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.

Patterns of nitrogen utilization in the soybean.

Abstract: The patterns of nitrate uptake, nitrate reductase activity in the leaves, and nitrogen fixation by the nodules were investigated in field-grown soybeans (Glycine max (L.) Merr.) over the growing season. The level of nitrate-reductase activity generally paralleled the concentration of nitrate in the leaf tissue over the entire growing season. A precipitous drop in both parameters was noted within 2–3 weeks after flowering. These parameters decreased by 80–95% at mid-pod fill, a stage where ovule (seed) development was in the logarithmic growth phase, placing a heavy demand on the plant for both energy and fixed nitrogen. The activity of nitrogen fixation of soybean root nodules bore a reciprocal relationship to that of nitrate reductase. The maximum levels of nitrogen fixation were reached at early pod fill when nitrate reductase activity had dropped to 25% of maximum activity. A rapid loss of nitrogen fixation activity occurred shortly after bean fill was initiated, again at a time when the ovules were developing at maximal rates. The total protein content of soybean leaves increased over the season to a maximum level at mid-pod fill. This was followed by a 50% drop over the next 3-week period when the plants approached senescence. This drop corresponded to that found for nitrogen fixation. A similar pattern was noted for watersoluble proteins in the leaf. These studies suggest that there is a close and competitive relationship between the processes of nitrate reduction and nitrogen fixation, with the latter process dominating as the major source of fixed nitrogen after the plants have flowered and initiated pods. At this transitional stage, both soil and environmental effects could cause pertrubation in these processes that could lead to a nitrogen stress causing flower and pod abscission. The rapid decay of nitrogen fixation at the time of midpod fill also suggests a competition between roots (nodules) and pods for available photosynthate. This competition appears to lead to the breakdown of foliar proteins and senescence.

Sulphate reduction and nitrogen fixation rates associated with roots, rhizomes and sediments from Zostera noltii and Spartina maritima meadows.

Abstract: Sulphate reduction rates (SRR) and nitrogen fixation rates (NFR) associated with isolated roots, rhizomes and sediment from the rhizosphere of the marine macrophytes Zostera noltii and Spartina maritima, and the presence and distribution of Bacteria on the roots and rhizomes, were investigated. Between 1% and 3% of the surface area of the roots and rhizomes of both macrophytes were colonized by Bacteria. Bacteria on the surfaces of S. maritima roots and rhizomes were evenly distributed, while the distribution of Bacteria on Z. noltii roots and rhizomes was patchy. Root- and rhizome-associated SRR and NFR were always higher than rates in the bulk sediment. In particular, nitrogen fixation associated with the roots and rhizomes was 41-650-fold higher than in the bulk sediment. Despite the fact that sulphate reduction was elevated on roots and rhizomes compared with bulk sediment, the contribution of plant-associated sulphate reduction to overall sulphate reduction was small (< or =11%). In contrast, nitrogen fixation associated with the roots and rhizomes accounted for 31% and 91% of the nitrogen fixed in the rhizosphere of Z. noltii and S. maritima respectively. In addition, plant-associated nitrogen fixation could supply 37-1,613% of the nitrogen needed by the sulphate-reducing community. Sucrose stimulated nitrogen fixation and sulphate reduction significantly in the root and rhizome compartments of both macrophytes, but not in the bulk sediment.

Methods for enhancing symbiotic nitrogen fixation

Abstract: Biological nitrogen fixation of leguminous crops is becoming increasingly important in attempts to develop sustainable agricultural production. However, these crops are quite variable in their effectiveness in fixing nitrogen. By the use of the 15N isotope dilution method some species have been found to fix large proportions of their nitrogen, while others like common bean have been considered rather inefficient. Methods for increasing N2 fixation are therefore of great importance in any legume work. Attempts to enhance nitrogen fixation of grain legumes has been mainly the domain of microbiologists who have selected rhizobial strains with superior effectiveness or competitive ability. Few projects have focused on the plant symbiont with the objective of improving N2 fixation as done in the FAO/IAEA Co-ordinated Research Programme which is being reported in this volume. The objective of the present paper is to discuss some possibilities available for scientists interested in enhancing symbiotic nitrogen fixation in grain legumes. Examples will be presented on work performed using agronomic methods, as well as work on the plant and microbial symbionts. There are several methods available to scientists working on enhancement of N2 fixation. No one approach is better than the others; rather work on the legume/Rhizobium symbiosis combining experience from various disciplines in inter-disciplinary research programmes should be pursued.