Controls of Benthic Nitrogen Fixation and Primary Production from Nutrient Enrichment of Oligotrophic, Arctic Lakes

We examined controls of benthic dinitrogen (N₂) fixation and primary production in oligotrophic lakes in Arctic Alaska, Toolik Field Station (Arctic Long-Term Ecological Research Site). Primary production in many oligotrophic lakes is limited by nitrogen (N), and benthic processes are important for whole-lake function. Oligotrophic lakes are increasingly susceptible to low-level, non-point source nutrient inputs, yet the effects on benthic processes are not well understood. This study examines the results from a whole-lake fertilization experiment in which N and P were added at a relatively low level (4 times natural loading) in Redfield ratio to a shallow (3 m) and a deep (20 m) oligotrophic lake. The two lakes showed similar responses to fertilization: benthic primary production and respiration (each 50–150 mg C m^?2 day^?1) remained the same, and benthic N₂ fixation declined by a factor of three- to fourfold by the second year of treatment (from ~0.35 to 0.1 mg N m^?2 day^?1). This showed that the response of benthic N₂ fixation was de-coupled from the nutrient limitation status of benthic primary producers and raised questions about the mechanisms, which were examined in separate laboratory experiments. Bioassay experiments in intact cores also showed no response of benthic primary production to added N and P, but contrasted with the whole-lake experiment in that N₂ fixation did not respond to added N, either alone or in conjunction with P. This inconsistency was likely a result of nitrogenase activity of existing N₂ fixers during the relative short duration (9 days) of the bioassay experiment. N₂ fixation showed a positive saturating response when light was increased in the laboratory, but was not statistically related to ambient light level in the field, leading us to conclude that light limitation of the benthos from increasing water-column production was not important. Thus, increased N availability in the sediments through direct uptake likely caused a reduction in N₂ fixation. These results show the capacity of the benthos in oligotrophic systems to buffer the whole-system response to nutrient addition by the apparent ability for significant nutrient uptake and the rapid decline in N₂ fixation in response to added nutrients. Reduced benthic N₂ fixation may be an early indicator of a eutrophication response of lakes which precedes the transition from benthic to water-column-dominated systems.     

Comparative study of N uptake and distribution in three lines of common bean (Phaseolus vulgaris L.) at early pod filling stage

Summary The uptake and distribution of¹⁵NH ₄ ⁺ ,¹⁵NO ₃ ⁻ and¹⁵N₂ was studied in greenhouse-grown beans (Phaseolus vulgaris L.) with a commercial cultivar and 2 recombinant inbred backcross lines;¹⁵N was supplied in the nutrient solution at the R3 (50% bloom) stage. Plants were harvested 1, 5 and 10 days after treatment, and were separated into nodules, roots, stems, mature leaflets, immature leaflets, and flowers/fruits. All 3 lines showed rapid increases in the N content of flowers/fruits after the R3 stage. However, the percentage N in these tissues decreased after the R3 stage. One of the recombinant lines showed a greater uptake of NH ₄ ⁺ than the other 2 lines. Rates of¹⁵N₂ fixation and NO ₃ ⁻ uptake were similar for all 3 lines, N₂ fixation estimated from total N content showed the 2 recombinant lines with 24 and 34 percent greater activity than the commercial cultivar. Distribution of¹⁵N at the whole plant level was similar for all 3 lines for a similar N source.¹⁵NO ₃ ⁻ was transported first to leaflets and the label then moved into flowers/fruits. Transport of fixed N₂ was from the nodules to roots, stems and into flowers/fruits; usually less than 10 percent entered the leaflets. This indicates that N₂ fixation furnishes N directly to flowers/fruits with over 50 percent of the fixed N being deposited into flowers/fruits within 5 days after treatment.     

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.     

Measurements of N2 fixation by common bean in Central Brazil as affected by different reference crops

An experiment was conducted at EMBRAPA/CNPAF, Goiânia, Goias, Brazil, on a typic haplustox soil to evaluate growth and N₂ fixation-related parameters of Phaseolus vulgaris L. Bean lines, which had been selected for N₂ fixation at CNPAF, including production cultivars, germplasm bank entries, and parents and progenies of a cross made to improve this characteristic. Wheat (Triticum aestivum L.) and dwarf sorghum (Sorghum bicolor (L.) Moench) were evaluated as non-N₂-fixing reference crops for difference method (DM) and ¹⁵N isotope dilution technique (IDT) estimates of N₂ fixation. IDT estimates ranges from 4 to 18 kg N₂ fixed ha^-1. High variability associated with low levels of N₂ fixation precluded definitive identification of the best N₂ fixing bean lines. Due to differences in growth cycle and in patterns and amounts of soil N uptake during the season, neither of the reference crops tested appears to be an adequate control for either DM or IDT estimates of N₂ fixation. However, ranking of lines for effectiveness in N₂ fixation could be performed without the use of any reference crops.     

Effect of nitrogen forms on growth,cell composition and N<Subscript>2</Subscript> fixation of <Emphasis Type="Italic">Cylindrospermopsis raciborskii</Emphasis> in phosphorus-limited chemostat cultures

The aim of this research was to test whether NH₄ ⁺ and NO₃ ⁻ affect the growth, P demand, cell composition and N₂ fixation of Cylindrospermopsis raciborskii under P limitation. Experiments were carried out in P-limited (200 μg l⁻¹ PO₄-P) chemostat cultures of C. raciborskii using an inflowing medium containing either 4,000 μg l⁻¹ NH₄-N, 4,000 μg l⁻¹ NO₃-N or no combined N. The results showed the cellular N:P and C:P ratios of C. raciborskii decreased towards the Redfield ratio with increasing dilution rate (D) due to the alleviation of P limitation. The cellular C:N and carotenoids:chlorophyll-a ratios also decreased with D, predominantly as a result of an increase in the chlorophyll-a and N content. The NH₄ ⁺ and NO₃ ⁻ supply reduced the P maintenance cell quota of C. raciborskii. Consequently, the biomass yield of the N₂-grown culture was significantly lower. The maximum specific growth rate of N₂-grown culture was also the lowest observed. It is suggested that these differences in growth parameters were caused by the P and energy requirement for heterocyte formation, nitrogenase synthesis and N₂ fixation. N₂ fixation was partially inhibited by NO₃ ⁻ and completely inhibited by NH₄ ⁺. It was probably repressed through the high N content of cells at high dissolved N concentrations. These results indicate that C. raciborskii is able to grow faster and maintain a higher biomass under P limitation where a sufficient supply of NH₄ ⁺ or NO₃ ⁻ is maintained. Information gained about the species-specific nutrient and pigment stoichiometry of C. raciborskii could help to access the degree of nutrient limitation in water bodies. Handling editor: Luigi Naselli-Flores     

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

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

The contribution of associative and symbiotic nitrogen fixation to the nitrogen nutrition of non-legumes

During the past 10 years estimates of N₂ fixation associated with sugar cane, forage grasses, cereals and actinorhizal plants grown in soil with and without addition of inoculum have been obtained using the ¹⁵N isotope dilution technique. These experiments are reviewed in this paper with the aim of determining the proportional and absolute contribution of N₂ fixation to the N nutrition of non-legumes, and its role as a source of N in agriculture. The review also identifies deficiencies in both the totality of data which are currently available and the experimental approaches used to quantify N₂ fixation associated with non-legumes.Field data indicate that associative N₂ fixation can potentially contribute agronomically-significant amounts of N (>30–40 kg N ha^-1 y^-1) to the N nutrition of plants of importance in tropical agriculture, including sugar cane (Saccharum sp.) and forage grasses (Panicum maximum, Brachiaria sp. and Leptochloa fusca) when grown in uninoculated, N-deficient soils. Marked variations in proportions of plant N derived from the atmosphere have been measured between species or cultivars within species.Limited pot-culture data indicate that rice can benefit naturally from associative N₂ fixation, and that inoculation responses due to N₂ fixation can occur. Wheat can also respond to inoculation but responses do not appear to be due to associative N₂ fixation. ¹⁵N dilution studies confirm that substantial amounts of N₂ can be fixed by actinorhizal plants.     

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.     

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

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

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

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

Nitrogen fixation associated with non-legumes in agriculture

Summary This review examines the nitrogen cycle in upland agricultural situations where nonlegume N₂-fixation is likely to be important for crop growth. Evidence for associative fixation is adduced from accumulation of N in the top 15 cm soil under grasses, from N balances for crop production obtained from both pot and field experiments, in tropical and temperate environments, measurements of nitrogen (C₂H₂ reduction) activity, uptake of¹⁵N₂ by plants and¹⁵N isotope dilution. Factors influencing the activity such as the provision of carbon substrate by the plant and the efficiency of its utilisation by the bacteria, plant cultivar, soil moisture and N levels, and inoculation with N₂-fixing bacteria are discussed. Crop responses to inoculation withAzospirillum are detailed. The breakdown of crop residues, particularly straw, can support large levels of N₂-fixation. Cyanobacteria as crusts on the soil surface also fix nitrogen actively in many environments. Fixation by the nodulated, non-legume treesCasuarina andParasponia has beneficial effects in some cropping systems in Asia. I conclude that nonlegume N₂-fixation makes a significant contribution to the production of some major cereal crops in both temperate and tropical environments.     

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.     

Measurement of N2 fixation in maize (Zea mays L.)—ricebean (Vigna umbellata [Thunb.] Ohwi and Ohashi) intercrops

The yield of N in maize (Zea mays L.) and ricebean (Vigna umbellata [Thumb.] Ohwi and Ohashi) were compared on a Tropoqualf soil in North Thailand in 1984 and 1985. Both species were grown in field plots in monoculture or as intercrops at a constant planting density equivalent to 8 maize or 16 ricebean plants per m². The contribution of symbiotic N₂ fixation to ricebean growth was estimated from measurements of the natural abundance of¹⁵N (δ¹⁵N) in shoot nitrogen and from analysis of ureides in xylem sap vacuumextracted from detached stems. The natural abundance of¹⁵N in the intercropped ricebean was found to be considerably less than that in monoculture in both growing seasons. Using maize and a weed (Ageratum conyzoides L.) as non-fixing¹⁵N reference plants the proportions (P ¹⁵N) of ricebean shoot N derived from N₂ fixation ranged from 0.27 to 0.36 in monoculture ricebean up to 0.86 when grown in a 75% maize: 25% ricebean intercrop. When glasshouse-derived calibration curves were used to calculate plant proportional N₂ fixation (Pur) from the relative ureide contents of field collected xylem exudates, the contribution of N₂ fixation to ricebean N yields throughout the 1985 growing season were greater in intercrop than in monocrop even at the lowest maize:legume ratio (25∶75). Seasonal patterns of sap ureide abundance indicated that N₂ fixation was greatest at the time of ricebean podset. The averagePur andP ¹⁵N in ricebean during the first 90 days of growth showed identical rankings of monocrop and intercrop treatments in terms of N₂ fixation, although the two sets ofP values were different. Nonetheless, seasonal estimates of N₂ fixation during the entire 147 days of legume growth determined from ureide analyses indicated that equivalent amounts of N could be fixed by ricebean in a 75∶25 intercrop and in monoculture despite the former being planted at one-quarter the density.     

Asymbiotic nitrogen fixation in the litter and surface soil of a Pinus laricio Poiret forest (Sila,Calabria, Italy)

Abstract

Nitrogen fixation was measured in a Corsican pine (Pinus laricio Poiret) forest in Calabria (Southern Italy). Acetylene reduction activity (ARA) and CO₂ production levels were determined by incubation of litter and superficial (0–5 cm) soil layer samples in the field, at monthly intervals. ARA variations were not correlated to those of substrate moisture, air temperature and microbial respiration. In fact N₂ fixation presented phases of different intensity which irregularly followed each other. Both litter and soil showed similar rates of N₂ fixation. Based on a C₂H₂:N₂ ratio of 3:1 0.8 Kg N ha^–1 y^–1 in each layer are fixed in the Pinus laricio forest, thus contributing to the N status of the soil in this nutrient–poor forest.     

Optimising biological N2 fixation by legumes in farming systems

Whether grown as pulses for grain, as green manure, as pastures or as the tree components of agro-forestry systems, the value of leguminous crops lies in their ability to fix atmospheric N₂, so reducing the use of expensive fertiliser-N and enhancing soil fertility. N₂ fixing legumes provide the basis for developing sustainable farming systems that incorporate integrated nutrient management. By exploiting the stable nitrogen isotope ¹⁵N, it has been possible to reliably measure rates of N₂ fixation in a wide range of agro-ecological field situations involving many leguminous species. The accumulated data demonstrate that there is a wealth of genetic diversity among legumes and their Rhizobium symbionts which can be used to enhance N₂ fixation. Practical agronomic and microbiological means to maximise N inputs by legumes have also been identified.     

Short-term effects of a dung pat on N2 fixation and total N uptake in a perennial ryegrass/white clover mixture

The short-term effects of a simulated cattle dung pat on N₂ fixation and total uptake of N in a perennial ryegrass/white clover mixture was studied in a container experiment using sheep faeces mixed with water to a DM content of 13%. We used a new ¹⁵N cross-labelling technique to determine the influence of dung-pat N on N₂ fixation in a grass/clover mixture and the uptake of dung N in grass and clover. The proportion of N in clover derived from N₂ fixation (%Ndfa) varied between 88–99% during the 16 weeks following application of the dung. There was no effect of dung on the %Ndfa in clover grown in mixture, whereas the %Ndfa in clover grown in pure stand decreased (nominal 2–3%) after dung application. Dung did not influence the amount of N₂ fixed, and the uptake of dung N in grass and clover proceeded at an almost constant rate. After 16 weeks, 10% of the applied dung N was taken up by grass and clover, 57% had been incorporated in the soil by faunal activity and 27% remained in residual dung on the soil surface. The dung N unaccounted for (7%) was probably lost by ammonia volatilisation and denitrification. The uptake of dung N in grass/clover mixtures in the field was similarly followed by using simulated ¹⁵N-labelled dung pats. The total dry matter production and N yields increased in the 0–30 cm distance from the edge of the dung patch, but the proportion of clover decreased. Thirteen months after application of the dung 4% of the applied dung N was recovered in the harvested herbage, 78% was recovered from the soil and the residual dung, and 18% was not accounted for. It is concluded that N₂ fixation in the dung patch border area in grass/clover mixtures is not influenced directly by the release of N from dung pats in the short term. However the amount of N₂ fixed may be reduced, if the growth of clover is reduced in the patch border area.     

Estimation of symbiotic N2 fixation in an Amazon floodplain forest

Sustainable management for existing Amazonian forests requires an extensive knowledge about the limits of ecosystem nutrient cycles. Therefore, symbiotic nitrogen (N₂) fixation of legumes was investigated in a periodically flooded forest of the central Amazon floodplain (Várzea) over two hydrological cycles (20 months) using the ¹⁵N natural abundance method. No seasonal variation in ¹⁵N abundance (δ ¹⁵N values) in trees which would suggest differences in N₂ fixation rates between the terrestrial and the aquatic phase was found. Estimations of the percentage of N derived from atmosphere (%Ndfa) for the nodulated legumes with Neptunia oleracea on the one side and Teramnus volubilis on the other resulted in mean %Ndfa values between 9 and 66%, respectively. More than half of the nodulated legume species had %Ndfa values above 45%. These relatively high N gains are important for the nodulated legumes during the whole hydrological cycle. With a %Ndfa of 4–5% for the entire Várzea forest, N₂ fixation is important for the ecosystem and therefore, has to be taken into consideration for new sustainable land-use strategies in this area.     

Growth and N2 fixation in mixed cropping of Medicago arborea and Atriplex halimus grown on a salt-affected soil using a 15N tracer technique

Abstract

The objective of this study was to evaluate dry matter (DM), nitrogen yield, N₂ fixation (Ndfa) and soil N uptake (Ndfs) in the shrubby medic (Medicago arborea) and saltbush (Atriplex halimus) grown in pots either solely or in a mixture on a salt-affected soil, using ¹⁵N dilution method. The combined DM of both species was considerably higher than that of solely grown shrubs. The inclusion of saltbush in the mixed cropping system decreased Ndfs by shrubby medic and enhanced % Ndfa without affecting amounts of N₂ fixed. It can be concluded that the use of mixed cropping system of shrubby medic and saltbush could be a promising bio-saline agricultural approach to utilize salt affected soils in terms of forage yield and N₂ fixation.     

Survival and colonization of inoculated bacteria in kallar grass rhizosphere and quantification of N2-fixation

Two experiments have been conducted, one in semi-solid Hoagland nutrient medium and the other in shallow pots containing saline soil. N₂-fixing bacteria belonging toAzospirillum, Azotobacter, Klebsiella andEnterobacter were inoculated separately on kallar grass grown in semi-solid nutrient medium. It was shown that inoculation affects root proliferation and also results in¹⁵N isotopic dilution. The % Ndfa ranged from 47–70 whereas no significant effect on the total nitrogen uptake was observed. The bacterial colonization of the root surface and the presence of enteric bacteria inside the root hair cells is reported. In a soil pot experiment, non-N₂-fixingPolypogon monspeliensis was used as a reference plant (control). A treatment receiving a high rate of nitrogen was also used as a non-N₂-fixing control.¹⁵N-labelled ammonium sulphate at 20 kg N ha^–1 and 90 kg N ha^–1 was used. The % Ndfa in the aerial parts of kallar grass was 12–15 whenP. monspeliensis was used as reference plant whereas 37–39% Ndfa was estimated when the treatment receiving high nitrogen fertilizer was used as a non-N₂-fixing control. These investigations revealed some problems of methodology which are discussed.