Cotton–cowpea intercropping and its N2 fixation capacity improves yield of a subsequent maize crop under Zimbabwean rain-fed conditions

Intercropping cotton (Gossypium hirsutum L.) and cowpea (Vigna unguiculata (L.) Walp) is one of the ways to improve food security and soil fertility whilst generating cash income of the rural poor. A study was carried out to find out the effect of cotton–cowpea intercropping on cowpea N₂-fixation capacity, nitrogen balance and yield of a subsequent maize crop. Results showed that cowpea suppressed cotton yields but the reduction in yield was compensated for by cowpea grain yield. Cowpea grain yield was significantly different across treatments and the yields were as follows: sole cowpea (1.6 Mg ha⁻¹), 1:1 intercrop (1.1 Mg ha⁻¹), and 2:1 intercrop (0.7 Mg ha⁻¹). Cotton lint yield was also significantly different across treatments and was sole cotton (2.5 Mg ha⁻¹), 1:1 intercrop (0.9 Mg ha⁻¹) and 2:1 intercrop (1.5 Mg ha⁻¹). Intercropping cotton and cowpea increased the productivity with land equivalence ratios (LER) of 1.4 and 1.3 for 1:1 and 2:1 intercrop treatments, respectively. There was an increase in percentage of N fixation (%Ndfa) by cowpea in intercrops as compared to sole crops though the absolute amount fixed (Ndfa) was lower due to reduced plant population. Sole cowpea had %Ndfa of 73%, 1:1 intercrop had 85% and 2:1 intercrop had 77% while Ndfa was 138 kg ha⁻¹ for sole cowpea, 128 kg ha⁻¹ for 1:1 intercrop and 68 kg ha⁻¹ for 2:1 intercrop and these were significantly different. Sole cowpea and the intercrops all showed positive N balances of 92 kg ha⁻¹ for sole cowpea and 1:1 intercrop, and 48 kg ha⁻¹ for 2:1 intercrop. Cowpea fixed N transferred to the companion cotton crop was very low with 1:1 intercrop recording 3.5 kg N ha⁻¹ and 2:1 intercrop recording 0.5 kg N ha⁻¹. Crop residues from intercrops and sole cowpea increased maize yields more than residues from sole cotton. Maize grain yield was, after sole cotton (1.4 Mg ha⁻¹), sole cowpea (4.6 Mg ha⁻¹), 1:1 intercrops (4.4 Mg ha⁻¹) and 2:1 intercrops (3.9 Mg ha⁻¹) and these were significantly different from each other. The LER, crop yields, %N fixation and, N balance and residual fertility showed that cotton–cowpea intercropping could be a potentially productive system that can easily fit into the current smallholder farming systems under rain-fed conditions. The fertilizer equivalency values show that substantial benefits do accrue and effort should be directed at maximizing the dry matter yield of the legume in the intercrop system while maintaining or improving the economic yield of the companion cash crop.     

Evaluation of N2-fixation and nitrogen economy of a maize/cowpea intercrop system using15N dilution methods

Yields of above ground biomass and total N were determined in summer-grown maize and cowpea as sole crops or intercrops, with or without supplementary N fertilizer (25 kg N ha⁻¹, urea) at an irrigated site in Waroona, Western Australia over the period 1982–1985. Good agreement was obtained between estimates of N₂ fixation of sole or intercrop cowpea (1984/85 season) based on the¹⁵N natural abundance and¹⁵N fertilizer dilution techniques, both in the field and in a glasshouse pot study. Field-grown cowpea was estimated to have received 53–69% of its N supply from N₂-fixation, with N₂-fixation onlyslightly affected by intercropping or N fertilizer application. Proportional reliance on N₂-fixation of cowpea in glasshouse culture was lower (36–66%) than in the field study and more affected by applied N. Budgets for N were drawn up for the field intercrops, based on above-ground seed yields, return of crop residues, inputs of fixed N and fertilizer N. No account was taken of possible losses of N through volatilization, denitrification and leaching or gains of N in the soil from root biomass. N₂-fixation was estimated tobe 59 kg N ha⁻¹ in the plots receiving no fertilizer N, and 73 kg N ha⁻¹ in plots receiving 25 kg N ha⁻¹ as urea. Comparable fixation by sole cowpea was higher (87 and 82 kg N ha⁻¹ respectively) but this advantage was outweighed by greater land use efficiency by the intercrop than sole crops.     

Nitrogen-15 balances for urea and neem coated urea applied to lowland rice following two cowpea cropping systems

Little is known about whether the high N losses from inorganic N fertilizers applied to lowland rice (Oryza sativa L.) are affected by the combined use of either legume green manure or residue with N fertilizers. Field experiments were conducted in 1986 and 1987 on an Andaqueptic Haplaquoll in the Philippines to determine the effect of cowpea [Vigna unguiculata (L.) Walp.] cropping systems before rice on the fate and use efficiency of¹⁵N-labeled, urea and neem cake (Azadirachta indica Juss.) coated urea (NCU) applied to the subsequent transplanted lowland rice crop. The pre-rice cropping systems were fallow, cowpea incorporated at the flowering stage as a green manure, and cowpea grown to maturity with subsequent incorporation of residue remaining after grain and pod removal. The incorporated green manure contained 70 and 67 kg N ha⁻¹ in 1986 and 1987, respectively. The incorporated residue contained 54 and 49 kg N ha⁻¹ in 1986 and 1987, respectively. The unrecovered¹⁵N in the¹⁵N balances for 58 kg N ha⁻¹ applied as urea or NCU ranged from 23 to 34% but was not affected by pre-rice cropping system. The partial pressure of ammoniapNH₃, and floodwater (nitrate + nitrite)-N following application of 29 kg N ha⁻¹ as urea or NCU to 0.05-m-deep floodwater at 14 days after transplanting was not affected by pre-rice cropping system. In plots not fertilized with urea or NCU, green manure contributed an extra 12 and 26 kg N ha⁻¹, to mature rice plants in 1986 and 1987, respectively. The corresponding contributions from residue were 19 and 23 kg N ha⁻¹, respectively. Coating urea with 0.2g neem cake per g urea had no effect on loss of urea-N in either year; however, it significantly increased grain yield (0.4 Mg ha⁻¹) and total plant N (11 kg ha⁻¹) in 1987 but not in 1986.     

The response ofAzolla pinnata R. Brown to the split application of phosphorus and the transfer of assimilated phosphorus to flooded rice plants

A field experiment was conducted at the Bangladesh Rice Research Institute, Joydebpur, Dhaka during the late wet season. Basal application of P at both 5 and 10 kg ha⁻¹ significantly increased total biomass production and nitrogen fixation byAzolla pinnata R. Brown (local strain). Addition of both 5 and 10 kg P ha⁻¹ in equal splits at inoculation and at six day intervals thereafter during growth periods of 12, 24 and 36 days increased biomass production and nitrogen fixation by Azolla over that attained with the basal application. Biomass and nitrogen fixation using a split application of 5 kg P ha⁻¹ exceeded that attained with basal application of 10 kg P ha⁻¹ and split application of 10 kg P ha⁻¹ resulted in 0.58, 11.2, and 18.3 t ha⁻¹ more biomass, and 0.47, 18.9, and 18.3 more kg fixed N ha⁻¹ at 12, 24 and 36 days, respectively, than the same amount applied as a basal application. Analyses indicated that the critical level of dry weight P in Azolla for sustained growth was in the range of 0.15–0.17%. Compared with the control, where no P was added, and additional 30 and 36 kg N ha⁻¹ were fixed after 24 and 36 days, respectively, when P was provided at 10 kg ha⁻¹ using a split application. A separate field study showed that flooded rice plants received P from incorporated Azolla with about 28% of the P present in the supplied Azolla being incorporated into the rice plants.     

Nitrogen contribution of cowpea green manure and residue to upland rice

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

Measurement of N2 fixation in 30 cowpea (Vigna unguiculata L. Walp.) genotypes under field conditions in Ghana, using the15N natural abundance technique

In 2005 and 2006, 30 and 15 cowpea genotypes were respectively evaluated for plant growth and symbiotic performance at Manga in Northern Ghana, in order to identify N₂-fixing potential of these cowpea genotypes as source of N for cropping systems. The results showed differences in biomass production by the 30 or 15 cowpea genotypes. In 2005, cultivars Fahari, Mchanganyiko, IT97K-499-39, IT93K-2045-29 and IT84S-2246 produced the most shoot biomass, while Apagbaala, Brown Eye, ITH98-46, Vita 7 and Iron Grey produced the least. Of the 15 genotypes tested in 2006, cv. TVu11424 produced the largest amount of biomass, and ITH98-46, the least. Isotopic analysis of¹⁵N in plant parts also revealed significant differences in δ¹⁵N of the cowpea genotypes studied. As a result, the percent N derived from fixation (% Ndfa) also differed among the cowpea genotypes tested in 2005, with only 5 out of the 30 cultivars obtaining over 50% of their N from symbiotic fixation. Whether expressed as mg N.plant^?1 or kg N.ha^?1, the levels of N₂ fixation by the cowpea genotypes varied considerably during 2005 and 2006, with values of N contribution ranging from 14.1 kg N.ha^?1 by cv. TVu1509 to 157.0 kg N.ha^?1 by IT84S-2246 in 2005. The amounts of N-fixed in 2006 ranged from 16.7 kg N.ha^?1 by cv. ITH98-46 to 171.2 kg N.ha^?1 by TVu11424, clearly indicating genotypic differences in symbiotic N yield. Re-evaluating 15 out of the 30 cowpea genotypes for N₂ fixation in 2006, revealed higher % Ndfa values (>50%) in all (15 cowpea genotypes) relative to those tested in 2005, indicating greater dependence on N₂ fixation for their N nutrition even though, the actual amounts of fixed-N were lower in 2006. This was due, in part, to reduced plant biomass as a result of very late sampling in 2006, close to physiological maturity (72 DAP in 2006 vs. 46 DAP in 2005) when considerable leaf matter was lost. The amount of N-fixed in 2006 can therefore be considered as being under-estimated.     

Exchange of N-gases at the Höglwald Forest – A summary

During 4 years continuous measurements of N-trace gas exchange were carried out at the forest floor-atmosphere interface at the Höglwald Forest that is highly affected by atmospheric N-deposition. The measurements included spruce control, spruce limed and beech sites. Based on these field measurements and on intensive laboratory measurements of N₂-emissions from the soils of the beech and spruce control sites, a total balance of N-gas emissions was calculated. NO₂-deposition was in a range of –1.6 –2.9 kg N ha^–1 yr^–1 and no huge differences between the different sites could be demonstrated. In contrast to NO₂-deposition, NO- and N₂O-emissions showed a huge variability among the different sites. NO emissions were highest at the spruce control site (6.4–9.1 kg N ha^–1 yr^–1), lowest at the beech site (2.3–3.5 kg N ha^–1 yr^–1) and intermediate at the limed spruce site (3.4–5.4 kg N ha^–1 yr^–1). With regard to N₂O-emissions, the following ranking between the sites was found: beech (1.6–6.6 kg N ha^–1 yr^–1) >> spruce limed (0.7–4.0 kg N ha^–1 yr^–1) > spruce control (0.4–3.1 kg N ha^–1 yr^–1). Average N-trace gas emissions (NO, NO₂, N₂O) for the years 1994–1997 were 6.8 kg N ha^–1 yr^–1 at the spruce control site, 3.6 kg N ha^–1 yr^–1 at the limed spruce site and 4.5 kg N ha^–1 yr^–1 at the beech site. Considering N₂-losses, which were significantly higher at the beech (12.4 kg N ha^–1 yr^–1) than at the spruce control site (7.2 kg N ha^–1 yr^–1), the magnitude of total gaseous N losses, i.e. N₂-N + NO-N + NO₂-N + N₂O-N, could be calculated for the first time for a forest ecosystem. Total gaseous N-losses were 14.0 kg N ha^–1 yr^–1 at the spruce control site and 15.5 kg N ha^–1 yr^–1 at the beech site, respectively. In view of the huge interannual variability of N-trace gas fluxes and the pronounced site differences in N-gas emissions it is concluded that more research is needed in order to fully understand patterns of microbial N-cycling and N-gas production/emission in forest ecosystems and mechanisms of reactions of forest ecosystems to the ecological stress factor of atmospheric N-input.     

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.     

Phosphorus fertilization and tillage effect on dinitrogen fixation in soybeans

Summary Tillage has been shown to affect the uptake of phosphorus (P) and yield of soybeans, [Glycine max (L.) Merr.], but there is little information concerning the effects of P fertilization on nitrogen (N₂) fixation in soybeans under no-tillage. Two field experiments were conducted in 1980 and 1981 to determine the effects of soil P on N₂ fixation under no-tillage and to study the interaction of P fertilization and tillage of N₂ fixation, nutrient uptake, and yield of soybeans. In Exp. I, P was applied in 1977 at five rates up to 384 kg P ha⁻¹ and the effects of residual soil P were evaluated in 1980 and 1981 under no-tillage management. Nitrogen fixation rates, as measured by acetylene reduction assay, were significantly affected by soil P in Exp. I, but the assay proved to be a poor technique for estimating total plant N in these tests. Acetylene reduction rates and plant P increased rapidly as soil P increased from 2 to 20 mg kg⁻¹, with little additional increase above 20 mg P kg⁻¹. In Exp. II, rates (0, 32, 64, and 128 kg P ha⁻¹) and time (fall, spring and fall plus spring) of P application were compared under conventional tillage and no tillage. However, plant P increased with increasing levels of applied P. Applied P had no affect on acetylene reduction rates but rates were greater for no-tillage than conventional tillage at the V9 and R5 stages of growth in 1981. Plant uptake of P was more efficient under no-tillage than under conventional tillage in 1980 and 1981. Application of 64 kg P ha⁻¹ under no-tillage resulted in equivalent plant P levels as the 128 kg P ha⁻¹ applied under conventional tillage.     

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.     

Nitrogen fixation and nitrogen balance studies in Rhizobium-VA mycorrhiza inoculated legume-grass association by15N isotope dilution technique under a field condition

A mixed pasture comprising of buffel grass and a legume siratro was studied under field condition for a two-year period to know the fodder yield increase, nitrogen fixation and nitrogen balance with and without the inoculation of VA mycorrhiza to grass and Rhizobium to legume component.¹⁵N dilution technique was followed using labelled ammonium sulphate. The data showed that during the first year of the above study combined inoculation of VA mycorrhiza and Rhizobium to grass and legume respectively significantly increased the total dry matter (DM) (23,900 kg ha^–1 yr^–1) and total N content (308 kg ha^–1 yr^–1) of the mixed pasture over the uninoculated mixture. However, the above increase due to combined inoculation was maximum during second year with respect to DM yield (28,200 kg ha^–1 yr^–1), but the total N harvested through grass-legume mixture was comparatively lower than the first year (297 kg ha^–1 yr^–1). The amount of biologically fixed N was highest in the first year (79 kg ha^–1 yr^–1) and showed a very drastic reduction at the end of second year (39 kg ha^–1 yr^–1). A positive nitrogen balance was observed in the grass-legume mixture irrespective of inoculation of VA mycorrhiza and/or Rhizobium.     

Fertilizer nitrogen budgets of two doses of Na15NO3 dressings split-applied to winter wheat in microplots on a loam soil

In a field experiment performed in microplots, winter wheat was fertilized at two different total N dressings (135 and 180 kg ha^–1) split-applied as Na¹⁵NO₃ in three equal applications at tillering, stem elongation, and flag leaf.No significant differences were found in the percentage recovery values for the entire plant at the three split applications between the two N dressings. The total percentage recovery of fertilizer N by the plant was high and practically equal at both fertilization levels (76.65% and 75.84% for 135 and 180 kg N ha^–1, respectively); crop yields were also similar. In contrast, gaseous losses calculated after drawing up the balance sheet were, in absolute values, higher for the tillering and stem elongation split applications when using the 180 kg N ha^–1 dressing (7.67 and 4.84 kg N ha^–1, respectively) than for the 135 kg N ha^–1 dressing (3.45 and 1.26 kg N ha^–1, respectively). They were found to be zero at flag leaf at both fertilization levels. The amount of applied fertilizer N did not influence the amount of N taken up from the soil which was about 143 kg ha^–1.     

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.     

Establishment of phosphate-solubilizing strains of Azotobacter chroococcum in the rhizosphere and their effect on wheat cultivars under green house conditions

A pot experiment was conducted in the green house to investigate the establishment of phosphate solubilizing strains of Azotobacter chroococcum, including soil isolates and their mutants, in the rhizosphere and their effect on growth parameters and root biomass of three genetically divergent wheat cultivars (Triticum aestivum L.). Five fertilizer treatments were performed: Control, 90 kg N ha^—1, 90 kg N + 60 kg P₂O₅ ha^—1, 120 kg N ha^—1 and 120 kg N + 60 kg P₂O₅ ha^—1. Phosphate solubilizing and phytohormone producing parent soil isolates and mutant strains of A. chroococcum were isolated and selected by an enrichment method. In vitro phosphate solubilization and growth hormone production by mutant strains was increased compared with soil isolates. Seed inoculation of wheat varieties with P solubilizing and phytohormone producing A. chroococcum showed better response compared with controls. Mutant strains of A. chroococcum showed higher increase in grain (12.6%) and straw (11.4%) yield over control and their survival (12—14%) in the rhizosphere as compared to their parent soil isolate (P4). Mutant strain M37 performed better in all three varieties in terms of increase in grain yield (14.0%) and root biomass (11.4%) over control.     

Effect of varying periods of inoculation ofAzolla pinnata R. Brown on its growth,nitrogen fixation and response to rice crop

Summary Inoculation of water fernAzolla pinnata R. Brown (Bangkok isolate) at the rate of 500kg fresh weight ha⁻¹ in rice fields at weekly intervals after planting in addition to 30 kg N ha⁻¹ as urea showed a decrease in its growth and N₂-fixation with delay in application. Use of Azolla up to 3 weeks after planting (WAP) during wet and 4 WAP during dry season produced significantly more grain yield than 30 kg N ha⁻¹, whereas its application upto one WAP produced more grain yield than 60 kg N ha⁻¹. Grain yield with Azolla applied at the time of planting was similar to that of 60 kg N treatment during the wet season. Higher grain yields in zero and one WAP Azolla treatments resulted due to increase in both number of panicles m⁻² and number of grains/panicle while the subsequent Azolla inoculations increased grain yield mainly by producing more number of grains/panicle. Dry matter and total N yields at maturity of rice crop were more with Azolla application upto 3 WAP during wet and 2 WAP during dry season while the reduction in sterility (%) was observed upto one WAP over 30 kg N ha⁻¹ during both seasons. Number of tillers m⁻² and dry matter production at maximum tillering and flowering were more than 30 kg N ha⁻¹ with the use of Azolla upto one WAP. Increased grain N yield was observed with the use of Azolla upto 4 WAP during two seasons whereas straw N yield increased upto one WAP during wet and 2 WAP during dry season.     

Influence of Azolla management on the growth,yield of rice and soil fertility

A field experiment conducted at Central Rice Research Institute, Cuttack, during three successive seasons showed that with the 120-day-duration variety Ratna two dual crops ofAzolla pinnata R. Brown (Bangkok isolate) could be achieved 25 and 50 days after transplanting (DAT) by inoculating 2.0 t ha⁻¹ of fresh Azolla 10 and 30 DAT respectively. One basal crop of Azolla could also be grown using the same inoculum 20 days before transplanting (DBT) in fallow rice fields. The three crops of Azolla grown—once before transplanting and twice after transplanting—gave an average total biomass of 38–63 and 43–64 t ha⁻¹ fresh Azolla containing 64–90 and 76–94 kg N ha⁻¹ respectively in the square and rectangular spacings. Two crops of Azolla grown only as a dual crop, on the other hand, gave 26–39 and 29–41 t ha⁻¹ fresh Azolla which contained 44–61 and 43–59 kg N ha⁻¹ respectively. Growth and yield of rice were significantly higher in Azolla basal plus Azolla dual twice incorporated treatments than in the Azolla dual twice incorporation, Azolla basal plus 30 kg N ha⁻¹ urea and 60 kg N ha⁻¹ urea treatments. Azolla basal plus 30 kg N ha⁻¹ urea and 60 kg N ha⁻¹ urea showed similar yields but Azolla dual twice incorporation was significantly lower than those. The different spacing with same plant populations did not affect growth and yield significantly, whereas Azolla growth during dual cropping was 8.3 and 64% more in the rectangular spacing than in the square spacing in Azolla basal plus Azolla dual twice incorporation and Azolla dual twice incorporation treatments.     

The response of field grownPhaseolus vulgaris to Rhizobium inoculation and the quantification of N2 fixation using15N

Summary A field experiment was performed to assess the effects of Rhizobium inoculation and nitrogen fertilizer (100 kg N ha^–1) on four cultivars of Phaseolus beans; Carioca, Negro Argel, Venezuela 350 and Rio Tibagi. In the inoculated treatment 2.5 kg N ha^–1 of¹⁵N labelled fertilizer was added in order to apply the isotope dilution technique to quantify the contribution of N₂ fixation to the nutrition of these cultivars.Nodulation of all cultivars in the uninoculated treatments was poor, but the cultivars Carioca and Negro Argel were well nodulated when inoculated. Even when inoculated, nodulation of the cultivars Venezuela 350 and Rio Tibagi was poor and these cultivars showed little response to inoculation in terms of nitrogen accumulation or grain yield. The estimates of the contribution of N₂ fixation estimated using the isotope dilution technique, for the Carioca and Negro Argel cultivars, amounted to 31.7 and 18.4 kg N ha^–1 respectively. These two cultivars produced 991 and 883 kg ha^–1 of grain, respectively, when inoculated and 663 and 620 kg ha^–1 with the addition of 100 kg N ha^–1 of N fertilizer. The response to nitrogen was particularly poor due to high leaching losses in the very sandy soil at the experimental site.The Venezuela 350 and Rio Tibagi cultivars only responded to N fertilizer and not to inoculation with Rhizobium which stresses the great importance of selecting plant cultivars for nitrogen fixation in the field.     

Influence of pruning management on P and N distribution and use efficiency by N2 fixing and non-N2 fixing trees used in alley cropping systems

Pruning of hedgerow trees is an important management practice for the successful establishment of an alley cropping system. Although pruning affects biomass production, only meager evidence of this management on distribution of nutrients among the different plant organs after tree regrowth is available. This study examined the effect of pruning on the distribution and use efficiency of N and P in a N₂ fixing leguminous tree species, Gliricidia sepium, and two non-N₂ fixing leguminous tree species, Senna siamea and S. spectabilis, grown in a field on an Alfisol (low in P) at Fashola (Guinea Savanna Zone), Southwestern Nigeria. Four P rates, 0, 20, 40 and 80 kg P ha^–1 as single superphosphate were used and management treatments included pruned versus unpruned plants. The ¹⁵N isotope dilution technique was used to measure N₂ fixation in G. sepium. Partitioning of total P among different plant organs was influenced by plant species and pruning management, but was not affected by P application rates. The distribution of total P in the various plant organs followed that of dry matter yield while N partitioning had a different pattern. Pruned plants distributed about 118% more total P to branches and had a higher physiological P use efficiency (PPUE) than unpruned plants. Leaves were the biggest sink for total N and N allocation in the other plant organs was influenced by plant species and pruning management, G. sepium had relatively more of its total N and P partitioned into roots (about double that of the non-N₂ fixing trees) but had a lower PPUE. Unpruned and pruned G. sepium derived 35 and 54% respectively of their total N from atmospheric N2, with about 54% of the fixed N2 being allocated to leaves and roots. Results showed that N and P pools turned over in the branches during plant regrowth after pruning but the causative factors associated with this phenomenon were not clear.     

Rice yields as influenced by Azolla N2 fixation and urea N-fertilization

Application of 0, 30, 60, 90 and 120 kg N ha^–1 of urea (U) in split doses with (and without)Azolla pinnata, R. Brown was studied for three consecutive seasons under planted field condition. Fresh weight (FW), acetylene reduction activity (ARA) and N yield of Azolla were found to be maximum 14 days after inoculation (DAI). Among the different treatments, maximum Azolla growth was recorded in no N control. The FW, ARA and N yield of Azolla were inhibited increasingly with the increase in N levels. Irrespective of season, FW and N yield of Azolla were inhibited only a small extent with 90 kg N ha^–1 U, beyond which the inhibition was pronounced. ARA was inhibited only slightly up to 60 kg N ha^–1 of U. Grain yield and crop N uptake of rice increased significantly up to 90 kg N ha^–1 of U (alone or in combination with Azolla) in the dry seasons (variety IR 36) and up to 60 kg N ha^–1 U in the wet season (variety CR 1018).     

Denitrification and total N losses from an irrigated sandy-clay loam under maize–wheat cropping system

Denitrification and total N losses were quantified from an irrigated field cropped to maize and wheat, each receiving urea at 100 kg N ha^-1. During the maize growing season (60 days), the denitrification loss measured directly by acetylene inhibition-soil cover method amounted 2.72 kg N ha^-1 whereas total N loss measured by ¹⁵N balance was 39 kg ha^-1. Most (87%) of the denitrification loss under maize occurred during the first two irrigation cycles. During the wheat growing season (150 days), the denitrification loss directly measured by acetylene inhibition-soil cover and acetylene inhibition-soil core methods was 1.14 and 3.39 kg N ha^-1, respectively in contrast to 33 kg N ha^-1 loss measured by ¹⁵N balance. Most (70-88%) of the denitrification loss under wheat occurred during the first three irrigation cycles. Soil moisture and NO ₃ ^- -N were the major factors limiting denitrification under both crops. Higher N losses measured by ¹⁵N balance than C₂H₂ inhibition method were perhaps due to underestimation of denitrification by C₂H₂ inhibition method and losses other than denitrification, most probably NH₃ volatilization.