Molybdenum reserves of seed,and growth and N2 fixation by Phaseolus vulgaris L.

Summary Plants grown from seed with high (1.5–7.3 g Mo seed^-1) and low (0.07–1.4 g Mo seed^-1) Mo contents were grown in the presence and absence of Mo in growth media (perlite) or in a flowing-solution culture, in a controlled environment. Neither the high (1.5 g Mo seed^-1) nor the low (0.1 g Mo seed^-1) Mo content in seed from a small-seeded genotype (BAT 1297) was able to prevent Mo deficiency (reduced shoot, root and nodule dry weight, N₂ fixation and seed production) in growth media without an external supply of Mo, whereas both the high (7.3 g Mo seed^-1) and the low (0.07 g Mo seed^-1) contents in seed were able to prevent Mo deficiency in a large-seeded genotype (Canadian Wonder). Responses to Mo treatment by the Two genotypes were inconsistent between the growth media and solution culture experiments. Seed with a large Mo content (3.5 g Mo seed^-1) from the Canadian Wonder genotype was unable to prevent Mo deficiency (reduced shoot and nodule dry weight and N₂-fixation) in a solution culture without an external source of Mo, whereas both the large (1.7 g Mo seed^-1) and the small (0.13 g Mo seed^-1) contents in seed prevented a deficiency in BAT 1297. Growing plants from seed with a small Mo content, without additional Mo, reduced the seed Mo content by 83–85% and seed production by up to 38% in both genotypes. Changes in seed size and increases in shoot, root and nodule dry weight occurred, but varied with the genotype and growth conditions. These effects were also observed in some cases where plants were grown with additional Mo, demonstrating that the amount of Mo in the seed sown can influence plant nutrition irrespective of the external Mo supply. Nodule dry weight, total N content of shoots and seed production were improved by using seed with a small Mo content (1.64–3.57 g Mo seed^-1) on acid tropical soils in Northern Zambia. Plants of both the large- and small-seeded genotypes grown from seed with a small Mo content (<1.41 g Mo seed^-1) had a smaller nodule weight, accumulated less N and produced less seed. The viability of seed with a small Mo content was lower (germination up to 50% less) than that of seed with a large Mo content.     

Comparative growth and symbiotic performance of four Acacia mangium provenances from Papua New Guinea in response to the supply of phosphorus at various concentrations

Plant growth performance, the P content in root and nodule tissues, and nodulation and N₂-fixing ability were studied in four provenances of Acacia mangium from Papua New Guinea following different levels of P fertilizer application. A. mangium did not seem to need high levels of P for growth and N₂ fixation. The response by this leguminous tree to the P supply varied significantly according to provenance and to P concentrations in the culture solution. The provenances of A. mangium were classified into three types according to their P response: (1) Growth performance, nodulation, and N₂ fixation of plants were stimulated as concentrations of P increased (provenance PH 482); (2) the maximal effect of P on plant growth was found only at P concentrations higher than 500 M (provenance PH 484); and (3) the plant response to P fertilization was low, even with nutrient solutions containing P concentratins higher than 500 M (provenances PH 483 and PH 485). Provenance PH 483 was distinguished by its low nodulating ability. However, this provenance grew well, probably because of its high N₂ fixation efficiency as expressed by specific acetylene reduction activity and its high P content in nodule tissues. Therefore, in certain cases, these two parameters may be useful criteria in selecting leguminous plants for field use. Statistical analyses of the study results showed that the effect of the factor P supply on N₂ fixation efficiency and nodule development was only significant at P concentrations lower than 250 M whereas the effect of the factor plant provenance was significant regardless of the P concentration used. This observation emphasizes the value of provenance screening in the identification of plants for use in a wide range of soil types.     

Growth and Nitrogen Fixation in Dhaincha/Sorghum and Dhaincha/Sunflower Intercropping Systems Using 15Nitrogen and 13Carbon Natural Abundance Techniques

A field experiment on dhaincha, sunflower, and sorghum plants grown in monocropping and intercropping systems was conducted to evaluate growth and nitrogen (N₂) fixation using ¹³carbon (C) and ¹⁵N natural abundance techniques. Intercropping of sesbania/sorghum showed a greater efficiency than monocropping in producing dry matter during the entire growth period, whereas the efficiency of producing dry matter in the sesbania/sunflower intercropping was similar to that in the monocropping system. Moreover, sorghum plants (C4) were more competitive than sesbania (C3) for soil N uptake, whereas sesbania seemed to be more competitive than its associated sunflower (C3). Nitrogen uptake in the mixed stand of sesbania/sorghum was improved as a result of the increase in soil N uptake by the component sorghum and the greater root nodule activity of component sesbania without affecting the amount of N₂ fixed. The Δ ¹³C in plant materials was affected by plant species and the cropping system.     

Effect of encapsulated calcium carbide on dinitrogen,nitrous oxide,methane, and carbon dioxide emissions from flooded rice

Summary The efficiency of N use in flooded rice is usually low, chiefly due to gaseous losses. Emission of CH₄, a gas implicated in global warming, can also be substantial in flooded rice. In a greenhouse study, the nitrification inhibitor encapsulated calcium carbide (a slow-release source of acetylene) was added with 75, 150, and 225 mg of 75 atom % ¹⁵N urea-N to flooded pots containing 18-day-old rice (Oryza sativa L.) plants. Urea treatments without calcium carbide were included as controls. After the application of encapsulated calcium carbide, 3.6 g N₂, 12.4 g N₂O-N, and 3.6 mg CH₄ were emitted per pot in 30 days. Without calcium carbide, 3.0 mg N₂, 22.8 g N₂O-N, and 39.0 mg CH₄ per pot were emitted during the same period. The rate of N added had a positive effect on N₂ and N₂O emissions, but the effect on CH₄ emissions varied with time. Carbon dioxide emissions were lower with encapsulated calcium carbide than without. The use of encapsulated calcium carbide appears effective in eliminating N₂ losses, and in minimizing emissions of the greenhouse gases N₂O and CH₄ in flooded rice.     

Influence of oxygen on denitrification and aerobic respiration in soil

Summary The influence of the partial pressure of oxygen on denitrification and aerobic respiration was investigated at defined P02 values in a mull rendzina soil. The highest denitrification and respiration rates obtained in remoistened, glucose- and nitrate-amended soil were 43 1 N₂0 h^–1g^–1 soil and 130 1 O₂ h^–1g^–1 soil, respectively. At -55 kPa matric water potential, corresponding to 40% water saturation, N₂0 was produced only below P0₂ 40 hPa. The K _m, for O₂ was 3.0 x 10^–6 M. Formation of N₂O and consumption of O₂ occurred simultaneously with half maximum rates at P0₂ 6.7–13.3 hPa. Nitrite accumulated in soil below 40 hPa and increased with decreasing pO₂. The upper threshold for N₂0 formation in amended soil was P0₂ 33–40 hPa (39-47 M O₂).     

Nitrogen fixation and CO2 exchange in soybeans (Glycine max L.) inoculated with mixed cultures of different microorganisms

N₂ fixation, photosynthesis of whole plants and yield increases in soybeans inoculated with mixed cultures of Bradyrhizobium japonicum 110 and Pseudomonas fluorescens 20 or P. fluorescens 21 as well as Glomus mosseae were found in pot experiments in gray forest soil carried out in a growth chamber. The effects of pseudomonads and vesicular-arbuscular (VA) mycorrhizal fungus on these parameters were found to be the same. Dual inoculation of soybeans with mixed cultures of microorganisms stimulated nodulation, nitrogenase activity of nodules and enhanced the amount of biological nitrogen in plants as determined by the ¹⁵N dilution method in comparison to soybeans inoculated with nodule bacteria alone. An increased leaf area in dually infected soybeans was estimated to be the major factor increasing photosynthesis. P. fluorescens and G. mosseae stimulated plant growth, photosynthesis and nodulation probably due to the production of plant growth-promoting substances. Increasing phosphorus fertilizer rates within the range of 5–40 mg P 100 g^-1 1:1 (v/v) soil: sand in a greenhouse experiment led to a subsequent improvement in nodulation, and an enhancement of N₂ fixation and yield in soybeans dually inoculated with B. japonicum 110 and P. fluorescens 21. These indexes were considerably higher in P-treated plants inoculated with mixed bacterial culture than in plants inoculated with nodule bacteria alone.     

Quantification and potential availability of non-symbiotically fixed 15N in soil

Summary Non-symbiotic N₂ fixation was studied under laboratory conditions in two soils from Pakistan (Hafizabad silt loam and Khurrarianwala silt loam) and one from Illinois, USA (Drummer silty clay loam) incubated in a ¹⁵N-enriched atmosphere. N₂ fixation was greatest with the Drummer soil (18–122 g g^–1 soil, depending upon the soil treatment) and lowest with the Khurrarianwala soil (4–81 g g^–1 soil). Fixation was increased by the addition of glucose, a close correlation being observed between the amount of glucose added and the amount of N₂ fixed in the three soils (r = 0.96). Efficiency of N₂ fixation varied with soil type and treatment and was greatest in the presence of added inorganic P. Application of Mo apparently had a negative effect on the amount and efficiency of N₂ fixation in all the soils. The percentage of non-symbiotically fixed ¹⁵N in potentially mineralizable form (NH ₄ ⁺ -N released in soil after a 15-day incubation period under anaerobic conditions) was low (2%–18%, depending upon the soil treatment), although most of the fixed N (up to 90%) was recovered as forms hydrolysable with 6N HCl. Recovery in hydrolysable forms was much greater for the fixed N than for the native soil N, indicating that the former was more available for uptake by plants.     

Influence of mycorrhiza vs. soluble phosphate on growth,nodulation, and N2 fixation (15N) in alfalfa under different levels of water potential

Summary The legume Medicago sativa (+Rhizobium melilott) was grown under controlled conditions to study the interactions between soluble P in soil (four levels), or a mycorrhizal inoculum, and the degree of water potential (four levels) in relation to plant development and N₂ fixation. ¹⁵N-labelled ammonium sulphate was added to each pot for a qualitative estimate of N₂ fixation, in order to rank the effects of the different treatments.Dry-matter yield, nutrient content and nodulation increased with the amount of plant-available P in the soil, and decreased as the water stress increased, for each P-level. The mycorrhizal effect on dry matter, N yield, and on nodulation was little affected by the water potential. Since P uptake was affected by the water content in mycorrhizal plants, additional mechanisms, other than those mediated by P, must be involved in the mycorrhizal activity.There was a positive correlation between N yield and nodulation for the different P levels and the mycorrhizal treatment at all water levels. A high correlation between plant unlabelled N content and atom% ¹⁵N excess was also found for all levels of P. In mycorrhizal plants, however, the correlation between unlabelled N yield and ¹⁵N was lower. This suggests that mycorrhiza supply plants with other N sources in addition to those derived from the improvement on N₂ fixation.     

Dinitrogen (C2H2) fixation in relation to nitrogen fertilization of grey alder [Alnus incana (L.) Moench.] plantations in a peat bog

Summary Nitrogenase activity was measured in young grey alder plantations in a peat bog in central Sweden. The stands were treated in three ways: (1) daily irrigation during the growing season with a complete nutrient solution, including N; (2) application of bark ash or wood ash before planting; and (3) fertilization every second year with solid PK fertilizers. Acetylene reduction assays were performed on (1) detached nodules and attached nodules, either on (2) whole enclosed plants or (3) enclosed nodules. The acetylene reduction rate for the enclosed plants showed a maximum in July when mean values of nearly 80 mol C₂H₄ (g nodule dry matter)^-1 h^-1 were reached. No diurnal patterns were observed. The irrigated stands, with an N supply, showed overall nitrogenase activities that corresponded well with those of the other treatments. Only in the case of temporarily increased soil nutrient concentrations in the irrigated stands did the nitrogenase activity fall considerably. In 6-to 7-year-old intensively managed irrigated stands N₂-fixation was estimated as 85–115 kg N ha^-1 year^-1 which was about 55% of the total N uptake of the trees.     

Phytotron studies to compare nitrogen losses from corn-planted soil by the 15-N balance or direct dinitrogen and nitrous oxide measurements

Summary Containers filled with soil mixed with potassium nitrate highly enriched in ¹⁵N were planted with corn (Zea mays L.) and kept in a phytotron under controlled conditions for 79 days. Soil water content was normally maintained at exactly 60% water-holding capacity (–33 kPa), but it was increased several times to 85% (–5 kPa) for short periods to favour denitrification. The soil headspace was sealed from the phytotron atmosphere and aerated by a continuous stream of air. Nitrous oxide emission was measured by estimating the N₂O concentration differences in the air entering and leaving the containers. Emission of N₂ was estimated by mass spectroscopy from changes in the N₂ composition in the temporarily enclosed soil headspace. Both methods were carefully checked for accuracy by different tests. At specific times during the experiment the distribution of ¹⁵N between plants and soil was determined and a ¹⁵N balance established. Emission of N gases peaked at times of increased water content and reached maxima of 149 and 142 g N pot^–1 day^–1 for N₂O and N₂, respectively. While N losses of 5% ± 2% were indicated by the ¹⁵N balance, only 1.1% ± 0.3% loss from 2.7 g applied N was estimated from the N₂O and N₂ measurements after 79 days. Possible reasons for these differences are discussed.     

Physiological and symbiotic characteristics of Rhizobium fredii isolated from subtropical-tropical soils

Summary Physiological and symbiotic characteristics were identified in Rhizobium fredii isolated from subtropical-tropical soils. The generation times of R. fredii Taiwan isolated-SB 357 and -SB 682 were 1.7 and 2.5 h, respectively. These strains were associated with acid production in yeast-extract mannitol medium. They were able to use hexoses, pentose, sucrose, trehalose and raffinose. Strain SB 357 can resist a high concentration of kanamycin (100 g ml^–1 and penicillin (400 g ml^–1). It can tolerate up to 2.34% NaCl and 1031.3 mosmol kg^–1 (23.4 bars). The growth rate of R. fredii SB 357 under the concentration of approximately 450 mosmol kg^–1 (10.2 bars) was not affected by salinity, but responded to osmotic pressure. Both strains (SB 357 and SB 682) isolated from subtropical-tropical soils were able to form an effective N₂-fixing symbiosis with the US soybean cv Clark lanceolate leaflet.     

Competition among strains of Bradyrhizobium and vesicular-arbuscular mycorrhizae for groundnut (Arachis hypogaea L.) root infection and their effect on plant growth and yield

Summary Strains of Bradyrhizobium influenced root colonization by a species of vesicular-arbuscular mycorrhizae (VAM), and species of VAM influenced root nodulation by strains of Bradyrhizobium in pot experiments. In a field experiment, the effects of VAM on competition amongst inoculated bradyrhizobia were less evident, but inoculation with Bradyrhizobium strains increased root colonization by VAM. Certain VAM/Bradyrhizobium inoculum strain combinations produced higher nodule numbers. Plants grown without Bradyrhizobium and VAM, but supplied with ammonium nitrate (300 g ml^–1) and potassium phosphate (16 g ml^–1), produced higher dry-matter yields than those inoculated with both symbionts in the pot experiment. Inoculation with either symbiont in the field did not result in higher pod and haulm yields at harvest.ICRISAT Journal Article No. 886     

Effect of bulk density and soil water tension on denitrification in the rhizosphere of spring wheat (Triticum vulgare

Summary Pot experiments were carried out to study the influence of bulk density (D _b), soil water tension (pF) and presence of plants (spring wheat) on denitrification in a low-humus B_t-horizon of a udalf. Pots of only 5-cm depth were found to be most suitable for the experiments when using the acetylene inhibition method. Almost homogeneous soil compaction between 1.1 and 1.6g soil cm^–3 was achieved by a Proctor tamper. Water tensions were adjusted by means of ceramic plates on which negative pressure was applied. No denitrification was detected in unplanted pots. With planted pots and increasing bulk density denitrification increased more in pots with 14-day-old plants than in pots with 7-day-old plants. With 14-day-old plants N₂O emission pot^–1 increased steadily from 2 mol at D _b 1.1 to 8 mol at D _b 1.6, when soil moisture was adjusted to pF 1.5, although root growth was impaired by higher bulk density. From an experiment with different bulk densities and water tensions it could be deduced that the air-filled porosity ultimately determined the rate of denitrification. When low water tension was applied for a longer period, water tension had an overriding effect on total denitrification. Denitrification intensity, however, i.e. the amount of N₂O g^–1 root fresh weight, was highest when low water tension was accompanied by high bulk density. The results suggest that the increase in denitrification intensity at oxygen stress is partly due to higher root exudation.     

Mineralization of carbon and nitrogen from fresh and anaerobically stored sheep manure in soils of different texture

A sandy loam soil was mixed with three different amounts of quartz sand and incubated with (¹⁵NH₄)₂SO₄ (60 g N g^-1 soil) and fresh or anaerobically stored sheep manure (60 g g^-1 soil). The mineralization-immobilization of N and the mineralization of C were studied during 84 days of incubation at 20°C. After 7 days, the amount of unlabelled inorganic N in the manure-treated soils was 6–10 g N g^-1 soil higher than in soils amended with only (¹⁵NH₄)₂SO₄. However, due to immobilization of labelled inorganic N, the resulting net mineralization of N from manure was insignificant or slightly negative in the three soil-sand mixtures (100% soil+0% quartz sand; 50% soil+50% quartz sand; 25% soil+75% quartz sand). After 84 days, the cumulative CO₂ evolution and the net mineralization of N from the fresh manure were highest in the soil-sand mixutre with the lowest clay content (4% clay); 28% fo the manure C and 18% of the manure N were net mineralized. There was no significant difference between the soil-sand mixtures containing 8% and 16% clay, in which 24% of the manure C and -1% to 4% of the manure N were net mineralized. The higher net mineralization of N in the soil-sand mixture with the lowest clay content was probably caused by a higher remineralization of immobilized N in this soil-sand mixture. Anaerobic storage of the manure reduced the CO₂ evolution rates from the manure C in the three soil-sand mixtures during the initial weeks of decomposition. However, there was no effect of storage on net mineralization of N at the end of the incubation period. Hence, there was no apparent relationship between net mineralization of manure N and C.     

The use of nitrogen-15 natural abundance and nitrogen yield of non-nodulating isolines to estimate nitrogen fixation by soybeans (Glycine max L.) across three elevations

Summary Dissimilarities in soil N uptake between N₂-fixing and reference non-N₂-fixing plants can lead to inaccurate N₂ fixation estimates by N difference and ¹⁵N enrichment methods. The natural ¹⁵N abundance ( ¹⁵N) method relies on a stabilized soil ¹⁵N pool and may provide reliable estimates of N₂ fixation. Estimates based on the ¹⁵N and differences in N yield of nodulating and non-nodulating isolines of soybean were compared in this study. Five soybeans from maturity groups 00, IV, VI, and VIII and their respective non-nodulating isolines were grown at three elevations differing in ambient temperature and soil N availability. Despite large differences in phenological development and N yield between the non-nodulating isolines, the ¹⁵N values measured on seeds were relatively constant within a site. The ¹⁵N method consistently produced lower N₂ fixation estimates than the N difference method, but only in three of the 15 observations did they differ significantly. The average crop N derived from N₂ fixation across sites and maturity groups was 81% by N difference compared to 71% by ¹⁵N. The magnitude of difference between the two methods increased with increasing proportions of N derived from N₂ fixation. These differences between the two methods were not related to differences in total N across sites or genotypes. The low N₂ fixation estimates based on ¹⁵N might indicate that the nodulating isolines had assimilated more soil N than the non-nodulating ones. A lower variance indicated that the estimates by N difference using non-nodulating isolines were more precise than those by ¹⁵N. Since the differences between the estimates were large only at high N₂ fixation levels (low soil N availability), either method may be used in most situations when a non-nodulating isoline is used as the reference plant. The ¹⁵N method may have a comparative advantage over N difference and ¹⁵N enrichment methods in the absence of a suitable non-N₂-fixing reference plant such as a non-nodulating isoline.     

Soybean nodulation and nitrogen fixation on soil amended with plant residues

Residues from some tree species may contain allelopathic chemicals that have the potential to inhibit plant growth and symbiotic N₂-fixing microorganisms. Soybean [Glycine max (L.) Merr] was grown in pots to compare nodulation and N₂-fixation responses of the following soil amendments: control soil, leaf compost, red oak (Quercus rubra L.) leaves, sugar maple (Acer saccharum Marsh) leaves, sycamore (Platanus occidentalis L.) leaves, black walnut (Juglans nigra L.) leaves, rye (Secale cereale L.) straw, and corn (Zea mays L.) stover. Freshly fallen leaves were collected from urban shade trees. Soil was amended with 20 g kg^-1 air-dried, ground plant materials. Nodulating and nonnodulating isolines of Clark soybean were grown to the R2 stage to determine N₂-fixation by the difference method. Although nodulation was not adversely affected, soybean grown on leaf-amended soil exhibited temporary N deficiency until nodulation. Nodule number was increased by more than 40% for soybean grown on amended soil, but nodule dry matter per plant generally was not changed compared with control soil. Nonnodulating plants were severely N deficient and stunted as a consequence of N immobilization. Nodulating soybean plants grown on leaf or crop residue amended soil were more dependent on symbiotically fixed N and had lower dry matter yields than the controls. When leaves were composted, the problem of N immobilization was avoided and dry matter yield was not reduced. No indication of an allelopathic inhibition on nodulation or N₂-fixation from heavy application of oak, maple, sycamore, or walnut leaves to soil was observed.     

Time-course of nitrogen fixation in two bambara groundnut (Vigna subterranea L. Verdc.) cultivars

TheA-value method, involving the application of a higher¹⁵N rate to a reference non-N₂-fixing plant, was used to assess the magnitude of N₂ fixation in two bambara groundnut cultivars at four growth stages [vegetative, 0–47 days after planting (DAP); early pod-filling, 47–99 DAP; mid-pod-filling, 99–120 DAP; physiological maturity, 120–148 DAP). The cultivars were Ex-Ada, a bunchy type, and CS-88-11, a slightly spreading type. They were grown on a loamy sand. Uninoculated Ex-Ada and CS-88-11 were used as reference plants to measure the N₂ fixed in the inoculated bambara groundnuts. In this greenhouse study, soil was the major source of N in bambara groundnuts during vegetative growth, and during this period it accounted for over 80% of the N accumulaed in the plants. However, N₂ fixation became the major source of plant N during reproductive growth. There were significant differences between the two cultivars in the ability to fix N₂, and at physiological maturity, almost 75% of the N in CS-88-11 was derived from the atmosphere compared to 55% in Ex-Ada. Also, the total N fixed in CS-88-11 at physiological maturity was almost double that in Ex-Ada. Our data indicate that the higher N₂ fixation in CS-88-11 was due to two factors, a higher intensity of N₂ fixation and a longer active period of N₂ fixation. The results also suggest that bambara groundnut genotypes could be selected for higher N₂ fixation in farining systems.     

Estimation of Deposition Velocities for 85Sr, 131I, 137Cs on Spinach,Radish and Beans Leaves in a Tropical Region Under Simulated Fallout Conditions

Poly dispersive aerosols of SrCl₂, NaI, CsCl having a size distribution of 2.33 m (Activity Median Aerodynamic Diameter, [AMAD]) with a geometric standard deviation of 1.83 m are used for the determination of deposition velocity for ⁸⁵Sr, ¹³¹I and ¹³⁷Cs aerosols on tropical spinach (Spinicia olericia), radish (Raphanous sative) and beans (Phasolous valugeries) plant leaves. The experiments were carried out in a specially designed exposure chamber to simulate deposition under accidental releases of radioactivity in tropical environment. The rates of particles deposition are expressed as a function of plant surface area and of plant dry weight. The deposition velocities obtained are in the range of 10^–6 to 10^–7 m/s. These values differ significantly from those in temperate regions. The lower values for deposition velocities (v _g) for the tropical environment is attributed due to high humidity. Therefore, the deposition of suspended aerosols on leaf surfaces caused by impaction and Brownian diffusion becomes slower in tropical environment and resulted into lower value of deposition velocities.     

Estimating N2 fixation by field-grown lupins (Lupinus angustifolius L.) using soil and plant 15N enrichment

Summary Biological N₂ fixation was estimated in a field experiment following the addition of NH₄Cl or KNO₃ to unconfined microplots (1.5 m²) at 2.5 g N m^-2 (10 atom% ¹⁵N). A model of total N and ¹⁵N accumulation in lupins and decreasing ¹⁵N enrichment in the KCl-extractable soil-N pool (0–0.15 m depth) was used to estimate the proportion of N in lupins derived from biological N₂ fixation. Estimates of N₂ fixation derived from the model were compared with ¹⁵N isotope-dilution estimates obtained using canola, annual ryegrass, and wheat as nonfixing reference plants. Biomass, total N accumulation, or ¹⁵N enrichment in the lupin and reference crops did not differ whether NH _inf4 ^sup+ or NO _inf3 ^sup- was added as the labelled inorganic-N source. The decrease in soil ¹⁵N enrichment was described by first-order kinetics, whereas total N and ¹⁵N accumulation in the lupins were described by logistical equations. Using these equations, the uptake of soil N by lupins was estimated and was then used to calculate fixed N₂. Estimates of N₂ fixation derived from the model increased from 0 at 50 days after sowing to a maximum of 0.79 at 190 days after sowing. Those based on the ¹⁵N enrichment of the NO _inf3 ^sup- pool were 10% higher than those based on the mineral-N pool. ¹⁵N isotope-dilution estimates of N₂ fixation ranged from 0.37 to 0.55 at 68 days after sowing and from 0.71 to 0.77 at 190 days after sowing. Reference plant-derived values of N₂ fixation were all higher than modelled estimates during the early states of growth, but were similar to modelled estimates at physiological maturity. The use of the model to estimate N₂ derived from the atmosphere has the intrinsic advantage that the need for a non-fixing reference plant is avoided.     

Reduction of bacterial growth by a vesicular-arbuscular mycorrhizal fungus in the rhizosphere of cucumber (Cucumis sativus L.)

Summary Cucumber was grown in a partially sterilized sand-soil mixture with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum or left uninoculated. Fresh soil extract was places in polyvinyl chloride tubes without propagules of mycorrhizal fungi. Root tips and root segments with adhering soil, bulk soil, and soil from unplanted tubes were sampled after 4 weeks. Samples were labelled with [³H]-thymidine and bacteria in different size classes were measured after staining by acridine orange. The presence of VAM decreased the rate of bacterial DNA synthesis, decreased the bacterial biomass, and changed the spatial pattern of bacterial growth compared to non-mycorrhizal cucumbers. The [³H]-thymidine incorporation was significantly higher on root tips in the top of tubes, and on root segments and bulk soil in the center of tubes on non-mycorrhizal plants compared to mycorrhizal plants. At the bottom of the tubes, the [³H]-thymidine incorporation was significantly higher on root tips of mycorrhizal plants. Correspondingly, the bacterial biovolumes of rods with dimension 0.28–0.40×1.1–1.6 m, from the bulk soil in the center of tubes and from root segments in the center and top of tubes, and of cocci with a diameter of 0.55–0.78 m in the bulk soil in the center of tubes, were significantly reduced by VAM fungi. The extremely high bacterial biomass (1–7 mg C g^-1 dry weight soil) was significant reduced by mycorrhizal colonization on root segments and in bulk soil. The incorporation of [³H]-thymidine was around one order of magnitude lower compared to other rhizosphere measurements, probably because pseudomonads that did not incorporate [³H]-thymidine dominated the bacterial population. The VAM probably decreased the amount of plant root-derived organic matter available for bacterial growth, and increased bacterial spatial variability by competition. Thus VAM plants seem to be better adapted to compete with the saprophytic soil microflora for common nutrients, e.g., N and P, compared to non-mycorrhizal plants.