Nitrogen Fixation in Soil: Environmental Factors | Crop Plants | Botany

The following environmental factors influence N₂ fixation: 1. Carbon-N Ratio 2. Mineral Nutrients 3. Pesticides 4. Weather Factors 5. Calcium (Ca) and pH 6. Carbon Dioxide.

Factor # 1. Carbon-N Ratio:

High amounts of N in relation to C in the soil or other media represses nif expression, thus reducing nodulation and/or nitrog­enase activity. Nodulation and N₂ fixation in legumes are inhibited by available N. While NH₃ greatly decreases N₂ fixation in legumes, it is completely inhibitory to fixation in the free-living organisms.

The repressive effect of available N depends to an appreciable extent on the N₂-fixing species and the environment. Moderate amounts of available N have been reported to benefit the establishment of some legumes. However, the practical benefits from N fertilization of legumes are usually small to nil. For example, Hinson (1975) obtained significantly more tops and roots from N- fertilized soybean in a pot experiment, but the nodule number and mass were reduced.

In field experiments, vegetative growth was greater on N-fertilized plots, but there was no effect on seed yield. However, improved seed yields from added N have been reported on cowpea and garden pea. In general, garden bean (Phaseolus vulgaris), unlike most legumes, responds to N fertilization, if the soil N is low.

Factor # 2. Mineral Nutrients:

Mineral requirements of N₂-fixing organisms are essentially the same as those of other plants. Particular attention should be given to soils deficient in molybdenum, iron, or sulfur, since these are mineral components of nitrogenase.

Nitrogenase activity is also responsive to other macroelements. Lynd et al. (1981) found that nodule growth, nitrogenase activity, and the support enzymes of nitrogenase (e.g., glutamate synthetase, or GOGAT) in hairy vetch were significantly increased by potassium fertiliza­tion.

Legume growth and N₂ fixation is also responsive to phos­phorus. Copper was observed to be necessary for nodulation, probably be­cause of its role in the cytochrome system and oxidative respiration. Generally Rhizobium-legume associations: are sensitive to low pHs, particularly legumes adapted to temperate climates.

Factor # 3. Pesticides:

Certain pesticides, especially mercury fungicides for seed treatment, can reduce the number of N₂-fixing organisms and decrease nodula­tion.

Factor # 4. Weather Factors:

Heat and drought can reduce the population of bac­teria and reduce N₂ fixation, as suggested by the wide spread of reported N yields from legumes. Cold (5°C) reduces N₂ fixation virtually to zero. This effect was primarily due to reduced nodulation rather than to re­duced nitrogenase activity. Pea nodulated well at 26°C but not at 20°C. Temperature effects on N₂ fixation varied greatly, de-pending on Rhizobium-legume association.

Rhizobium-temperate legume associations were effective at temperatures as low as 7°C, whereas tropical associations ceased fixation at temperatures lower than 20°C.

Temperate legume symbiosis has an optimum temperature of 20 to 25°C; that of tropical legumes is about 35 to 40°C. Most temperate legumes have a Mediterranean origin, evolving in a climate characterized by mild, moist winters and dry summers conducive to cool-season growth.

A soil moisture equivalent to 25 to 75% of field capacity was found to be optimum for soybean and alfalfa symbiosis. Nodule mois­ture content must remain above 80% for nodule survival.

With soybean, soil moisture near field capacity was optimum at a high greenhouse temperature, but neither soil moisture nor depth of inoculum placement had an effect at moderate temperatures. Excess moisture or waterlogging gener­ally reduced N₂ fixation, probably by reducing root respiration and ATP pro­duction. Flooding did not reduce nitrogenase activity in Aeschynomene, whereas a moisture deficit did.

Factor # 5. Calcium (Ca) and pH:

The effect of pH on N₂ fixation can be either direct or indirect. Acid soils became devoid of Rhizobium. Also, the nodules formed in acid soils were often from ineffective strains of bacteria. The pH of the medium also directly affects nodule formation; nodules began to appear on roots in a nu­trient solution in 3 to 5 days at a relatively high pH. The pH- sensitive phase in nodulation may be that of root curling prior to infection.

Calcium is essential for plant and nodule meristem growth. The Ca re­quirement for the legume itself is much lower than the requirement for sym­biosis, especially in bacteria-temperate legume associations. Without ade­quate Ca, aberrant growth and aborted nodule meristems occurred. R. meliloti, micro-symbiont for sweet clover and alfalfa, has a high pH requirement.

Factor # 6. Carbon Dioxide:

Normally, the atmosphere of N₂-fixing bacteria is much richer in CO₂ (10× -100×) and lower in O₂ than ambient air. Rhizobia in pure culture required the presence of CO₂ for optimal growth of the culture; a CO₂ content of 4% enhanced N₂ fixation. Conditions favoring good root growth and respiration probably remove the need for additional CO₂, but rhizobia are known to be capable of fixing some CO₂, probably via pep carboxylase.