Soil and plant mechanisms involved in soybean inoculation

Nodule formation

Nodules form on soybean roots based on a communication process between the host root and bacteria. The process begins in the root with the production and release of compounds (isoflavonoids) which are recognized by the bacteria. The B. japonicum respond by expressing “nod factors” (lipochitooligosaccharides) which eventually lead to root hair modification allowing the B. japonicum to enter. After the bacteria have invaded the root hair, they multiply rapidly. Nodules, which individually house thousands of B. japonicum cells, can be visible within one week after the time of infection.

A well-nodulated soybean plant should have five to seven nodules on the tap root two weeks after emergence. To evaluate nodule performance, cut a nodule in half and examine the interior. If it is pink to bright red, it is actively fixing nitrogen, but if it is white or green, the nodule is ineffective or hasn’t become active yet. Once nodules are formed they can capture atmospheric nitrogen (N2) and convert it to ammonia (NH3). Ammonia proceeds through a series of biochemical conversions within the nodule to a form of nitrogen called ureides. Ureides are then transported to the above-ground plant for use.

Soil factors affecting B. japonicum

The soil environment (pH, temperature, texture, water content and residual nitrogen levels) will strongly impact the ability of B. japonicum to infect soybean root hairs, develop nodules and fix nitrogen. The optimum soil pH is 5-8 and optimum root temperature for nitrogen fixation is 77-86º F. In this temperature range, bacteria can begin actively fixing nitrogen within seven days of forming nodules. In cooler soils, the process will slow down considerably.

Soil type also affects bacteria growth. Soils with lower soil water holding capacity and wider fluctuations in soil moisture won’t be favorable to bacteria growth. Excess soil water is also detrimental to B. japonicum because it reduces the amount of oxygen in the soil which the bacteria needs for survival. If a field is flooded for seven days, nitrogen fixation can be inhibited up to 55%. In certain environmental conditions, such as severe drought or flooding, the plant will increase its proportion of nitrogen derived from fertilizer sources compared to the amount fixed by B. japonicum.

The effect of residual nitrogen in the soil cannot be overemphasized as this plays a significant role in determining the amount of nodules formed as well as the amount of nitrogen fixed throughout the season. Although B. japonicum are efficient nitrogen producers, the soybean still acts as a host and is depleted of energy resources by the bacteria. Thus, if soil residual nitrogen is high, we would not expect to see as many nodules as you would with deficient or normal nitrogen levels.

Insert side note in a box: Throughout the growing season soybean uses large amounts of nitrogen -- up to 315 pounds per acre. At 50 to 75 days after emergence (approximately R2 to R5), the soybean plant derives 60-70% of its needed nitrogen from nitrogen fixation.

Nitrogen is highly mobile in the soybean plant. When it is exposed to nitrogen fertilizer, the productivity and growth of nodules decreases within a day. This impact becomes greater as the rate of nitrogen fertilizer increases. B. japonicum responds to the demands of the above-ground plant by increasing or decreasing nitrogen fixation, depending on how much nitrogen is used by the plant. Understanding why this occurs is important to understanding why applying nitrogen fertilizer is detrimental to nodule formation and nitrogen fixation. In a few instances, yield increases have been observed when nitrogen fertilizer is applied during the growing season; however, under non-severe field conditions, a yield response is not expected from applying nitrogen fertilizer, even during the plant’s reproductive stages.

Lori J. Abendroth
Former Agronomy Research and Extension Associate
Roger W. Elmore
Adjunct Professor of Agronomy, Former Extension Crops Specialist

Copyright 2006 by the University of Nebraska Board of Regents. All rights reserved.

Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.

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