Drought Reduces Soybean Rhizobia Cells in Soil

Drought Reduces Soybean Rhizobia Cells in Soil

January 21st, 2013

In keeping with ABM’s mission of educating the farmer, this blog post was drafted to address the effects of last year’s (2012) drought on Bradyrhizobia populations for the upcoming planting season.

An article published in the August 14- August 20 edition of The Ohio State University C.O.R.N. Newsletter, titled “Survival of Soybean Rhizobia Cells in Soil,” written by Jim Beuerlein and Harold Watters, advises inoculating soybeans and other legume seeds heading into the 2013 planting season due to reduced rhizobia populations following the 2012 drought.

In response to this perfectly logical advice, Dan Custis, CEO of ABM states, “The 2012 growing season was a challenge, to say the least, for the American Farmer. Here in the Mid-West, we experienced the hottest summer on record. This put tremendous pressure on crops above ground and even more pressure on the microbes in the rhizosphere, or root zone. Specifically, the Bradyrhizobia that the soybean depends upon to nodulate the root system were greatly reduced.”

Below are the comments of Dr. David Kuykendall, retired USDA Senior Research Scientist and Senior Consulting Scientist with ABM:

“As an expert I was asked to comment on the advisability of inoculating soybeans in 2013 due to reduced survival of soybean-nodulating Bradyrhizobium japonicum in field soil following the 2012 drought. A recent article by Jim Beuerlein and Harold Watters described how the widespread drought in crop production areas of the central U.S. in 2012 have justifiably caused concern for the crop production forecast for this year, 2013. I agree fully that it seems extremely likely that the dry soil surface conditions and high soil temperatures experienced over broad areas were harsh enough to have dramatically reduced useful populations of beneficial soil bacteria known microbiologically as B. japonicum.

“Soybean-nodulating Bradyrhizobium bacterial cells persist or survive well in moist soil and soil temperatures below 90 degrees F.  The drought in 2012 caused the top half of a foot (6 inches) of soil to stay extremely dry and hot for a prolonged period.  Such stressful environmental conditions cause the death of 99.9% or more of the bradyrhizobia cells.  Therefore, it is reasonable to suppose that normal populations of millions of soybean-nodulating bacteria per ounce of soil could have likely been reduced in population size to only as few as a hundred, or perhaps a thousand, residual soil rhizobia cells remaining per ounce of soil in many Midwestern soybean fields in 2012 due to the drought.

“Although some cells survive such extreme environmental conditions, Bauerlein and Watters reasonably surmise that those cells will have likely drifted genetically into “survival mode” and, as is typical of persistent feral populations of soybean-nodulating bacteria, are likely lacking in potential to reduce nitrogen into a form utilizable by soybean plants. They conclude ‘That means the surviving rhizobia population will likely be less productive next year than in previous years. That reduced productivity should translate into increased yield responses to inoculating soybeans and other legume seeds in the spring of 2013.’ Indeed inoculation of soybeans with a good genetically enhanced commercial inoculant would be particularly well advised this year, more so than any other year that most can recall.” Dr. L. David Kuykendall, Senior Consulting Scientist, Advanced Biological Marketing.

*Dr. L. David Kuykendall is a Consulting Senior Scientist working with ABM in the development of microbiological technologies. His professional career has spanned more than 36 years, 25 of which with the USDA as a Senior Research Scientist Earning a PhD in Microbiology in 1976 at the age of 23, Dr. Kuykendall has authored 107 scientific journal articles and book chapters. His published work has received more than 1,500 literature citations.