 2.6 kg N ha⁻¹ day⁻¹ in June. For clover in pure stand the average N₂ fixation rate was greater and varied between 0.5 and 3.3 kg N ha⁻¹ day⁻¹, but with the same seasonal pattern as for clover in mixture. The amount of N fixed in the mixture was 23, 187 and 177 kg N ha⁻¹ in the seeding, first and second production year, respectively, whereas pure stand clover fixed 28, 262 and 211 kg N ha⁻¹ in the three years. The apparent transfer of clover N to grass was negligible in the seeding year, but clover N deposited in the rhizosphere or released by turnover of stolons, roots and nodules, contributed 19 and 28 kg N ha⁻¹ to the grass in the first and second production year, respectively. This revised version was published online in June 2006 with corrections to the Cover Date.     

Assessment of genetic variability for N2 fixation between and within provenances of Leucaena leucocephala and Acacia albida estimated by 15N labelling techniques

Nitrogen fixed in 13 provenances of Acacia albida and 11 isolines of Leucaena leucocephala inoculated with effective Rhizobium strains was measured by ¹⁵N techniques and the total N difference method. In the test soil, on the average, L. leucocephala derived about 65% of its total N from atmospheric N₂ fixation compared to about 20% by A. albida. Significant differences in the percentage of N derived from atmospheric N₂ (% Ndfa) occurred, between provenances or isolines within species. The % Ndfa ranged from 37 to 74% within L. leucocephala and from 6 to 37 within A. albida; (equivalent to 20–50 mg N plant^–1 and 4–37 mg N plant^–1 for the two species over three months, respectively) and was correlated with the nodule mass (r=0.91). The time course of N₂ fixation of three selected provenances (low, intermediate and good fixers) was followed at 12 weekly intervals over a 36 week period. The % Ndfa of all provenances and isolines increased with time; and except for one of the L. leucocephala provenances, % Ndfa was similar within species at the 36 weeks harvest. There was a significant correlation between % Ndfa and the amount of N₂ fixed (r=0.96). Significant interactions occurred between provenances and N treatments and often growth of uninoculated but N fertilized plants was less variable than for inoculated unfertilized plants.     

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.     

The15N methodology in estimating N2 fixation by vetch and pea grown in pure stand or in mixtures with oat

Cereal-legume mixtures are frequently the best management decision for forage production instead of growing crops in pure stands. Nitrogen fertilization of cereal-legume mixtures is questionable since combined nitrogen could depress N₂ fixation by legumes. The objectives of this study were (1) to examine the effect of N fertilization on N₂ fixation by vetch and field peas in pure and in mixed stands with oats, and (2) to examine if there is any transfer of N from legumes to associated cereals. The field experiment was conducted for two growing seasons. The treatments were pure stands of vetch, pea and oats, and the mixtures of the two legumes with oats at the seeding ratios 90:10 and 75:25, fertilized with labelled¹⁵N at the rates of 15 and 90 kg N ha⁻¹. Nitrogen fertilization of 90 kg N ha⁻¹ suppressed N₂ fixation in both legumes grown in pure and in mixed stands. Crops grown in mixtures in many instances had lower atom %¹⁵N excess. Whether this was due to high N₂ fixation in the case of legume and transfer in the case of oat or the differences were due to practical problems of the¹⁵N technique is not clearly shown by the results, so based on the literature the aspect is discussed as well as the precautions which should be considered in using the¹⁵N technique in such studies.     

Field measurement of symbiotic nitrogen fixation in an established lucerne ley using15N and an acetylene reduction method

Summary Lucerne is an important forage legume in the south and south-east of Sweden on well-drained soils. However, data is lacking on the apparent amount of nitrogen derived through N₂ fixation by field-grown lucerne. This report provides basic information on the subject. The experiment was performed in a lucerne ley grown 40 km north of Uppsala. The input of nitrogen through fixation to the above-ground plant material of an established lucerne (Medicago sativa L.) ley was estimate by¹⁵N methodology during two successive years. The amount of fixed N was 242 kg N ha^–1 in 1982 and 319 kg N ha^–1 in 1983. The proportion of N derived from the atmosphere (%Ndfa) was 70% and 80% for the two years respectively. The first harvest in both years contained a lower proportion fixed N. Both N₂ fixation and dry matter production were enhanced during the second year, particularly in the first harvest. The Ndfa was 61% in the first harvest in 1982, compared to 72% Ndfa during the same period in 1983. This demonstrates the strong influence of environment on both dry matter production and N₂ fixation capacity of the lucerne.In addition anin situ acetylene reduction assay was used in 1982 to measure the seasonal distribution of the N₂ fixation and in 1983 to study the effect of soil moisture on the N₂ fixation process. The seasonal pattern showed great dependence on physiological development and harvest pattern of the lucerne ley. The maximum rate of N₂ fixation occurred at the bud or early flower stage of growth and was followed by a rapid decline as flowering proceeded. After harvest the nitrogenase activity markedly decreased and remained low during at least two weeks until regrowth of new shoots began. Irrigation doubled the nitrogenase activity of the lucerne in late summer 1983, when soil moisture content in the top soil was near wilting point. No changes in nitrogenase activity did occur in response to watering earlier during the summer, when the soil matric potential was around –0.30 MPa.     

Nitrogen fixation by white clover in pastures grazed by dairy cows: Temporal variation and effects of nitrogen fertilization

Effects of rate of nitrogen (N) fertilizer and stocking rate on production and N₂ fixation by white clover (Trifolium repens L.) grown with perennial ryegrass (Lolium perenne L.) were determined over 5 years in farmlets near Hamilton, New Zealand. Three farmlets carried 3.3 dairy cows ha^–1 and received urea at 0, 200 or 400 kg N ha^–1 yr^–1 in 8–10 split applications. A fourth farmlet received 400 kg N ha^–1 yr^–1 and had 4.4 cows ha^–1.There was large variation in annual clover production and total N₂ fixation, which in the 0 N treatment ranged from 9 to 20% clover content in pasture and from 79 to 212 kg N fixed ha^–1 yr^–1. Despite this variation, total pasture production in the 0 N treatment remained at 75–85% of that in the 400 N treatments in all years, due in part to the moderating effect of carry-over of fixed N between years.Fertilizer N application decreased the average proportion of clover N derived from N₂ fixation (P_N; estimated by ¹⁵N dilution) from 77% in the 0 N treatment to 43–48% in the 400 N treatments. The corresponding average total N₂ fixation decreased from 154 kg N ha^–1 yr^–1 to 39–53 kg N ha^–1 yr^–1. This includes N₂ fixation in clover tissue below grazing height estimated at 70% of N₂ fixation in above grazing height tissue, based on associated measurements, and confirmed by field N balance calculations. Effects of N fertilizer on clover growth and N₂ fixation were greatest in spring and summer. In autumn, the 200 N treatment grew more clover than the 0 N treatment and N₂ fixation was the same. This was attributed to more severe grazing during summer in the 0 N treatment, resulting in higher surface soil temperatures and a deleterious effect on clover stolons.In the 400 N treatments, a 33% increase in cow stocking rate tended to decrease P_N from 48 to 43% due to more N cycling in excreta, but resulted in up to 2-fold more clover dry matter and N₂ fixation because lower pasture mass reduced grass competition, particularly during spring.     

Grain yield, symbiotic N2 fixation and interspecific competition for inorganic N in pea-barley intercrops

The effect of mixed intercropping of field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.), compared to monocrop cultivation, on the yield and crop-N dynamics was studied in a 4-yr field experiment using ¹⁵N-isotope dilution technique. Crops were grown with or without the supply of 5 g ¹⁵N-labeled N m^-2. The effect of intercropping on the dry matter and N yields, competition for inorganic N among the intercrop components, symbiotic fixation in pea and N transfer from pea to barley were determined. As an average of four years the grain yields were similar in monocropped pea, monocropped and fertilized barley and the intercrop without N fertilizer supply. Nitrogen fertilization did not influence the intercrop yield, but decreased the proportion of pea in the yield. Relative yield totals (RYT) showed that the environmental sources for plant growth were used from 12 to 31% more efficiently by the intercrop than by the monocrops, and N fertilization decreased RYT-values. Intercrop yields were less stable than monocrop barley yields, but more stable than the yield of monocropped pea. Barley competed strongly for soil and fertilizer N in the intercrop, and was up to 30 times more competitive than pea for inorganic N. Consequently, barley obtained a more than proportionate share of the inorganic N in the intercrop. At maturity the total recovery of fertilizer N was not significantly different between crops, averaging 65% of the supplied N. The fertilizer N recovered in pea constituted only 9% of total fertilizer-N recovery in the intercrop. The amount of symbiotic N₂ fixation in the intercrop was less than expected from its composition and the fixation in monocrop. This indicates that the competition from barley had a negative effect on the fixation, perhaps via shading. At maturity, the average amount of N₂ fixation was 17.7 g N m^-2 in the monocrop and 5.1 g N m^-2 in the intercropped pea. A higher proportion of total N in pea was derived from N₂ fixation in the intercrop than in the monocrop, on average 82% and 62%, respectively. The ¹⁵N enrichment of intercropped barley tended to be slightly lower than of monocropped barley, although not significantly. Consequently, there was no evidence for pea N being transferred to barley. The intercropping advantage in the pea-barley intercrop is mainly due to the complimentary use of soil inorganic and atmospheric N sources by the intercrop components, resulting in reduced competition for inorganic N, rather than a facilitative effect, in which symbiotically fixed N₂ is made available to barley.Abbreviations MC monocrop - IC intercrop - PMC pea monocrop - BMC barley monocrop - PIC pea in intercrop - BIC barley in intercrop     

Seasonal accumulation and partitioning of nitrogen by lentil

Although wheat (Triticum aestivum L.) is the dominant crop of the semi-arid plains of Canada and the western United States, lentil (Lens culinaris Medik.) has become an important alternative crop. Sources and seasonal accumulation of N must be understood in order to identify parameters that can lead to increased N₂-fixing activity and yield. Inoculated lentil was grown in a sandy-loam soil at an irrigated site in Saskatchewan, Canada. Wheat was used as the reference crop to estimate N₂ fixation by the A-value approach. Lentil and wheat received 10 and 100 kg N ha⁻¹ of ammonium nitrate, respectively. Crops were harvested six times during the growing season and plant components analyzed. During the first 71 days after planting the wheat had a higher daily dry matter and N accumulation compared to lentil. However, during the latter part of the growing season, daily dry matter and N accumulation were greater for lentil. The maximum total N accumulation for lentil at maturity was 149 kg ha⁻¹. In contrast, wheat had a maximum N accumulation of 98 kg ha⁻¹ in the Feekes 11.1 stage, or 86 days after planting. The maximum daily rates of N accumulation were 3.82 kg N ha⁻¹ day⁻¹ for lentil and 2.21 kg N ha⁻¹ day⁻¹ for wheat. The percentage of N derived from N₂ fixation (% Ndfa) ranged from 0 at the first harvest to 92 % at final harvest. Generative plant components had higher values for % Ndfa than the vegetative components which indicates that N in the reproductive plant parts was derived largely from current N₂ fixation and lentil continued to fix N until the end of the pod fill stage. At final harvest, lentil had derived 129 kg N ha⁻¹ from N₂ fixation with maximum N₂-fixing activity (4.4 kg N ha⁻¹ day⁻¹) occurring during the early stages of pod fill. Higher maximum rates of N₂-fixing activity than net N accumulation (3.82 kg N ha⁻¹ day⁻¹) may have been caused by N losses like volatilization. In addition, lentil provided a net N contribution to the soil of 59 kg ha⁻¹ following the removal of the grain.     

Dinitrogen-fixation by three neotropical agroforestry tree species under semi-controlled field conditions

Cultivating dinitrogen-fixing legume trees with crops in agroforestry is a relatively common N management practice in the Neotropics. The objective of this study was to assess the N₂ fixation potential of three important Neotropical agroforestry tree species, Erythrina poeppigiana, Erythrina fusca, and Inga edulis, under semi-controlled field conditions. The study was conducted in the humid tropical climate of the Caribbean coastal plain of Costa Rica. In 2002, seedlings of I. edulis and Vochysia guatemalensis were planted in one-meter-deep open-ended plastic cylinders buried in soil within hedgerows of the same species. Overall tree spacing was 1 × 4 m to simulate a typical alley-cropping design. The ¹⁵N was applied as (NH₄)₂SO₄ at 10% ¹⁵N atom excess 15 days after planting at the rate of 20 kg [N] ha⁻¹. In 2003, seedlings of E. poeppigiana, E. fusca, and V. guatemalensis were planted in the same field using the existing cylinders. The ¹⁵N application was repeated at the rate of 20 kg [N] ha⁻¹ 15 days after planting and 10 kg [N] ha⁻¹ was added three months after planting. Trees were harvested 9 months after planting in both years. The ¹⁵N content of leaves, branches, stems, and roots was determined by mass spectrometry. The percentage of atmospheric N fixed out of total N (%N_f) was calculated based on ¹⁵N atom excess in leaves or total biomass. The difference between the two calculation methods was insignificant for all species. Sixty percent of I. edulis trees fixed N₂; %N_f was 57% for the N₂-fixing trees. Biomass production and N yield were similar in N₂-fixing and non-N₂-fixing I. edulis. No obvious cause was found for why not all I. edulis trees fixed N₂. All E. poeppigiana and E. fusca trees fixed N₂; %N_f was ca. 59% and 64%, respectively. These data were extrapolated to typical agroforestry systems using published data on N recycling by the studied species. Inga edulis may recycle ca. 100 kg ha⁻¹ a⁻¹ of N fixed from atmosphere to soil if only 60% of trees fix N₂, E. poeppigiana 60–160 kg ha⁻¹ a⁻¹, and E. fusca ca. 80 kg ha⁻¹ a⁻¹.     

Biological nitrogen fixation in four Gliricidia sepium genotypes

A field experiment was conducted at the Coconut Research Institute in Sri Lanka to examine the biological nitrogen fixation potential of three Gliricidia sepium provenances (OFI 14/84, 17/84, 12/86) and a local landrace (designated LL), using the ¹⁵N isotope dilution method. There was marked variation in dry matter, total N, nodulation and ¹⁵N enrichment among the Gliricidia genotypes (=0.001), and the dry matter yield of Cassia siamea (syn. Senna siamea), the non-N₂ fixing reference plant was higher than for G. sepium. In all cases, highest biomass and total N were aboveground, with roots on average accounting for < 20 % of total dry matter or the total N in plants. Atom % ¹⁵N excess was highest in C. siamea, and lowest in OFI 14/84. Although atom % ¹⁵N excess was lower in Gliricidia leaves than in the other organs (all of which had similar ¹⁵N enrichments), values of % N derived from atmospheric N₂ fixation (% Ndfa) calculated for any individual organ or for the whole plant were similar. This was because the relative distribution of ¹⁵N in the different parts of the fixing plant followed the same trend as in the reference plant. There were significant differences (p=0.01) in N₂ fixation between the Gliricidia genotypes. The values ranged from 17.8 g N tree^-1 (equivalent to 86 kg N ha^-1 at 5000 trees ha^-1) in OFI 12/86 to 61.7g N tree^-1 (equivalent to 309 kg N ha^-1) in OFI 14/84. Although most of this variability was due to differences in both % Ndfa and total N in plant, amount of N fixed was more correlated with total N in plant (r=0.935) than with % Ndfa (r=0.707). On average, % Ndfa in all three G. sepium provenances and LL was about 55 % or 34.6 g N tree^-1 (equivalent to some 166 kg N ha^-1) in the 9 months within which N₂ fixation was measured. This represents a substantial contribution of N into the soil-plant system.     

15N-Determined effect of inoculation with N2 fixing bacteria on nitrogen assimilation in Western Canadian wheats

Summary A two-year field study was undertaken using¹⁵N isotope techniques to differentiate between stimulation of N uptake and N₂ fixation in Western Canadian cultivars of spring wheat (Triticum aestivum L. emend Thell) and durum (T. turgidum L. emend Bowden) in response to inoculation with N₂-fixing bacteria. Bacterial inoculation either had no effect or lowered the % N derived from the fertilizer and the fertilizer use efficiency. Despite the depression of fertilizer uptake, inoculants did not alter the relative uptake from soil and fertilizer-N pools indicating that bacterial inoculation did not alter rooting patterns. Nitrogen-15 isotope dilution indicated that N₂ fixation did occur. In 1984, % plant N derived from the atmosphere (% Ndfa) due to inoculation with Bacillus C-11-25 averaged 23.9% while that withAzospirillum brasilense ATCC 29729 (Cd) averaged 15.5%. In 1985, higher soil N levels reduced these values by approximately one-half. Cultivar x inoculant interactions, while significant, were not consistent across years. However, these interactions did not affect cultivars ‘Cadet’ and ‘Rescue’. In agreement with previous results, ‘Cadet’ performed well with all inoculants in both years while ‘Rescue’ performed poorly. Among 1984 treatments, the N increament in inoculated plants was positively correlated with % Ndfa but no such correlation existed in 1985. N₂ fixation averaged over all cultivars and strains was 17.9 and 6.7 kg N fixed ha⁻¹ in 1984 and 1985, respectively. Highest rates of N₂ fixation were estimated at 52.4 kg N ha⁻¹ for ‘Cadet’ in 1984 and 31.3 kg N ha⁻¹ for ‘Owens’ in 1985, both inoculated with Bacillus C-11-25, an isolate from southern Alberta soils. Inoculation with either ofAzospirillum brasilense strain Cd (ATCC29729) or 245 did not result in as consistent or as high N₂ fixation, suggesting that these wheats had not evolved genetic compatability with this exogenous microorganism. These agronomically significant amounts of N₂ fixation occurred under optimally controlled experimental conditions in the field. It is yet to be determined if N₂ fixation would occur in response to bacterial inoculation under dryland conditions commonly occurring in Western Canada. Contribution from Agriculture Canada Research Station, Lethbridge, Alberta, Canada.     

Measurement of N2-fixation in field-grown pigeonpea [Cajanus cajan (L.) Millsp.] using15N-labelled fertilizer

Two experiments were carried out from 1981 to 1983 in Vertisol field at ICRISAT Center, Patancheru, India to measure N₂-fixation of pigeonpea [Cajanus cajan (L.) Millsp.] using the¹⁵N isotope dilution technique. One experiment examined the effect of control of a nodule-eating insect on fixation while another in vestigated the effect of intercroping with cereals on fixation and the residual effect of pigeonpea on a succeeding cereal crop. Although both experiments indicated that at least 88% of the N in pigeonpea was fixed from the atmosphere, one result is considered fortuitous in view of the differential rates of growth of the legume and the control, sorghum [Sorghum bicolor (L.) Moench]. The difference method of calculation in dieated negative fixation and the results emphasized the problem of finding a suitable nonfixing control. In a second experiment, when all plants were confined to a known volume of soil to which¹⁵N fertilizer was added in the field, these problems were overcome, and isotope dilution and difference methods gave similar results of N₂-fixation of about 90%. In intercropped pigeonpea 96% of the total N was derived from the atmosphere. This estimate might be an artifact. There was no evidence of benefit from N fixed by pigeonpea to intercropped sorghum plants. Plant tissue¹⁵N enrichments of cereal crops grown after pigeonpea indicated that the cereal derived some N fixed by the previous pigeonpea. Thus residual benefits to cereals are not only an effect of ‘sparing’ of soil N.     

Nitrogen fertilization effects on dinitrogen fixation as influenced by legume species and proportion in legume-grass mixtures in Uruguay

Grasses grown in mixture with nodulated legumes often are N-limited, but N fertilization may result in reductions of N₂ fixation and legume stands. We studied N-fertilizer effects on N₂ fixation for three binary legume-grass mixtures in Uruguay. Replicated swards of white clover (Trifolium repens L.) (WC), red clover (Trifolium pratense L.) (RC), or birdsfoot trefoil (Lotus corniculatus L.) (BT), each in combination with tall fescue (Festuca arundinacea Schreb) (TF) at two legume proportions were sown in 1983 (Exp. 1) and 1984 (Exp. 2). In the fall of 1984, N treatments at 100 kg ha⁻¹ and controls were randomly assigned to subplots in Exp. 1 (established swards) and in Exp. 2 (at seeding). The soil for both experiments was a fine, montmorillonitic, mesic, Typic Argiudolls. Herbage fixed-N was estimated by ¹⁵N isotope-dilution with pure stands of TF as reference. In both experiments, N fertilization reduced the proportion of legume N derived from air (% Ndfa) and increased herbage yield only during the first 18 to 20 weeks after application. Fertilizer-N reduced annual fixed-N yield from 178 to 148 kg ha⁻¹ in Exp. 1 and from 65 to 29 kg ha⁻¹ in Exp. 2 Fixed-N yield for BT was markedly reduced by N in both experiments (33 to 53%), whereas for the clovers reduction was lesser in Exp. 1 (9 to 13%) than in Exp. 2 (46 to 64%). Negative effects of N on % Ndfa were more evident for the high legume proportion. We conclude that fertilization with 100 kg N ha⁻¹ reduced % Ndfa only for the immediate 18 to 20 weeks after application. Fertilizer-induced reduction of fixed-N yields lasted longer because of a more prolonged depression of legume proportion, especially for BT and for newly seeded swards. Journal Paper no. J.-13327 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, U.S.A. (Project 2281). Supported in part by the Facultad de Agronomía, Montevideo, Uruguay; and the International Atomic Energy Agency, Vienna, Austria (Project URU/5/012). Journal Paper no. J.-13327 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, U.S.A. (Project 2281). Supported in part by the Facultad de Agronomía, Montevideo, Uruguay; and the International Atomic Energy Agency, Vienna, Austria (Project URU/5/012).     

Variability in nitrogen fixation of common bean (Phaseolus vulgaris L.) intercropped with maize

Thirty one selected bean lines were evaluated in the field for ability to support N₂ fixation when intercropped with maize which received 0, 30 and 60 kg N ha^–1 as ammonium sulphate. The amount of fixed N₂ was estimated using the natural variation of ¹⁵N and wheat as the standard non-fixing crop. Nitrogen as low as 15 kg N ha^–1 at sowing suppressed nodule weight and activity (acetylene reduction activity) but not nodule number, suggesting that the main effect of mineral N was on nodule development and function. ¹⁵N data revealed a high potential of the bean genotypes to fix N₂, with the most promising ones averaging between 50–60% of seed N coming from fixation. Bean lines CNF-480, Puebla-152, Mexico-309, Negro Argel, CNF-178, Venezuela-350 and WBR22-3, WBR22-50 and WBR22-55 were ranked as good fixers.     

Symbiotically fixed nitrogen from field- grown white and red clover mixed with ryegrasses at low levels of15N-fertilization

A field study was carried out near Zürich (Switzerland) to determine the yield of symbiotically fixed nitrogen (¹⁵N dilution) from white clover (Trifolium repens L.) grown with perennial ryegrass (Lolium perenne L) and from red clover (Trifolium pratense L.) grown with Italian ryegrass (Lolium multiflorum Lam.). A zero N fertilizer treatment was compared to a 30 kg N/ha per cut regime (90 to 150 kg ha⁻¹ annually). The annual yield of clover N derived from symbiosis averaged 131 kg ha⁻¹ (49 to 227 kg) without N fertilization and 83 kg ha⁻¹ (21 to 173 kg) with 30 kg of fertilizer N ha⁻¹ per cut in the seeding year. Values for the first production year were 308 kg ha⁻¹ (268 to 373 kg) without N fertilization and 232 kg ha⁻¹ (165 to 305 kg) with 30 kg fertilizer N ha⁻¹ per cut. The variation between years was associated mainly with the proportion of clover in the mixtures. Apparent clover-to-grass transfer of fixed N contributed up to 52 kg N ha⁻¹ per year (17 kg N ha⁻¹ on average) to the N yield of the mixtures. Percentage N derived from symbiosis averaged 75% for white and 86% for red clover. These percentages were affected only slightly by supplemental nitrogen, but declined markedly during late summer for white clover. It is concluded that the annual yield of symbiotically fixed N from clover/grass mixtures can be very high, provided that the proportion of clover in the mixtures exceeds 50% of total dry mass yield.     

Seasonal N2 fixation by cool-season pulses based on several15N methods

Summary Accurate estimates of N₂ fixation by legumes are requisite to determine their net contribution of fixed N₂ to the soil N pool. However, estimates of N₂ fixation derived with the traditional¹⁵N methods of isotope dilution and A_N value are costly.Field experiments utilizing¹⁵N-enriched (NH₄)₂SO₄ were conducted to evaluate a modified difference method for determining N₂ fixation by fababean, lentil, Alaska pea, Austrian winter pea, blue lupin and chickpea, and to quantify their net contribution of fixed N₂ to the soil N pool. Spring wheat and non-nodulated chickpea, each fertilized with two N rates, were utilized as non-fixing controls.Estimates of N₂ fixation based on the two control crops were similar. Increasing the N rate to the controls reduced A_N values 32, 18 and 43% respectively in 1981, 1982 and 1983 resulting in greater N₂ fixation estimates. Mean seasonal N₂ fixation by fababean, lentil and Austrian winter pea was near 80 kg N ha^–1, pea and blue lupin near 60 kg N ha^–1, and chickpea less than 10 kg N ha^–1. The net effects of the legume crops on the soil N pool ranged from a 70 kg N ha^–1 input by lentil in 1982, to a removal of 48 kg N ha^–1 by chickpea in 1983.Estimates of N₂ fixation obtained by the proposed modified difference method approximate those derived by the isotope dilution technique, are determined with less cost, and are more reliable than the total plant N procedure.Scientific paper No. 6605. College of Agriculture and Home Economics Research Center, Washington State University, Pullman, WA 99164, U.S.A.     

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.     

Measurements of nitrogen fixation in fababean at different N fertilizer rates using the 15N isotope dilution and ‘A-value’ methods

The ¹⁵N isotope dilution and A-value methods were used to measure biological nitrogen (N₂) fixation in field grown fababean (Vicia faba L.), over a 2-year period. Four N rates, 20, 100, 200 and 400 kg N ha^–1 were examined. The two isotope methods gave similar values of % N derived from the atmosphere (%Ndfa). With 20 kg N ha^–1, %Ndfa in fababean was about 85% in both years. Increasing the N rate to 100 kg N ha^–1 decreased N₂ fixation slightly to 75%. Further reductions in N₂ fixed to 60 and 43% occurred where 200 and 400 kg N ha^–1 were applied, respectively. Thus even higher rates of N than normally applied in farming practice could not completely suppress N₂ fixation in fababean.We also devised one equation for both the isotope dilution and A-value approaches, thereby (i) avoiding the need for different calculations for the ¹⁵N isotope methods, and (ii) showing once again that the isotope dilution and A-value methods are mathematically and conceptually identical.     

Row spacing effects of N2-fixation,N-yield and soil N uptake of intercropped cowpea and maize

In the tropics, cowpea is often intercropped with maize. Little is known about the effect of the intercropped maize on N₂-fixation by cowpea or how intercropping affects nitrogen fertilizer use effiency or soil N-uptake of both crops. Cowpea and maize were grown as a monocrop at row spacings of 40, 50, 60, 80, and 120 cm and intercropped at row spacing of 40, 50, and 60 cm. Plots were fertilized with 50 kg N as (NH₄)₂SO₄; microplots within each plot received the same amount of¹⁵N-depleted (NH₄)₂SO₄. Using the¹⁵N-dilution method, the percentage of N derived from N₂-fixation by cowpea and the recovery of N-fertilizer and soil N-uptake was measured for both crops at 50 and 80 days after planting.Significant differences in yield and total N for cowpea and maize at both harvest periods were dependent on row spacing and cropping systems. Maize grown at the closer row spacing accumulated most of its N during the first 50 days after planting, whereas maize grown at the widest row spacing accumulated a significant portion of its N during the last 30 days before the final harvest, 80 days after planting.Overall, no significant differences in the percentage of N derived from N₂-fixation for monocropped or intercropped cowpea was observed and between 30 and 50% of its N was derived from N₂.At 50 DAP, fertilizer and soil N uptake was dependent on row spacing with maize grown at the narrowest row spacing having a higher fertilizer and soil N recovery than maize grown at wider spacings. At 50 and 80 DAP, intercropped maize/cowpea did not have a higher fertilizer and soil N uptake than monocropped cowpea or maize at the same row spacing. Monocropped maize and cowpea at the same row spacing took up about the same amount of fertilizer or soil N. When intercropped, maize took up twice as much soil and fertilizer N as cowpea. Apparently intercropped cowpea was not able to maintain its yield potential.Whereas significant differences in total N for maize was observed at 50 and 80 DAP, no significant differences in the atom %¹⁴N excess were observed. Therefore, in this study, the atom %¹⁴N excess of the reference crop was yield independent. Furthermore, the similarity in the atom %¹⁴N excess for intercropped and monocropped maize indicated that transfer of N from the legume to the non-legume was small or not detectable.