Cold Temperatures Delay Nodulation and Reduce Nitrogen Fixation

The 2009 growing season has been one of the coolest in recent memory. Cool, wet conditions cause numerous problems for soybeans, including slow growth, low pod set, increased disease, and lower yields. Nitrogen fixation was also significantly inhibited or delayed by the cool soil temperatures.

Soybeans are a subtropical species. For optimal symbiotic activity, the soil temperature should be between 25-30°C. Poor nitrogen fixation is most evident in first-time soybean fields. There were numerous first-time fields where ample inoculant was applied, but nodulation did not occur. In other cases, nodulation did occur but not until early-August. Poor nodulation occurred across a wide geography, from North Dakota to Pennsylvania. It also occurred with several different inoculant products, so it was not a product failure. In a few cases, even second-time soybean fields failed to nodulate properly. Biological nitrogen fixation is essential for both first-time fields and fields with a history of soybeans, since it converts gaseous nitrogen in the air (N₂) to a form of nitrogen the plant can use.

How Does Nodulation Occur?

When soybean plants need nitrogen, they secrete chemical signals (flavanoids) into the soil from the roots. These signals are picked up by the rhizobia, which in return send a chemical signal back to the root. The signals sent back are lipochitooligosaccharides (Nod factors) which elicit nodulation in the plant. Within 10-14 days of colonization, a nodule will become visible. The return signal prepares the root for infection by the bacterium. Infection can only occur where root hairs are present. The Nod factor causes root hairs to curl and pick up rhizobia and allows them to invade the root. As the bacterial cells divide, they form a small tumor-like structure called a nodule.

Why Was Nodulation Poor This Year?

There are a number of factors that influence nodulation, nodual growth, and nitrogen fixation. These factors include too much or too little moisture, soil nitrate levels, soil pH, diseases, organic matter, soil temperature, as well as the rhizobial quality. Extremely cool temperatures along with excess moisture are largely to blame for poor nodulation this year.

The onset of N₂ fixation in soil temperatures between 17 - 25 °C was delayed by 2.5 days for each degree decrease in temperature in McGill University research ⁽¹⁾. Below 17 °C, each degree delayed the onset of N₂ fixation by 7.5 days. A root zone temperature of 17 °C seems to be the critical temperature for soybean nodulation and N fixation. By 49 days after inoculation, plants at temperatures between 17 - 25 °C were fixing some nitrogen, but plants at 15 °C were not fixing any nitrogen. A decrease of only 2 °C, from 21 °C to 19 °C, made an important difference in the time to onset of N₂ fixation, total N accumulation within the plant, and overall growth. Other research shows that nodulation can cease when temperatures fall to 10°C ⁽²⁾ and that a root zone temperature of 15 °C restricts both infection and nodule development, and delays the onset of N₂ fixation by 4-6 weeks ⁽³⁾. Plants with a root zone temperature of 15 °C had only fixed 9% of the nitrogen fixed by plants at 25 °C, 6 weeks after inoculation.

This helps us understand why some soybeans did not nodulate until late-July or early-August this year. No-till fields, especially those with large amounts of crop residue, suffered more from a lack of nodulation, because these soils are generally cooler by about 2°C

Soil Nitrate and N Fixation

High nitrate levels also caused some problems. Nodule formation is inhibited by the high soil nitrate levels. If the soybean plant picks up too much nitrogen early in the season, it will delay or prevent nodulation. The reduction of atmospheric N₂ to ammonia is energetically expensive, and costs more photosynthate than simply taking up nitrate. Therefore, the plant will naturally consume nitrates before attempting to nodulate. Nitrogen fertilization (at amounts greater than very small "starter" fertilizer rates) does not pay in soybeans because of the inability to develop and sustain N₂ fixation in the presence of soil nitrates. Applying nitrogen fertilizer simply reduces the amount of N₂ fixed from the air.

What About Next Year?

Temperatures in Ontario in June and July are usually sufficient for proper nodulation, so under normal conditions this will not be a significant problem. In first-time soybean fields, use two inoculant products, such as a peat and a liquid, at the high rate with good coverage. This helps to increase the number of live bacteria available for nodulation. Insecticide / fungicide seed treatments will impact the viability of inoculants. Refer to the inoculant label. In fields that have had a well nodulated crop in the past, shallow spring time tillage can increase soil temperatures. In our 2009 trials, the use of an inoculant also significantly increased the number of nodules, even in fields that had previously grown a well nodulated crop.

1. Zhang F, Lynch D. H, and Smith D.L. (1995) Impact of low root temperatures in soybean on nodulation and nitrogen fixation. Env. And Exp. Botony, Vol 35, no3 pp. 279-285.
   
   
2. Maatthews D.J. and Hayes P. (1982) Effect of root zone temperature on early growth, nodulation and nitrogen fixation in soya beans. F. Agric. Sci 98, 371-376.
   
   
3. Lynch D.H. and Smith D. L. (1993) Soybean nodulation and N₂ fixation as affected by period of exposure to a low root zone temperature. Physiol. Plant. 88, 212-220.