Optimizing isotopic measurement of potential free‐living nitrogen fixation in soil

Nitrogen (N) fixation in soils is closely linked to microbially mediated molybdenum (Mo) cycling. Therefore, elucidating the mechanisms and factors that affect Mo bioavailability is crucial for understanding N fixation. Here, we demonstrate that long-term (26 years) manure fertilization increased microbial diversity and content of short-range ordered iron (oxyhydr)oxides that raised Mo bioavailability (by 2.8 times) and storage (by ∼30%) and increased the abundance of nifH genes (by ∼14%) and nitrogenase activity (by ∼60%). Nanosized iron (oxyhydr)oxides (ferrihydrite, goethite, and hematite nanoparticles) play a dual role in soil Mo cycling: (i) in concert with microorganisms, they raise Mo bioavailability by catalyzing hydroxyl radical (HO•) production via the Fenton reactions and (ii) they increase Mo retention by association with the nanosized iron (oxyhydr)oxides. In summary, long-term manure fertilization raised the stock and bioavailability of Mo (and probably also of other micronutrients) by increasing iron (oxyhydr)oxide reactivity and intensified asymbiotic N fixation through an increased abundance of nifH genes and nitrogenase activity. This work provides a strategy for increasing biological N fixation in agricultural ecosystems.

Carbon type and quantity regulate soil free-living nitrogen fixation through restructuring diazotrophic community

Biological nitrogen fixation in barren soils of a high-vanadium region: Roles of carbon and vanadium

Effect of 2,3,6-TBA on [formula omitted] nitrogen fixation in soil

The effect on nitrogen fixation of 100, 200, 300, 400 and 500 μg metolachlor per gram of soil was studied in nonsterile soil incubated under aerobic conditions and sterilized soil inoculated with Azotobacter chroococcum. Metolachlor at concentrations of 400 and 500 μg/g significantly reduced dinitrogen fixation in nonsterile soil and sterile soil inoculated with Azotobacter sp. However, this inhibitory effect on nitrogenase activity disappeared after 24 h, showing that acrobic diazotrophs populations present in the soil can tolerate high concentration of this herbicide without significant changes in their nitrogenase activity.

The rhizosphere contributes disproportionately to free-living nitrogen fixation in subalpine forest soils

5 - Nitrogen Fixation in Soil and Plant Systems

Biological nitrogen fixation is a fundamental component of the nitrogen cycle and is the dominant natural process through which fixed nitrogen is made available to the biosphere. While the process of nitrogen fixation has been studied extensively with a limited set of cultivated isolates, examinations of nifH gene diversity in natural systems reveal the existence of a wide range of noncultivated diazotrophs. These noncultivated diazotrophs remain uncharacterized, as do their contributions to nitrogen fixation in natural systems. We have employed a novel 15N2-DNA stable isotope probing (5N2-DNA-SIP) method to identify free-living diazotrophs in soil that are responsible for nitrogen fixation in situ. Analyses of 16S rRNA genes from 15N-labeled DNA provide evidence for nitrogen fixation by three microbial groups, one of which belongs to the Rhizobiales while the other two represent deeply divergent lineages of noncultivated bacteria within the Betaproteobacteria and Actinobacteria, respectively. Analysis of nifH genes from 15N-labeled DNA also revealed three microbial groups, one of which was associated with Alphaproteobacteria while the others were associated with two noncultivated groups that are deeply divergent within nifH cluster I. These results reveal that noncultivated free-living diazotrophs can mediate nitrogen fixation in soils and that 15N2-DNA-SIP can be used to gain access to DNA from these organisms. In addition, this research provides the first evidence for nitrogen fixation by Actinobacteria outside of the order Actinomycetales.

Veterinary and growth-promoting antibiotics are widely used in animal husbandry and accumulate in manure-fertilized soils. However, the impact of these antibiotics on symbiotic nitrogen fixation is poorly understood. We investigated the effect of the veterinary antibiotics oxytetracycline and sulfamethazine, and a combination of both, on nitrogen fixation in alfalfa ( L.) inoculated with . In a pot experiment, was grown in soils fertilized with fresh manure that contained environmentally relevant antibiotic concentrations (0.2, 2, and 200 mg kg). Nodulation, nitrogen fixation, and nutrient concentrations were determined in the alfalfa plants and soils after 12 wk. Compared with the antibiotic-free control, symbiotic nitrogen fixation increased significantly in soils mixed with manure containing 2 and 200 mg kg oxytetracycline (20.1 and 20.8% increase, respectively) and a mixture of 200 mg kg oxytetracycline and sulfamethazine (12.4% increase). The measured plant- and soil-related parameters failed to explain the observed increase in nitrogen fixation. However, using concentration levels that accurately reflect common agricultural practices, we obtained results that directly contradict other experiments conducted under unrealistically high antibiotic concentrations.

ABSTRACTRaised‐field agricultural systems were exploited extensively by pre‐Columbian Andean civilizations and now are used less extensively by contemporary Bolivian and Peruvian farmers as a method for farming the perennially wet soils of the intermontane pampas of the Andean high altiplano. The raised‐field agricultural systems are linear or semi‐linear raised planting platforms interspersed with shallow, parallel canals filled with water. The elevated planting platforms raise the critical rooting zone of crops such as potatoes, quinoa, or barley above perennially saturated soils. A hypothesized advantage of the raised‐field system is that agricultural production can be increased by exploiting the ability of bacteria and Azolla fililculoides to fix nitrogen in the canals adjacent to the planting platforms.To test this hypothesis, nitrogen fixation was estimated in situ in the water column of canals, in the soils of planting platforms, in the soils of a dry hillside field, and in the sediments and water column of marshes and totora beds. Nitrogen fixation was significantly greater in the canals of the raised‐field systems and in marshes compared to planting platforms and the hillside field. These data support the theory that pre‐Hispanic agricultural prosperity of the Andes, as well as other regions of Central and South America, was partially supported by extensive development of raised‐field agriculture and probable exploitation of nitrogen fixation.

We examined rates of free-living (nonsymbiotic) nitrogen fixation in a successional sequence of forest types in the Piedmont of North Carolina. Annual nitrogen input through this process did not differ among ecosystems, averaging roughly 0.002 kg/ha annually. The rate in a 20- to 30-year-old stand did not increase with irrigation or additions of nitrogen, phosphorus, or molybdenum. Free-living nitrogen fixation in these forests should not be expected to compensate for nitrogen losses through natural processes or management activities. For. Sci. 33(2):548-551.

Introduction. Mass Spectrometry. Optical Emmission Spectrometry. Natural Abundance of 15N: Fractional Contribution of Two Sources to a Common Sink an Use of Isotope Discrimination. Nitrogen Fixation in Soil and Plant Systems. Nitrogen Fixation in Flooded Rice Soils, Aquatic and Sediment Systems. Nitrification and Dentrification. Mineralization and Assimilation in Soil and Plant Systems. Mineralization and Assimilation in Aquatic, Sediment, And Wet Land Systems. 13N Techniques. Chapter References. Index.

Biological nitrogen fixation was studied on Truelove Lowland, Devon Island, N. W. T., in three different habitats. Raised beach ridges, resulting from postglacial uplift, form well-defined xeric habitats, which are dominated by dwarf shrubs and cushion plants. Hummocky sedge meadows with associated mosses and forbs comprise the second intensively studied habitat. Waterlogged wet sedge meadows with moss were studied less intensively.Nitrogen fixation was estimated using acetylene-reduction assay. Incubation temperatures were moderated by burying the jars in soil pits (10 cm) on site.Available nitrogen was determined by microkjeldahl analysis of KCl soil extracts.Symbiotic nitrogen fixation by vascular plants is nil on Truelove Lowland. One lichen species, Peltigera aphthosa, reduced acetylene (5.1 μμmol∙mg−1∙h−1). Nostoc commune, a prominent blue-green alga on meadow soils, reduced acetylene at a rate 10 times that of P. aphthosa. An estimated 30 and 7 mg N∙m−2∙year−1 was fixed by bacteria and soil algae in beach ridge soil in 1971 and 1972 respectively, and 380 and 120 mg N∙m−2∙year−1 was fixed in meadow soils in the same years.

Divergent responses of biological nitrogen fixation in soil, litter and moss to temperature and moisture in a karst forest, southwest China

The increasing use of chemical fertilizers causes the loss of natural biological nitrogen fixation in soils, water eutrophication and emits more than 300 Mton CO2 per year. It also limits the success of external bacterial inoculation in the soil. Nitrogen fixing bacteria can be inhibited by the presence of ammonia as its presence can inhibit biological nitrogen fixation. Two aerobic sludges from wastewater treatment plants (WWTP) were exposed to high ammonium salts concentrations (>450 mg L−1 and >2 dS m−1). Microbial analysis after treatment through 16S pyrosequencing showed the presence of Fluviicola sp. (17.70%), a genus of the Clostridiaceae family (11.17%), and Azospirillum sp. (10.42%), which were present at the beginning with lower abundance. Denaturing gradient gel electrophoresis (DGGE) analysis based on nifH genes did not show changes in the nitrogen-fixing population. Nitrogen-Fixing Bacteria (NFB) were identified and associated with other microorganisms involved in the nitrogen cycle, presumably for survival at extreme conditions. The potential use of aerobic sludges enriched with NFB is proposed as an alternative to chemical fertilizer as this bacteria could supplement nitrogen to the plant showing competitive results with chemical fertilization.