Magnitude Of Nitrogen Fixation Research Articles

Toward resolving disparate accounts of the extent and magnitude of nitrogen fixation in the Eastern Tropical South Pacific oxygen deficient zone

AbstractExamination of dinitrogen (N2) fixation in the Eastern Tropical South Pacific oxygen deficient zone has raised questions about the range of diazotrophs in the deep sea and their quantitative importance as a source of new nitrogen globally. However, technical considerations in the deployment of stable isotopes in quantifying N2 fixation rates have complicated interpretation of this research. Here, we report the findings of a comprehensive survey of N2 fixation within, above and below the Eastern Tropical South Pacific oxygen deficient zone. N2 fixation rates were measured using a robust 15N tracer method (bubble removal) that accounts for the slow dissolution of N2 gas and calculated using a conservative approach. N2 fixation was only detected in a subset of samples (8 of 125 replicated measurements) collected within suboxic waters (< 20 μmol O2 kg−1) or at the oxycline. Most of these detectable rates were measured at nearshore stations, or where surface productivity was high. These findings support the hypothesis that low oxygen/high organic carbon conditions favor non‐cyanobacterial diazotrophs. Nevertheless, this study indicates that N2 fixation is neither widespread nor quantitatively important throughout this region.

The representation of cyanobacteria life cycle processes in aquatic ecosystem models

Two simplified versions of a numerical life cycle model for diazotrophic cyanobacteria (of the order Nostocales) are developed and evaluated. Both consider two-life cycle stages, one growing, nitrogen-fixing stage and one stage that combines the resting, germinating and vegetative stages. The versions differ in the vertical resolution of the non-diazotrophic stage: version 1 collects the biomass in one layer at the bottom, version 2 considers sinking and rising of biomass explicitly. The results of the two versions are compared with a complex cyanobacteria life cycle model which describes four different life cycle stages each with two internal quotas for energy and nitrogen. The two simplified approaches show a good agreement with respect to the main characteristics of cyanobacteria dynamics (timing and duration of blooms, magnitude of nitrogen fixation, interannual variability). Our model study shows that both simplified approaches are suitable to be implemented into three-dimensional coastal or lake models.

Magnitude of Nitrogen Fixation by Lentil at Different Rates of Phosphorous Using 15N Technique

AbstractTwo lentil cultivars, UJ1 and ILL, have been introduced into the farming system of the Middle East. The influence of P on their potential to fix N2 under drought conditions is lacking. A factorial field experiment was carried out at Taibeh (500 mm yr−1) and Muru (300 mm yr−1), where three rates of P, two lentil cultivars and barley were included. Phosphorus was the main plot, while lentil and barley were grown randomly in the subplots. A typical experiment treated with unlabeled 100 kg N ha−1 with similar P rates was conducted at the Taibeh site. Both cultivars on each site did not differ significantly at different levels of P regarding the biological yield. At each P level, both cultivars derived similar nitrogen percentages from atmosphere (per cent Ndfa), except at Taibeh with the intermediate rate of P, where ILL derived (66.1 %) compared to UJ1 (40.3 %). At Taibeh, the average percentages of N in the grain and straw were 4.17 % and 1.14%, respectively, and were significantly higher than at the Muru site (3.38 %, 1.29 %). The relatively drought‐like conditions at Muru reduced percentage Ndfa to ∼28 but this was increased by P addition. Nitrogen addition reduced partitioning of N (N index) from ∼0.70 % to ∼0.55 % and decreased P percentage in the grain from ∼0.40 % to ∼0.31 % and in straw from 0.11% to 0.07 % due to early maturation. In spite of the indigenous Rhizobium efficiency to fix N2, only 52.0 % and 42.3 % of the plant N was derived from the atmosphere at Taibeh and Muru, respectively, causing depletion of soil N reservoir.

Effectiveness of inoculation on biological nitrogen fixation and water consumption by lentil under rainfed conditions

The magnitude of nitrogen fixation and the export of plant N, to the grain and straw, is important to assess the potential of lentil to contribute N to the subsequent cereal in a crop rotation. A field experiment was conducted for 2 consecutive years on a clay soil (very fine, smectitic, thermic, Typic Chromoxerert) at Moshagar, Jordan, in order to assess the effect of inoculation on biological N-fixation and yield of two lentil cultivars, and to determine their water consumption. The study included two lentil cultivars (UJ1, ILL), two rhizobial inoculants (719, 735) and one rate of nitrogen fertilizer. The results showed that UJ1 and ILL cultivars obtained 80 and 83% respectively of their N requirements from the atmosphere. The indigenous strains nodulating lentils were as efficient in N-fixation as the selected inoculant strains. Only in terms of dry matter (grain and straw), was there a significant lentil cultivar effect, UJ1 being more productive than ILL, under all conditions. The UJ1 cultivar exported more N into the grain, straw, and the whole shoot. The nitrogen harvest index was higher with the indigenous rhizobial strains under heat-stressed conditions than with the inoculant strains. Cropping the land with lentil under existing practices would deplete the soil N by 2–11 kgNha −1. Water consumption differences between lentil cultivars were insignificant and were not affected by inoculation.

Significance of nitrogen fixation in fish ponds

The magnitude of nitrogen fixation in an inorganic fertilized and cereal fed system (fish pond) was measured by an in situ application of the acetylene reduction method. The effects of inorganic fertilization were intensively examined. In comparison with natural lakes, the fish ponds exhibited a relatively low fixation rate. The midday nitrogen fixation rate was usually less than 30 μg at N·m −2·h −1. The total income was equal to 5.7 kg N·ha −1 or 0.575 g N·m −2 for the growing season. Fish-induced bioperturbation and nitrogen fertilization, in the form of inorganic and organic fertilizers and foods, are considered to be the major regulatory factors in nitrogen fixation.

Physiology and chemical composition of nitrogen-fixing phytoplankton in the central North Pacific Ocean

The magnitude and physiological characteristics of biological nitrogen fixation have been studied in the oligotrophic waters of the North pacific gyre. The filamentous blue-green algae Trichodesmium spp. and Richelia intracellularis were the important nitrogen-fixing phytoplankton. Most of the nitrogen fixation occurs in the upper 40 m of the water column, with detectable fixation as deep as 90 m, which corresponds to about the 1 % light depth. There was no evidence of photoinhibition of nitrogen fixation, although CO2 reduction was depressed slightly at the highest light levels. The rate of nitrogen fixation in the water column varied throughout the day, being highest in mid-morning and in late afternoon. Relatively high fixation rates were also found during periods of darkness. Elevated oxygen concentrations had a marked inhibitory effect on rates of nitrogen fixation, a pO2 of 0.4 atm causing a 75% inhibition. Data from studies of nitrogen fixation and assimilation rates of 15N-labelled nitrate, ammonium, and urea indicate that nitrogen fixation furnished about 3% of the total daily fixed nitrogen requirement for phytoplankton growth. Studies with isolated colonies of Trichodesmium spp. indicated that 100% of their nitrogen requirement was met by nitrogen fixation. Chemical composition of the Trichodesmium colonies showed that the C:N ratio was 4.1 and that their phosphorus content relative to carbon or nitrogen was much lower than that of the total particulate material in the water column. Elevated ratios of carbon: adenosine triphosphate (ATP) also suggest that phosphorus deficiency may be limiting the growth of Trichodesmium. The magnitude of nitrogen fixation in the gyre is seasonally dependent, with high rates in late summer and autumn. At these times the water column is stratified, with phosphate and nitrate barely detectable in the upper 100 m. Our data suggest that during these months of stratification, biological fixation of nitrogen amounts to about 33 μg-at N/m2/day.