Nitrogen fertilisation reduces grass-induced N2 fixation of tree seedlings from semi-arid savannas

Aims

Coexistence of trees and grasses in nutrient-poor arid savannas may result in competition for soil N. While grasses may be more effective than woody plants in acquiring N from the soil, some leguminous woody species rely on N₂ fixation. We assessed the role of N₂ fixation in the N-budget of Acacia mellifera seedlings by varying N supply and grass competition.

Methods

The contribution of N₂ fixation to the N-budget of Acacia mellifera seedlings with varying N supply and grass competition was determined by measuring growth, nutrient concentrations, and ¹⁵N values.

Results

Tree seedlings were 4-fold taller and had 20-fold more biomass in the absence of grass. Tree foliar δ¹⁵N was lower with (?0.25?±?0.2‰, n?=?9) than without grasses (5.2?±?0.1‰, n?=?64). The contribution of N₂-fixation to the N budget decreased with increasing N supply. Greater reliance on N₂-fixation by trees in the presence of grasses did not result in greater biomass accumulation or tissue [N] relative to tree seedlings grown without grass competition. Tree seedlings competing with grass had significantly more negative δ¹³C (?29.5?±?0.6‰) than seedlings without grass competition (?28.8‰?±?0.5‰).

Conclusions

Induction of N₂-fixation by grass may have resulted from competition for nutrients. N₂-fixation enables tree seedlings to compensate for limited soil N and survive grass competition at a critical and vulnerable developmental stage of germination and establishment.     

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.     

Symbiotic nitrogen fixation in eight Acacia senegal provenances in dryland clays of the Blue Nile Sudan estimated by the 15N natural abundance method

The symbiotic biological N₂fixation by Acacia senegal was estimated using the ¹⁵N natural abundance (δ ¹⁵N) procedure on eight provenances collected from different environments and soil types grown in a clay soil in the Blue Nile region, Sudan. Balanites aegyptiaca (a non-legume) was used as a non-N₂-fixing reference plant to allow ¹⁵N-based estimates of the proportion of the Acacia N derived from atmospheric N₂ (N_dfa) to be calculated. Results show variation in leaf δ ¹⁵N between A. senegal and the reference plant and among years. The relative δ ¹⁵N values (‰) were higher in B. aegyptiaca than in the N₂-fixing acacia provenances. Provenances originally collected from clay soils fixed little N in the first year, but the amount fixed increased as the trees aged. All provenances showed a decrease in δ ¹⁵N with age. The N_dfa varied between 24% (Mazmoom provenance) and 61% (Rahad provenance) 4 years after planting. There was no significant difference in δ ¹⁵N between provenance groups based on soil type or rainfall at original growing site. The amount of N_dfa increased significantly with age in all provenances. The above-ground contribution of fixed N to foliage growth in a 4-year-old A. senegal was highest in the Rahad sand–soil provenance (46.7 kg N ha⁻¹) and lowest in the Mazmoom clay-soil provenance (28.7 kg N ha⁻¹). Our study represents the first use of the δ ¹⁵N method for estimating the N input by A. senegal to the clay plain soils of the gum belt in the Sudan.     

Natural15N abundance as a method of estimating the contribution of biologically fixed nitrogen to N2-fixing systems: Potential for non-legumes

The¹⁵N abundance of plants usually closely reflects the¹⁵N abundance of their major immediate N source(s); plant-available soil N in the case of non-N₂-fixing plants and atmospheric N₂ in the case of N₂ fixing plants. The¹⁵N abundance values of these sources are usually sufficiently different from each other that a significant and systematic difference in the¹⁵N abundance between the two kinds of plants can be detected. This difference provides the basis for the natural¹⁵N abundance method of estimating the relative contribution of atmospheric N₂ to N₂-fixing plants growing in natural and agricultural settings. The natural¹⁵N abundance method has certain advantages over more conventional methods, particularly in natural ecosystems, since disturbance of the system is not required and the measurements may be made on samples dried in the field. This method has been tested mainly with legumes in agricultural settings. The tests have demonstrated the validity of this method of arriving at semi-quantitative estimates of biological N₂-fixation in these settings. More limited tests and applications have been made for legumes in natural ecosystems. An understanding of the limits and utility of this method in these systems is beginning to emerge. Examples of systematic measurements of differences in¹⁵N abundance between non-legume N₂-fixing systems and neighbouring non-fixing systems are more unusual. In principle, application of the method to estimate N₂-fixation by nodulated non-legumes, using the natural¹⁵N abundance method, is as feasible as estimating N₂-fixation by legumes. Most of the studies involving N₂-fixing non-legumes are with this type of system (e.g., Ceanothus, Chamabatia, Eleagnus, Alnus, Myrica, and so forth). Resuls of these studies are described. Applicability for associative N₂-fixation is an empirical question, the answer to which probably depends upon the degree to which fixed N goes predominantly to the plant rather than to the soil N pool. The natural¹⁵N abundance method is probably not well suited to assessing the contribution of N₂-fixation by free-living microorganisms in their natural habitat, particularly soil microorganisms.This work was supported in part by subcontracts under grants from the US National Science Foundation (DEB79-21971 and BSR821618)     

An exotic Australian <Emphasis Type="Italic">Acacia</Emphasis> fixes more N than a coexisting indigenous <Emphasis Type="Italic">Acacia</Emphasis> in a South African riparian zone

Acacia mearnsii is an introduced Australian acacia in South Africa and has invaded more than 2.5 million ha, primarily establishing in rangeland and riparian areas. Because acacias have the capability to fix N, A. mearnsii invasions may fundamentally change N dynamics in invaded systems. This study compares biological N₂-fixation in the alien invasive A. mearnsii and the native A. caffra growing in a grassland riparian zone in the Komati Gorge Reserve, Mpumalanga, South Africa. A ¹⁵N natural abundance field survey suggested that both mature alien and native acacias fix N under current conditions in the riparian zone. Significantly depleted δ¹⁵N was observed in both acacias relative to reference species, although variation in δ¹⁵N was not correlated with N concentrations. Calculated contributions of N₂-fixation (%Ndfa) suggest that alien acacias fix significantly more of their N than native acacias (~75 ± 5% SE and 53 ± 9% SE, respectively). There was a larger variation in δ¹⁵N and %Ndfa in the native acacia, suggesting relatively high plasticity in its N₂-fixation contributions. This plasticity was interpreted as a facultative N₂-fixation strategy for the native acacia, while the N₂-fixation strategy of the alien acacia remained unclear. Our results emphasize the importance of potentially elevated N inputs through N₂-fixation by invasive legumes in invaded landscapes. Furthermore, they suggest that N₂-fixation by invasive acacias may not respond to fine-scale patchiness in soil N in the same manner as native acacias, making them potential contributors to N excess in Southern Africa.     

Symbiotic N2-fixation by the cover crop Pueraria phaseoloides as influenced by litter mineralization

The perennial legume Pueraria phaseoloides is widely used as a cover crop in rubber and oil palm plantations. However, very little knowledge exists on the effect of litter mineralization from P. phaseoloides on its symbiotic N₂-fixation. The contribution from symbiotic N₂-fixation (Ndfa) and litter N (Ndfl) to total plant N in P. phaseoloides was determined in a pot experiment using a ¹⁵N cross-labeling technique. For determination of N₂-fixation the non-fixing plant Axonopus compressus was used as a reference. The experiment was carried out in a growth chamber during 9 weeks with a sandy soil and 4 rates of ground litter (C/N=16,2.8% N). P. phaseoloides plants supplied with the highest amount of litter produced 26% more dry matter and fixed 23% more N than plants grown in soil with no litter application, but the percentage of Ndfa decreased slightly, but significantly, from 87 to 84%. The litter N uptake was directly proportional to the rate of application and constituted 10% of total plant N at the highest application rate. Additionally, a positive correlation was found between litter N uptake and the amount of fixed N₂. The total recovery of litter N in plants averaged 26% at harvest (shoot + root) and was not affected by the quantity added. A parallel incubation experiment also showed that, as an average of all litter levels, 26% of the litter N was present in the inorganic N pool. The amounts of fertilizer and soil N taken up by plants decreased with litter application, probably due to microbial immobilization and denitrification. It is concluded that, within the litter levels studied, litter mineralization will result in a higher amount of N₂-fixed by P. phaseoloides.     

Symbiotic N2-fixation in alpine tundra: ecosystem input and variation in fixation rates among communities

Annual inputs of symbiotic N₂-fixation associated with 3 species of alpine Trifolium were estimated in four alpine communities differing in resource supplies. We hypothesized that fixation rates would vary according to the degree of N, P, and water limitation of production, with the higher rates of fixation in N limited communities (dry meadow, moist meadow) and lower rates in P and water limited communities (wet meadow, fellfield). To estimate N₂-fixation rates, natural abundance of N isotopes (¹⁵N) were measured in field collected Trifolium and reference plants and in Trifolium plants grown in N-free medium in a growth chamber. All three Trifolium species relied on a large proportion of atmospherically-fixed N₂ to meet their N requirements, ranging from 70 to 100%. There were no apparent differences in the proportion of plant N derived from fixation among the communities, but differences in the contribution of the Trifolium species to community cover resulted in a wide range of annual N inputs from fixation, from 127 mg m^–2 year^–1 in wet meadows to 810 mg m^–2 year^–1 in fellfields. Annual spatially integrated input of symbiotic N₂-fixation to Niwot Ridge, Colorado was estimated at 490 mg m^–2 year^–1 (5 kg ha^–1 year^–1), which is relatively high in the context of estimates of net N mineralization and N deposition.     

Interactions between nitrate uptake and N2 fixation in white clover

Summary White clover (Trifolium repens L.) plants grown in pots and supplied with the same concentration x days of¹⁵N labelled nitrate, but in contrasting patterns and doses had similar N concentrations but differed in the proportions devived from N₂ fixation and nitrate. N₂-fixation and nodule dry weight responded rapidly (2–3 days) to changes in nitrate availability. Plants exposed frequently to small doses of nitrate took up more nitrate (and hence relied less on N₂-fixation) and had greater dry weights and shoot: root ratios than those exposed to larger doses less often. In mixed ryegrass (Lolium perenne L.)/clover communities clover's ability to either successfully compete for nitrate or fix N₂ gave it consistently higher N concentrations than grass whether they were given high or low nitrate nutrient. This higher N concentration was accompanied by greater dry weights than grass in the low nitrate swards but not where high levels of nitrate were applied.     

Different sensitivity of Phragmites australis and Glyceria maxima to high availability of ammonium-N

The ability to cope with NH₄⁺-N was studied in the littoral helophytes Phragmites australis and Glyceria maxima, species commonly occupying fertile habitats rich in NH₄⁺ and often used in artificial wetlands. In the present study, Glyceria growth rate was reduced by 16% at 179 μM NH₄⁺-N, and the biomass production was reduced by 47% at 3700 μM NH₄⁺-N compared to NO₃⁻-N. Similar responses were not found in Phragmites. The amounts (mg g⁻¹ dry wt) of starch and total non-structural carbohydrates (TNC) in rhizomes were significantly lower in NH₄⁺ (8.9; 12.2 starch; 20.1; 41.9 TNC) compared to NO₃⁻ treated plants (28.0; 15.6 starch; 58.5; 56.3 TNC) in Phragmites and Glyceria, respectively. In addition, Glyceria showed lower amounts (mg g⁻¹ dry wt) of soluble sugars, TNC, K⁺, and Mg²⁺ in roots under NH₄⁺ (5.6; 14.3; 20.6; 1.9) compared to NO₃⁻ nutrition (11.6; 19.9; 37.9; 2.9, for soluble sugars, TNC, K⁺, and Mg²⁺, respectively), while root internal levels of NH₄⁺ and Ca²⁺ (0.29; 4.6 mg g⁻¹ dry wt, mean of both treatments) were only slightly affected. In Phragmites, no changes in soluble sugars, TNC, Ca²⁺, K⁺, and Mg²⁺ contents of roots (7.3; 14.9; 5.1; 17.3; 2.6 mg g⁻¹ dry wt, means of both treatments) were found in response to treatments. The results, therefore, indicate a more pronounced tolerance towards high NH₄⁺ supply in Phragmites compared to Glyceria, although the former may be susceptible to starch exhaustion in NH₄⁺-N nutrition. In contrast, Glyceria's ability to colonize fertile habitats rich in NH₄⁺ is probably related to the avoidance strategy due to shallow rooting or to the previously described ability to cope with high NH₄⁺ levels when P availability is high and NO₃⁻ is also provided.     

Estimation of symbiotically fixed nitrogen in field grown soybeans: An application of natural15N/14N abundance and a low level15N-tracer technique

Summary Plants from agricultural and natural upland ecosystem were investigated for¹⁵N content to evaluate the role of symbiotic N₂-fixation in the nitrogen nutrition of soybean. Increased yields and lower δ¹⁵N values of nodulating soybeansvs, non-nodulating isolines gave semi-quantitative estimates of N₂ fixation. A fairly large discrepancy was found between estimations by δ¹⁵N and by N yield at 0 kg N/ha of fertilizer. More precise estimates were made by following changes in plant δ¹⁵N when fertilizer δ¹⁵N was varied near¹⁵N natural abundance level. Clearcut linear relationships between δ¹⁵N values of whole plants and of fertilizer were obtained at 30 kg N/ha of fertilizer for three kinds of soils. In experimental field plots, nodulating soybeans obtained 13±1% of their nitrogen from fertilizer, 66±8% from N₂ fixation and 21±10% from soil nitrogen in Andosol brown soil; 30%, 16% and 54% in Andosol black soil; 7%, 77% and 16% in Alluvial soil, respectively. These values for N₂ fixation coincided with each corresponding estimation by N yield method. Other results include: 1)¹⁵N content in upland soils and plants was variable, and may reflect differences in the mode of mineralization of soil organics, and 2) nitrogen isotopic discrimination during fertilizer uptake (δ¹⁵N of plant minus fertilizer) ranged from −2.2 to +4.9‰ at 0–30 kg N/ha of fertilizer, depending on soil type and plant species. The proposed method can accurately and relatively simply establish the importance of symbiotic nitrogen fixation for soybeans growing in agricultural settings.     

Discrimination against 15N among recombinant inbred lines of Phaseolus vulgaris L. contrasting in phosphorus use efficiency for nitrogen fixation

Although isotopic discrimination processes during nitrogen (N) transformations influence the outcome of ¹⁵N based quantification of N₂ fixation in legumes, little attention has been given to the effects of genotypic variability and environmental constraints such as phosphorus (P) deficiency, on discrimination against ¹⁵N during N₂ fixation. In this study, six Phaseolus vulgaris recombinant inbred lines (RILs), i.e. RILs 115, 104, 34 (P deficiency tolerant) and 147, 83, 70 (P deficiency sensitive), were inoculated with Rhizobium tropici CIAT899, and hydroaeroponically grown with P-sufficient (250 μmol P plant⁻¹ week⁻¹) versus P-deficient (75 μmol P plant⁻¹ week⁻¹) supply. Two harvests were done at 15 (before nodule functioning) and 42 (flowering stage) days after transplanting. Nodulation, plant biomass, P and N contents, and the ratios of ¹⁵N over total N content (¹⁵N/Nt) for shoots, roots and nodules were determined. The results showed lower ¹⁵N/Nt in shoots than in roots, both being much lower than in nodules. P deficiency caused a larger decrease in ¹⁵N/Nt in shoots (−0.18%) than in nodules (−0.11%) for all of the genotypes, and the decrease in shoots was greatest for RILs 34 (−0.33%) and 104 (−0.25%). Nodule ¹⁵N/Nt was significantly related to both the quantity of N₂ fixed (R² = 0.96***) and the P content of nodules (R² = 0.66*). We conclude that the discrimination against ¹⁵N in the legume N₂-fixing symbiosis of common bean with R. tropici CIAT899 is affected by P nutrition and plant genotype, and that the ¹⁵N/Nt in nodules may be used to screen for genotypic variation in P use efficiency for N₂ fixation.     

Luminostat operation: a tool to maximize microalgae photosynthetic efficiency in photobioreactors during the daily light cycle?

The luminostat regime has been proposed as a way to maximize light absorption and thus to increase the microalgae photosynthetic efficiency within photobioreactors. In this study, simulated outdoor light conditions were applied to a lab-scale photobioreactor in order to evaluate the luminostat control under varying light conditions. The photon flux density leaving the reactor (PFD_out) was varied from 4 to 20 μmol photons m⁻² s⁻¹and the productivity and photosynthetic efficiency of Chlorella sorokiniana were assessed.Maximal volumetric productivity (1.22 g kg⁻¹ d⁻¹) and biomass yield on PAR photons (400-700 nm) absorbed (1.27 g mol⁻¹) were found when PFD_out was maintained between 4 and 6 μmol photons m⁻² s⁻¹. The resultant photosynthetic efficiency was comparable to that already reported in a chemostat-controlled reactor. A strict luminostat regime could not be maintained under varying light conditions. Further modifications to the luminostat control are required before application under outdoor conditions.     

Synthesis and characterization of gold(III) complexes with alkyldiamine ligands

A series of gold(III) metalacycle of five-, six- and seven-membered ring was prepared by reacting Auric acid (HAuCl₄ · 3H₂O) with 1 equiv. unsubstituted ethylenediamine (en), propylene diamine (pn) and butylenediamine (bn) ligands and with some N-mono-substituted as well as N,N′-disubstituted ethylenediamine ligands. The general formula of these complexes is [Au(alkyldiamine)Cl₂]Cl. These complexes are characterized by melting point and elemental analysis, while structural analysis was done by spectroscopic techniques such as UV-Vis, Far-IR, IR spectroscopy, ¹H and ¹³C solution as well as ¹³C and ¹⁵ N solid-state NMR. The solid-state ¹⁵ N NMR shows that the chemical shift difference between free and bound ligand decreases as bn > pn > en, indicating stronger Au-N bond for bn complex compared to pn and en. UV-Vis shows relative stability of the Au(III) complexes of unsubstituted ethylenediamine with respect to N,N′-di-substituted ethylenediamine. Far-IR data show the six-membered metalacycle gold(III) alkanediamine complexes to be more stable. Spectroscopic data are evaluated by comparisons with calculated data of the built and optimized structure by gaussian03 at the RB3LYP level with LanL2DZ bases set.     

Nitrogen nutrition of Canna indica: Effects of ammonium versus nitrate on growth, biomass allocation, photosynthesis, nitrate reductase activity and N uptake rates

The effects of inorganic nitrogen (N) source (NH₄⁺, NO₃⁻ or both) on growth, biomass allocation, photosynthesis, N uptake rate, nitrate reductase activity and mineral composition of Canna indica were studied in hydroponic culture. The relative growth rates (0.05-0.06 g g⁻¹ d⁻¹), biomass allocation and plant morphology of C. indica were indifferent to N nutrition. However, NH₄⁺ fed plants had higher concentrations of N in the tissues, lower concentrations of mineral cations and higher contents of chlorophylls in the leaves compared to NO₃⁻ fed plants suggesting a slight advantage of NH₄⁺ nutrition. The NO₃⁻ fed plants had lower light-saturated rates of photosynthesis (22.5 μmol m⁻² s⁻¹) than NH₄⁺ and NH₄⁺/NO₃⁻ fed plants (24.4-25.6 μmol m⁻² s⁻¹) when expressed per unit leaf area, but similar rates when expressed on a chlorophyll basis. Maximum uptake rates (V_max) of NO₃⁻ did not differ between treatments (24-35 μmol N g⁻¹ root DW h⁻¹), but V_max for NH₄⁺ was highest in NH₄⁺ fed plants (81 μmol N g⁻¹ root DW h⁻¹), intermediate in the NH₄NO₃ fed plants (52 μmol N g⁻¹ root DW h⁻¹), and lowest in the NO₃⁻ fed plants (28 μmol N g⁻¹ root DW h⁻¹). Nitrate reductase activity (NRA) was highest in leaves and was induced by NO₃⁻ in the culture solutions corresponding to the pattern seen in fast growing terrestrial species. Plants fed with only NO₃⁻ had high NRA (22 and 8 μmol NO₂⁻ g⁻¹ DW h⁻¹ in leaves and roots, respectively) whereas NRA in NH₄⁺ fed plants was close to zero. Plants supplied with both forms of N had intermediate NRA suggesting that C. indica takes up and assimilate NO₃⁻ in the presence of NH₄⁺. Our results show that C. indica is relatively indifferent to inorganic N source, which together with its high growth rate contributes to explain the occurrence of this species in flooded wetland soils as well as on terrestrial soils. Furthermore, it is concluded that C. indica is suitable for use in different types of constructed wetlands.     

Characterization and antimicrobial activity of water-soluble N-(4-carboxybutyroyl) chitosans against some plant pathogenic bacteria and fungi

Water-soluble N-(4-carboxybutyroyl) chitosan derivatives with different degrees of substitution (DS) were synthesized to enhance the antimicrobial activity of chitosan molecule against plant pathogens. Chitosan in a solution of 2% aqueous acetic acid-methanol (1:1, v/v) was reacted with 0.1, 0.3, 0.6 and 1 mol of glutaric anhydride to give N-(4-carboxybutyroyl) chitosans at DS of 0.10, 0.25, 0.48 and 0.53, respectively. The chemical structures and DS were characterized by ¹H and ¹³C NMR spectroscopy, which showed that the acylate reaction took place at the N-position of chitosan. The synthesized derivatives were more soluble than the native chitosan in water and in dilute aqueous acetic acid and sodium hydroxide solutions. The antimicrobial activity was in vitro investigated against the most economic plant pathogenic bacteria of Agrobacterium tumefaciens and Erwinia carotovora and fungi of Botrytis cinerea, Pythium debaryanum and Rhizoctonia solani. The antimicrobial activity of N-(4-carboxybutyroyl) chitosans was strengthened than the un-modified chitosan with the increase of the DS. A compound of DS 0.53 was the most active one with minimum inhibitory concentration (MIC) of 725 and 800 mg/L against E. carotovora and A. tumefaciens, respectively and also in mycelial growth inhibiation against B. cinerea (EC₅₀ = 899 mg/L), P. debaryanum (EC₅₀ = 467 mg/L) and R. solani (EC₅₀ = 1413 mg/L).     

Nitrogen fixation in Faidherbia albida,Acacia raddiana,Acacia senegal and Acacia seyal estimated using the 15N isotope dilution technique

A pot experiment was conducted in a greenhouse using the ¹⁵N isotope dilution method and two reference plants, Parkia biglobosa and Tamarindus indica to estimate nitrogen fixed in four Acacia species: A raddiana, A. senegal, A. seyal and Faidherbia albida (synonym Acacia albida). For the reference plants, the ¹⁵N enrichments in leaves, stems and roots were similar. With the fixing plants, leaves and stems had similar ¹⁵N enrichments; they were higher than the ¹⁵N enrichment of roots. The amounts of nitrogen fixed at 5 months after planting were similar using either reference plant. Estimates of the percentage of N derived from fixation (%Ndfa) for the above ground parts, in contrast to %Ndfa in roots, were similar to those for the whole plant. However, none of the individual plant parts estimated accurately total N fixed in the whole plant, and excluding the roots resulted in at least 30% underestimation of the amounts of N fixed. Between species, differences in N₂ fixation were observed, both for %Ndfa and total N fixed. For %Ndfa, the best were A. seyal (average, 63%) and A. raddiana (average, 62%), being at least twice the %Ndfa in A. senegal and F. albida. Because of its very high N content, A. seyal was clearly the best in total N fixed, fixing 1.62 g N plant^–1 compared to an average of 0.48 g N plant^–1 for the other Acacia species. Our results show the wide variability existing between Acacia species in terms of both %Ndfa and total N fixed: A. seyal was classified as having a high N₂ fixing potential (NFP) while the other Acacia species had a low NFP.     

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.     

Spectroscopic characterization of the polycrystalline copper(I) di-n-butyldithiophosphate cluster - Cu8[S2P(O-n-Bu)2]6(μ8-S): Solid-state P CP/MAS and static Cu NMR studies

A polycrystalline copper(I) O,O′-di-n-butyldithiophosphate cluster compound, Cu₈[S₂P(O-n-Bu)₂]₆(μ₈-S), was synthesized and characterized by ³¹P CP/MAS NMR at 8.46 T and static ⁶⁵Cu NMR at multiple magnetic field strengths (7.05, 9.4 and 14.1 T). The principal values of the ³¹P chemical shift tensor and the ⁶⁵Cu chemical shift and quadrupolar splitting parameters are presented. The data are compared to those for the analogous octa-nuclear cluster compounds [Cu₈(S₂P(OEt)₂)₆(μ₈-S)], [Cu₈(S₂P(O-n-Pr)₂)₆(μ₈-S)] and [Cu₈(S₂P(O-i-Bu)₂)₆(μ₈-S)]. The transverse relaxation time constant, T₂, for the [Cu₈(S₂P(O-n-Bu)₂)₆(μ₈-S)] cluster compound was found to be 160 ± 8 μs. Possible intra-molecular motions in the cluster structures in terms of size and branching of the hydrocarbon chains are discussed as reasons for the different ⁶⁵Cu NMR responses of the systems.     

Management of biological N2 fixation in alley cropping systems: Estimation and contribution to N balance

Alley cropping is being widely tested in the tropics for its potential to sustain adequate food production with low agricultural inputs, while conserving the resource base. Fast growth and N yield of most trees used as hedgerows in alley cropping is due greatly to their ability to fix N₂ symbiotically with Rhizobium. Measurements of biological N₂ fixation (BNF) in alley cropping systems show that some tree species such as Leucaena leucocephala, Gliricidia sepium and Acacia mangium can derive between 100 and 300 kg N ha^-1 yr^–1 from atmospheric N₂, while species such as Faidherbia albida and Acacia senegal might fix less than 20 kg N ha^-1 yr^-1. Other tree species such as Senna siamea and S. spectabilis are also used in alley cropping, although they do not nodulate and therefore do not fix N₂. The long-term evaluation of the potential or actual amounts of N₂ fixed in trees however, poses problems that are associated with their perennial nature and massive size, the great difficulty in obtaining representative samples and applying reliable methodologies for measuring N₂ fixed. Strategies for obtaining representative samples (as against the whole tree or destructive plant sampling), the application of ¹⁵N procedures and the selection criteria for appropriate reference plants have been discussed.Little is known about the effect of environmental factors and management practices such as tree cutting or pruning and residue management on BNF and eventually their N contribution in alley cropping. Data using the ¹⁵N labelling techniques have indicated that up to 50% or more of the tree's N may be below ground after pruning. In this case, quantification of N₂ fixed that disregards roots, nodules and crowns would result in serious errors and the amount of N₂ fixed may be largely underestimated. Large quantities of N are harvested with hedgerow prunings (>300 kg N ha^-1 yr^-1) but N contribution to crops is commonly in the range of 40–70 kg N ha^-1 season. This represents about 30% of N applied as prunings; however, N recoveries as low as 5–10% have been reported. The low N recovery in maize (Zea mays) is partly caused by lack of synchronization between the hedgerow trees N release and the associated food crop N demand. The N not taken up by the associated crop can be immobilized in soil organic matter or assimilated by the hedgerow trees and thus remain in the system. This N can also be lost from the system through denitrification, volatilization or is leached beyond the rooting zone. Below ground contribution (from root turnover and nodule decay) to an associated food crop in alley cropping is estimated at about 25–102 kg N ha^-1 season^-1. Timing and severity of pruning may allow for some management of underground transfer of fixed N₂ to associated crops. However many aspects of root dynamics in alley cropping systems are poorly understood. Current research projects based on ¹⁵N labelling techniques or ¹⁵N natural abundance measurements are outlined. These would lead to estimates of N₂ fixation and N saving resulting from the management of N₂ fixation in alley cropping systems.