Drought impact on soil rhizobia populations

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By SHAWN CONLEY

Severe drought conditions limited soybean production (6.2 percent per acre yield decline from 2011) across much of the U.S. in 2012.

Though 2012 will soon be behind us, the implications of the 2012 drought may progress well into the 2013 growing season. One area of concern among growers is the impact of the drought on soil microbial populations; most importantly Bradyrhizobium japonicum.

B. japonicum

Soybean live in a symbiotic (mutually beneficial) relationship with B. japonicum. The soybean plant provides nutrients and a protective growing environment for the rhizobia.

In turn, the rhizobia “fix” atmospheric nitrogen into ammonia (NH3), which can then be used by the soybean plant. For this relationship to exist and benefit the soybean plant, effective nitrogen-fixing bacteria must be present in the soil in relatively high numbers at planting time.

To date, little research has been published that quantifies the impact of soil desiccation (e.g. drought) on endemic B. japonicum populations.

Pena-Cabriales and Alexander (1979) reported a biphasic decline in Rhizobium japonicum numbers in soils undergoing drying. The initial rapid phase of R. japonicum loss coincided with major water loss (simulated drought) whereas the secondary and subsequently slower decline in numbers was governed by the soil water content of specific soils.

Pena-Cabriales and Alexander (1979) also noted that organic matter content provided little protection against R. japonicum desiccation and loss.

In a parallel study, Oso-Afiana and Alexander (1982) reported similar results when comparing strains of R. japonicum and cowpea rhizobia under desiccation. As one would expect, variation exists among Rhizobium spp. as well as individual isolates within species in their response to soil desiccation, though significant losses were evident regardless of species or isolate (Foulds, 1971; Trotman and Weaver, 1995).

Nitrogen

To further complicate the situation, most soybean acres will be planted into corn ground that was subjected to severe drought stress. This suggests that excess nitrogen may be present for the 2013 soybean crop.

In excess situations, soybean will generally utilize the background nitrogen prior to initiating maximum N fixation. This may lead to luxurious early season growth, which in fields with a history of white mold, may cause problems if weather conditions are conducive.

High soil N reserves may also lead to increased lodging. In either case, manage your soybean crop accordingly to minimize risk of white mold or lodging. This can be accomplished through variety selection (e.g. white mold tolerance, short statured soybean cultivars or good lodging tolerance), decreasing seeding rates, and proper scouting to time fungicide applications if needed.

I am not advocating that every soybean acre automatically receives an inoculant treatment, however, the small amount of research that is available on the topic does suggest that growers may expect lowered B. japonicum populations going into the 2013 growing season.

(The author is the Extension soybean and wheat specialist in Wisconsin. This information first appeared in the university’s Soy Report Dec. 19.)

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