Foliar chemical concentrations in red alder stands of various ages

Summary Six stands of red alder (Alnus rubra Bong.) were sampled to assess the relationship between stand age and foliar chemical concentrations. The stands ranged in age from 9 to 45 years, and were located near Olympia, Washington, on sites of similar soil, aspect, and slope. Foliage was collected in July, and concentrations of most essential nutrients and aluminum were determined. Foliar concentrations of these elements varied among the different stands. Highest concentrations of N, P, Ca, S, Fe, and Al were found in trees of the youngest stand. Concentrations of N, P, S, and Fe were negatively correlated with stand age. The strongest correlation was obtained with P (r=–0.96,P<0.01), followed by N (r=–0.85,P=0.03). Results suggest that declining growth and deterioration of red alder stands with age may be related, at least in part, to limiting supplies of essential nutrients, especially P, N, and S.     

Rhizosphere pH along different root zones of Douglas-fir (Pseudotsuga menziesii), as affected by source of nitrogen

Douglas-fir, grown on strongly acid soil (pH-H₂O 3.87), was fertilized with ammonium, nitrate or ammonium nitrate as N-source. Rhizosphere pH along the root axis was measured with microelectrodes. Pure ammonium supply resulted in acidification of the rhizosphere, almost along the entire root axis; only the extreme root tip was slightly alkaline compared with the bulk soil. With nitrate supply, the alkalization of the rhizosphere at the root tip was considerably stronger than with ammonium supply and the length of the alkalization zone greater, extending over the entire growth zone of the root. Acidification of the rhizosphere along the older parts of the root was less pronounced in the case of nitrate. It is concluded that nitrate nutrition enables the plant to protect its most essential root zone from the adverse effects of strong acidity by locally raising the rhizosphere pH.     

Nitrogen fixation (15N dilution) with soybeans under Thai field conditions

Controlled environment and field studies were conducted to determine relationships between various measurements of N₂ fixation using soybeans and to use these measures to evaluate a number ofBradyrhizobium japonicum strains for effectiveness in N₂ fixation in Thai soils.¹⁵N dilution measurements of N₂ fixation showed levels of fixation ranging from 32 to 161 kg N ha⁻¹ depending on bacterial strain, host cultivar and location. Midseason measures of N₂ fixation were correlated with each other, but not related measures taken at maturity. Ranking ofB. japonicum strains based on performance under controlled conditions in N-free media were highly correlated with rankings based on soybean seed yields and N₂ fixation under field conditions. This study showed that inoculation of soybeans with effectiveB. japonicum strains can result in significant increases in yield and uptake of N through fixation. The most effective strains tested for use in Thai conditions were those isolated from Thai soils; however, effective strains from other locations were also of benefit.     

Growth,N2 fixation and photosynthesis in a cyanobacterium,Trichodesmium sp., under Fe stress

Trichodesmium sp., isolated from the Great Barrier Reef lagoon, was cultured in artificial seawater media containing a range of Fe concentration. Fe additions stimulated growth, N₂ fixation, cellular chlorophyll a content, light-saturated chlorophyll a-specific gross photosynthetic capacity (P_m ^chla) and the dark respiration rate (R_d ^chla). Cell yields only doubled for 9 nM Fe relative to zero added Fe, whereas N₂ fixation increased 11-fold considerably for 450 nM Fe. The results suggest that N₂ fixation of Trichodesmium is more sensitive to Fe limitation than are the cell yields.     

N2 fixation and nitrogen allocation to above and below ground plant parts in red clover-grasslands

Leys, used for grazing or production of forage to be conserved as silage or hay, are very important crops in northern areas. In order to measure the N₂ fixation in leys of varying ages and during different parts of the season, detailed measurements were taken of yield, N₂ fixation and the amounts of N remaining in the field after harvesting red clover (Trifolium pratense L.)-grass leys at a site in northern Sweden, where they are generally harvested twice per growing season. Entire plants, including stubble and roots, were sampled at the time of first and second harvest and, in addition, at the end of the growing season in three neighbouring fields, carrying a first, a second and a third year ley, respectively. N₂ fixation was measured by both ¹⁵N isotope dilution (ID) and ¹⁵N natural abundance (NA) methods. The proportion of clover dry matter (DM) in the stands increased from the first to the second harvest, but the grasses dominated throughout the entire season, especially below ground. The N concentrations, in both herbage and whole plants, were about twice as high in the clover as in the grasses. Seasonal variations in N concentrations were minor, and total N contents followed the same trends as DM. The clover acquired nearly all of its N from N₂ fixation: the proportion of N in clover herbage derived from N₂ fixation was often >0.8 throughout the season. The variations in the amounts of N₂ fixed during the course of the season corresponded well to the seasonal changes in clover biomass. Amounts of fixed N₂ allocated to clover herbage during the whole season were in the range 4 to 6 g N m⁻² in this unusually rainy year. Calculations of daily N allocation rates to herbage showed that N uptake rates were similar, and high, in grasses during May–June and July–August, while N₂ fixation rates in clover were about 10-fold as high in July–August as in May–June, reflecting the need for N in clover growth. The proportion of N remaining in clover stubble and roots after the first and second harvests was about 60 and 25%, respectively, while about 60% of the N in grasses remained in stubble and roots after both harvests. The considerable amounts of biomass and N that were left in field after harvesting red clover-grass leys are important for re-growth of the plants and provide substantial N fertilization for the next crop in the crop rotation.     

Population dynamics and growth patterns for a cohort of northern red oak (Quercus rubra) seedlings

Summary I studied the survival and development of a 1986 cohort of northern red oak (Quercus rubra L.) seedlings growing under a variety of overstory and microsite conditions in a northern hardwood forest dominated by northern red oak, red maple (Acer rubrum L.) paper birch (Betula papyrifera Marsh.), and scattered white pine (Pinus strobus L.). Fifty naturally regenerating seedlings of oak were randomly selected in each of three canopy classes: no overstory, partial overstory, and complete overstory. Growth and mortality were measured for six years. Seedling height growth decreased with overstory density, with less growth evident with even a partial overstory. Seedling survival also declined with overstory density and depended on microtopography to a lesser extent. After six years, 92% of the seedlings survived in the open, compared to 54% under the partial overstory, and 36% under the complete overstory. The open environment, in which woody and herbaceous regrowth formed a low canopy reducing light intensities to about 50% of full sunlight, provided a favorable site for the growth and survival of northern red oak.     

The influence of long-term tillage systems on symbiotic N2 fixation of pea (Pisum sativum L.) and red clover (Trifolium pratense L.)

Pea as a grain legume and red clover as a forage legume in the seeding year were cultivated in two long-term differentiated tillage systems on a loess soil in Germany. A continuous conventional tillage system (plow; CT) and a continuous minimum tillage system (rotary harrow; MT) were established in 1970. With pea and red clover dry matter accumulation and N parameters (N accumulation, Ndfa, N-harvest-index, N balance) were investigated in 1998 and 1999. Differences in the N₂ fixation of pea due to the tillage system could clearly be shown whereas grain yields and total N accumulation were equal in both tillage systems and years. In both years a significantly (P < 0.05) higher Ndfa in the MT system was found at least in the final harvest (maturity of pea): 1998/1999, 0.42/0.54 in CT, 0.62/0.75 in MT. The differences in N₂ fixation of pea may be explained by the delayed soil N supply in MT at the beginning of the vegetative period. Simplified N balances of pea were -18 and –25 kg N ha^–1 in CT and –5 and +1 kg N ha^–1 in MT for 1998 and 1999, respectively. Red clover showed no significant differences in the DM and N accumulation between both tillage systems but a year dependent effect caused by different stubble and root yields between the years was apparent. With red clover slightly, but also significantly (P < 0.05) increased Ndfa values were found in the MT system compared to the CT system with 0.55/0.62 in CT (1998/1999) and 0.64/0.71 in MT. However, the difference in Ndfa between the tillage systems (9 percentage points) was much smaller with red clover than with pea (20 and 21 percentage points in 1998 and 1999, respectively). Soil N uptake of red clover using the longer growing season reflected the more adjusted N supply in both long-term differentiated tillage systems, whereas pea in using only a short-term vegetative period reacted stronger to the lower N mineralization in the MT system in springtime.     

Enhancing legume N2 fixation through plant and soil management

Atmospheric N₂ fixed symbiotically by associations between Rhizobium spp. and legumes represents a renewable source of N for agriculture. Contribution of legume N₂ fixation to the N-economy of any ecosystem is mediated by: (i) legume reliance upon N₂ fixation for growth, and (ii) the total amount of legume-N accumulated. Strategies that change the numbers of effective rhizobia present in soil, reduce the inhibitory effects of soil nitrate, or influence legume biomass all have potential to alter net inputs of fixed N. A range of management options can be applied to legumes growing in farming systems to manipulate N₂ fixation and improve the N benefits to agriculture and agroforestry.     

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

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

Biological N2 fixation by heterotrophic and phototrophic bacteria in association with straw

Much of the crop residues, including cereal straw, that are produced worldwide are lost by burning. Plant residues, and in particular straw, contain large amounts of carbon (cellulose and hemicellulose) which can serve as substrates for the production of microbial biomass and for biological N₂ fixation by a range of free-living, diazotrophic bacteria. Microorganisms with the dual ability to utilise cellulose and fix N₂ are rate, but some strains that utilize hemicellulose and fix N₂ have been found. Generally, cellulolysis and diazotrophy are carried out by a mixed microbial community in which N₂-fixing bacteria utilise cellobiose and glucose produced from straw by cellulolytic microorganisms. N₂-fixing bacteria include heterotrophic and phototrophic organisms and the latter are apparently more prominent in flooded soils such as rice paddies than in dryland soils. The relative contributions of N₂ fixed by heterotrophic diazotrophic bacteria compared with cyanobacteria and other phototrophic bacteria depend on the availability of substrates from straw decomposition and on environmental pressures. Measurements of asymbiotic N₂ fixation are limited and variable but, in rice paddy systems, rates of 25 kg N ha^-1 over 30 days have been found, whereas in dryland systems with wheat straw, in situ measurements have indicated up to 12 kg N ha^-1 over 22 days. Straw-associated N₂ fixation is directly affected by environmental factors such as temperature, moisture, oxygen concentration, soil pH and clay content as well as farm management practices. Modification of managements and use of inoculants offer ways of improving asymbiotic N₂ fixation.In laboratory culture systems, inoculation of straws with cellulolytic and diazotrophic microorganisms has resulted in significant increases in N₂ fixation in comparison to uninoculated controls and gains of N of up to 72 mg N fixed g^-1 straw consumed have been obtained, indicating the potential of inoculation to improve N gains in composts that can then be used as biofertilisers. Soils, on the other hand, contain established, indigenous microbial populations which tend to exclude inoculant microorganisms by competition. As a consequence, improvements in straw-associated N₂ fixation in soils have been achieved mostly by specific straw-management practices which encourage microbial activity by straw-decomposing and N₂-fixing microorganisms.Further research is needed to quantify more accurately the contribution of asymbiotic N₂ fixation to cropping systems. New strains of inoculants, including those capable of both cellulolytic and N₂-fixing activity, are needed to improve the N content of biofertilisers produced from composts. Developments of management practices in farming systems may result in further improvements in N₂ fixation in the field.     

15N2 measurement of nitrogen fixation by legumes and actinorhizals: theory and practice

A gas-tight chamber has been constructed to calibrate the ¹⁵N isotope dilution method against direct ¹⁵N₂ measurements. The theoretical basis for such estimates is given, and the practical problems associated with the experiments are discussed.     

Contribution of basal portion of shoot to N2 fixation associated with wetland rice

Summary Oryza sativa grown in flooded soil were transferred to water culture solution and acetylene reduction activities (ARA) of intact plants and rootless plants were measured for 5 h. Relative rate of ARA associated with the rootless wetland rice plant as compared with an intact plant varied from 8 to 100 percent, depending on the growth stage and varieties of rice and highest at the early stage (3 weeks after transplanting) for all varieties tested (IR26, Latisail, Khao Lo, and JBS236). ARA of shoots was associated with basal parts of the shoots about 3 cm from the base of wetland cultivated rice andOryza australiensis. Phyllospheric ARA was negligible except for senescent outer leaf sheaths. Microaerophilic N₂-fixing bacteria also inhabited basal parts of shoots (outer leaf sheaths and stems) of wetland rice. These findings suggest that N₂-fixation is partly associated with the shoots of wetland rice plants.     

Effects of N source on proton excretion,ionic balance and growth ofAlnus glutinosa (L.) Gaertner: comparison of N2 fixation with single and mixed sources of NO3 and NH4

Summary Non-nodulatedalnus glutinosa plants were grown for 6 weeks in nutrient solutions using 3 combined-N treatments (NO₃; NO₃/NH₄; and NH₄) at a total N level of 4 meq.l^–1, and growth was ccmpared with nodulated plants at zero N (N₂ fixation). Of the combined-N sources, 100 per cent NH₄ resulted in the highest dry matter yields when the solution pH was adjusted daily atc. 6. The dry matter yield was lowest with NO₃.During the first 3 weeks, the yield of the N₂-fixing plants was as high as that of the NH₄ plants, but fell relatively behind during the second 3-week period. These effects could be attributed to higher initial N contents and higher shoot:root ratios, respectively, in the N₂-fixing plants. Specific rates of N acquisition in the root were of a comparable order of magnitude for the combined-N and zero-N treatments.When NO₃ was taken up, it was almost completely reduced in the roots. Regardless of N source there was a large excess of cations (C) relative to inorganic anions (A) in the plants, which was presumed to be balanced by an equivalent amount of organic anions (C-A). The relatively small differences in generation of organic anions for the various modes of N supply indicated the relative importance of the proton pump when NH₄ or N₂ was the N source. Proton or hydroxyl-ion effluxes, calculated on the basis of plant analyses, were generally in good agreement with measured excretion values. The acidity generation with N₂ fixation amounted toc. 0.5 meq H⁺.mmol^–1 N_org, which was distinctly higher than the range of 0.1–0.2 mentioned by Raven and Smith⁴³ for dinitrogen-fixing plants.Without pH adjustment, specific rates of cation uptake and carboxylate generation were strongly depressed as the acidity increased, when NO₃/NH₄, NH₄ and N₂ were the N sources. Growth ofAlnus glutinosa appeared to be still normal at a pH ofc. 2.8. During the final 3 weeks, only the NH₄ plants ceased growing at a pH of 2.6.     

Seasonal N uptake and N2fixation by common and adzuki bean at various spacings

The adzuki bean (Vigna angularis (Wild.) Ohwi and Ohashi) and common bean (Phaseolus vulgaris L.) have a high physiological demand for N. A 2-year field study was conducted to investigate the seasonal change of available soil N and symbiotic N₂ fixation usage. The beans were seeded at two densities, 22.2 plants m^–2 with a row spacing of 0.3 m and 11.1 plants m^–2 with a row spacing of 0.6 m. The amount of fixed N₂ in the shoot was calculated using the ¹⁵N natural abundance method. The common bean demonstrated low N₂ fixation and the ability to accumulate high levels of soil N. Soil nitrate under the common bean was continually absorbed. The adzuki bean, on the other hand, had a remarkable peak of N accumulation in the early reproductive stage. This was mainly due to N₂ fixation, though the soil nitrate level was high. Narrowing the plant row spacing increased the dry matter yield of both species, but the origin of the increased N differed between the species. For the first 77 DAP in 1999 (73 DAP in 2000) the N increase for both beans was due to both soil and atmospheric N₂. At harvest, though, the increase of N in common bean was mainly due to soil N, while that in adzuki bean was mainly due to atmospheric N₂. It can be concluded that the low symbiotic N₂ fixation ability of common bean was due to its high soil N uptake ability and constant N accumulation, which enabled an efficient soil N absorption. Adzuki bean absorbed N mainly for a short period and depended more on symbiotically fixed N₂ and, in contrast to common bean, left a high level of NO₃-N remaining in the soil after cropping.     

Effect of different levels of mineral nitrogen on nodulation and N2 fixation of two cultivars of common bean (Phaseolus vulgaris L.)

An experiment was conducted under greenhouse conditions to evaluate the effect of mineral nitrogen on N₂ fixation of two cultivars of Phaseolus vulgaris L., Puebla 152 and Negro Argel. Nitrogen application was 0, 2.5, 12.5 and 25 mg N Kg^–1 of a vermiculite-sand-mixture at planting time. Shoot and root growth were elevated by nitrogen application at all growth stages. During vegetative growth (V 5) nodule dry weight and nitrogenase activity (acetylene reducing activity) per plant were reduced by nitrogen supply in both cultivars, but less in Negro Argel than in Puebla 152. At later stages nodulation in nitrogen-treated Puebla 152 did not differ from that in non-treated plants, whereas increased nodule number was found in Negro Argel at high nitrogen levels. The influence of mineral N on the total amount of nitrogen fixed in the two bean cultivars was only slightly different.     

Effects of low molybdenum seed on nodule initiation,development and N2 fixation in black gram (Vigna mungo L.)

In legumes, both increases and decreases in nodule number in response to Mo deficiency have been reported, but reasons for the different responses have not been proposed. The present study examined nodule initiation and development in black gram seedlings using two levels of seed Mo to induce Mo deficiency. In the first 11 days after inoculation, low levels of Mo in seed had no effect on nodule initiation or the number of nodules. At 13 days after inoculation, low Mo in seed depressed bacteroid concentration, leghaemoglobin concentration, nodule number and nodule fresh weight. Acetylene reduction activity was delayed by 2 days in plants grown from low Mo seed. We suggest that the delay in N₂ fixation in plants grown from low Mo seed was due to slower incorporation of Mo of soil origin into nitrogenase. We further suggest that restricted supply of essential metabolites to the nodules on plants from low Mo seed resulted in the slower maturation of early initiated nodules and the repression of formation of new nodules.     

Enhancing crop legume N2 fixation through selection and breeding

Legume N₂ fixation is variable, but nonetheless is a valuable process in world agriculture. There is great potential to increase the contribution by the crop legumes to the world's supply of soil.N. This will be achieved by (i) increasing the area of legumes sown by farmers; (ii) improved management of the crops in order that the major determinants of productivity, e.g. land area, water availability, are converted to harvested product with maximum efficiency; and (iii) genetic modification of the commonly-grown species to ensure high dependence of the legume crop on N₂ fixation at all levels of productivity. Currently-used methods for measuring N₂ fixation and for assessing heritability and repeatability of N₂ fixation in breeding and selection programs are reviewed. Results from research programs to define genetic variation in N₂ fixation and to enhance N₂ fixation through selection and breeding are presented with particular emphasis on common bean (Phaseolus vulgaris) and soybean (Glycine max).     

Symbiotic N2 fixation by soybean in organic and conventional cropping systems estimated by 15N dilution and 15N natural abundance

Nitrogen (N) is often the most limiting nutrient in organic cropping systems. N₂ fixing crops present an important option to improve N supply and to maintain soil fertility. In a field experiment, we investigated whether the lower N fertilization level and higher soil microbial activity in organic than conventional systems affected symbiotic N₂ fixation by soybean (Glycine max, var. Maple Arrow) growing in 2004 in plots that were since 1978 under the following systems: bio-dynamic (DYN); bio-organic (ORG); conventional with organic and mineral fertilizers (CON); CON with exclusively mineral fertilizers (MIN); non-fertilized control (NON). We estimated the percentage of legume N derived from the atmosphere (%Ndfa) by the natural abundance (NA) method. For ORG and MIN we additionally applied the enriched ¹⁵N isotope dilution method (ID) based on residual mineral and organic ¹⁵N labeled fertilizers that were applied in 2003 in microplots installed in ORG and MIN plots. These different enrichment treatments resulted in equal %Ndfa values. The %Ndfa obtained by NA for ORG and MIN was confirmed by the ID method, with similar variation. However, as plant growth was restricted by the microplot frames the NA technique provided more accurate estimates of the quantities of symbiotically fixed N₂ (Nfix). At maturity of soybean the %Ndfa ranged from 24 to 54%. It decreased in the order ORG > CON > DYN > NON > MIN, with significantly lowest value for MIN. Corresponding Nfix in above ground plant material ranged from 15 to 26 g N m^-2, with a decreasing trend in the order DYN = ORG > CON > MIN > NON. For all treatments, the N withdrawal by harvested grains was greater than Nfix. This shows that at the low to medium %Ndfa, soybeans did not improve the N supply to any system but removed significant amounts of soil N. High-soil N mineralization and/or low-soil P availability may have limited symbiotic N₂ fixation.     

The rock-phosphate solubilizing capacity ofPueraria javanica as affected by soil pH,superphosphate priming effect and symbiotic N2 fixation

Summary WhenPueraria javanica was grown on acid soil in a pot experiment, the legume’s acidifying effect, originating from an uptake pattern in which on an equivalence basis more nutritive cations than-anions were absorbed, was sufficient to mobilize rock phosphate even when added as very insoluble material. In neutral soil, a small quantity of triple superphosphate proved necessary to set into motion a chain of reactions in which the priming action of the TSP enables nodulation to take place followed, in order, by N₂ fixation, soil acidification and rock phosphate mobilization. More attention should be paid in tropical regions to mixed farming systems in which leguminous crops, like Pueraria, produce fodder for livestock whose faeces and urine, when properly collected, can be used for the manufacturing of biogas, after which the residues can serve as manure to food crops. Even when they prove unsuitable for beneficiation, many rock phosphates found in African and Latin American countries can be made useful as fertilizer for leguminous fodder crops.     

Adaptation of methods for evaluating N2 fixation in food legumes and legume cover crops

Methods for partitioning the nitrogen assimilated by nodulated legumes, between nitrogen derived from soil sources and from N₂ fixation, are described as applied in peninsular Malaysia. The analysis of nitrogenous components translocated from the roots to the shoots of nodulated plants in the xylem sap is outlined, with some precautions to be observed for applications in the tropics. Some examples of the use of the technique in surverying apparent N₂ fixation by tropical legumes, in studying interrow cropping in plantation systems and in assessing effects of experimental treatments on N₂ fixation by food legumes, are described. Techniques for assesing N₂ fixation by means of¹⁵N abundance have been used to show that applications of nitrogenous fertilizers commonly used in Malaysia for soybeans depress N₂ fixation, that similar results are obtained with natural abundance and¹⁵N-enrichment methods and that, in at least two locations in Malaysia, differences between the natural abundance of¹⁵N in plant-available soil nitrogen and in atmospheric N₂ are great enough to permit application to measurement of N₂ fixation by leguminous crops.