Nitrogen fixation

About: Nitrogen fixation is a research topic. Over the lifetime, 7940 publications have been published within this topic receiving 232921 citations. The topic is also known as: GO:0009399.

Absence of nitrogen fixation in clover grown on soil subject to long-term contamination with heavy metals is due to survival of only ineffective Rhizobium

Abstract: All strains of Rhizobium isolated from ineffective nodules on white clover (> 50 separate isolations) formed in metal-contaminated soils from a field experiment were demonstrated to be wholly ineffective in nitrogen fixation in plant infection tests on N-free nutrient agar. The plasmid profiles of these isolates were all very similar indicating a lack of genetic diversity in the population surviving in high concentrations of heavy metals. Isolates from comparable field plots of uncontaminated soil had a wide diversity of plasmid profiles. Inoculation of white clover at sowing with a large inoculum of effective Rhizobium leguminosarum bv. trifolii on metal contaminated soil resulted in effective N2-fixation. However, if the inoculated soils were held for 2 months in a moist condition before sowing with white clover, N2-fixation was not detected with inoculum doses of 107 cells pot−1 or less but was obvious where a very large inoculum of 1010 cells pot−1 was added. This indicates that white clover rhizobia are unable to survive (or at least unable to remain effective) in the presence of concentrations of heavy metals close to the current Commission of the European Communities guidelines for environmental protection. A rapid method of assessment of the toxic effects of pollutants on populations of Rhizobium spp is described.

Nitrogen fixation in the North Pacific Ocean

Abstract: Nitrogen fixation in the euphotic zone of the ocean was measured by C2H2 reduction and 15N2 incorporation associated with Trichodesmium sp. and also with Richelia intracellularis occurring within the cells of Rhizosolenia styliformis var. longispina, and R. cylindrus. The vertical distribution of N2 fixation activity, N2-fixing species, particulate matter and dissolved nutrients was measured. The effects of light intensity, sample concentration, length of incubation, and nutrient enrichment on the rates of C2H2 reduction were determined. Estimates of the importance of N2 fixation in adding previously uncycled nitrogen to the euphotic zone are given.

Defining new roles for plant and rhizobial molecules in sole and mixed plant cultures involving symbiotic legumes

Abstract: Summary The view that symbiotic legumes benefit companion and subsequent plant species in intercrop and rotation systems is well accepted. However, the major contributions made separately by legumes and their microsymbionts that do not relate to root-nodule N2 fixation have been largely ignored. Rhizobia (species of Rhizobium, Bradyrhizobium, Azorhizobium, Allorhizobium, Sinorhizobium and Mesorhizobium) produce chemical molecules that can influence plant development, including phytohormones, lipo-chito-oligosaccharide Nod factors, lumichrome, riboflavin and H2 evolved by nitrogenase. When present in soil, Nod factors can stimulate seed germination, promote plant growth and increase grain yields of legume and nonlegume crops, as well as stimulate increased photosynthetic rates following plant leaf spraying. Very low concentrations of lumichrome and H2 released by bacteroids also promote plant growth and increase biomass in a number of plant species grown under field and glasshouse conditions. Rhizobia are known to suppress the population of soil pathogens in agricultural and natural ecosystems and, in addition to forming nodule symbioses with rhizobia, the legume itself releases phenolics that can suppress pathogens and herbivores, solubilize nutrients, and promote growth of mutualistic microbes. Phytosiderophores and organic acid anions exuded by the host plant can further enhance mineral nutrition in the system. This review explores new insights into sole and mixed plant cultures with the aim of identifying novel roles for molecules of legume and microbial origin in natural and agricultural ecosystems.