Biofertilizers for Biofuels

12 Aug

With some general information on plant growth promoting rhizobacteria (PGPR) out of the way (see Soil Probiotics post), I wanted to review a nice PGPR article for today, my idea of a good time. 

Today’s article is by Marques, et al., and just came out in Soil Biology and Biochemistry this month (see full citation below) on the specific plant growth promotion abilities of six bacterial isolates on Zea mays – known in most English speaking countries as ‘corn’. The fact that the authors chose corn attracted my attention for a few reasons, one of which pertains to the new push for biofuels from corn. 

While I refuse to open the Pandora’s box of ethical issues on the use of corn for fuel when we have millions of people starving all over the world, I would actually like to consider for a moment just a few of the logistical hurdles.  Corn is a crop that requires a great deal of nutrient input (mostly nitrogen and phosphorus) and has very deep roots requiring subsurface tillage.  These two constraints alone lead to significant fossil fuel input for fertilizer production and land cultivation.  So, when we consider “carbon-inputs vs. carbon outputs”  the benefits of biofuel production with corn are outweighed by the heavy use of fossil fuels in the corn cultivation process.  But what if we could reduce the need for those fossil fuel inputs?

PGPR are known for several characteristics that could address these issues, such as:

  1.  Reducing ethylene production, which allows plants to develop longer roots and establish better during early growth stages (Glick et al., 1998);
  2. Enhancing nitrogen availability by nitrogen fixation (Fuhrmann and Wollum, 1989); and
  3. Solubilizing nutrients such as phosphorus and other trace elements which may be present, but not in forms readily available to the plant (Glick, 1995). 

Unfortunately, certain beneficial fungi and bacteria may be choosy or particular with regard to the plant species to which they confer benefit in this way, for instance a certain Glomus species may enhance biomass production (growth) in a prairie grass, but actually reduce growth in tomato. 

So the authors of this paper set out to find effective PGPR for corn.  They took soil from a heavy-metal contaminated site, which should theoretically give them a set of fairly hardy bacteria, and tested 320 bacterial strains for the above mentioned PGPR characteristics.  Of these 320 isolates, they chose 6 that clearly demonstrated these characteristics in the lab for further testing with corn in the greenhouse. 

Now this is where I run into my only bone to pick with this paper… 

They set up a series of pots in the greenhouse, each pot contained sterile soil and sterilized corn seeds.  They inoculated each ‘treatment ‘ pot with one of their isolates; the control simply consisted of sterilized corn seeds planted in sterile soil.  Above and beyond the fact that “sterilized soil” is an issue to and of itself, they didn’t try their isolates in “native” or unsterilized soil, nor did they try combinations of their chosen organisms.   This results in corn seedlings struggling to survive in sterile soil (control) measured against corn seedlings inoculated with an organism known to possess plant growth promoting characteristics with no microbial competition in the rhizosphere.

Now, I’m not against setting yourself up for success, but in the real world, we would be growing corn seedlings in a soil rife with pathogens and even other natural, native PGPR and the result of the chemical warfare in the corn rooting zone is anybody’s game.  Isn’t the more pressing question then, “Would these newly discovered PGPR from a metal-contaminated site stand the test of microbial competition and still provide growth enhancement for the corn plant?”  Just my thoughts on their experimental design.

PGPR for corn

So, their experiment very neatly demonstrated that almost all these six beneficial bacteria increased root and shoot growth and several also increased foliar phosphorus and nitrogen content over the control. They found positive correlations between the plant and bacterial traits (i.e. bacterial hormone production responsible for increased root elongation was positively correlated with corn root and shoot biomass). 

They clearly discovered PGPR species useful with corn, at least not deleterious to corn biomass production, both above and belowground.  But for their next study, I’d like to see a comparison of consortia (the 100-dollar word for the day, meaning groups working together) of microbes rather than one at a time, and to measure the growth promotion of those against the native microbial communities of a variety of soils, particularly soils that have been cropped to corn for years and years. 

 ___________________________________________________________

Fuhrmann, J.J. and A.G. Wollum (1989).  Nodulation competition among Bradyrhizobium japonicum strains as influenced by rhizosphere bacteria and iron availability.  Biology and Fertility of Soils, 7: 108-112.

Glick, et al. (1998).  A model for the lowering of plant ethylene concentrations by plant growth promoting bacteria.  Journal of Theoretical Biology, 190:63-68.

Glick, B.R. (1995).  The enhancement of plant growth by free living bacteria.  Canadian Journal of Microbiology, 41: 109-114.

Marques, et al. (2010).  Assessment of the plant growth promotion abilities of six bacterial isolates using Zea mays as indicator plant.  Soil Biology & Biochemistry, 42(8): 1229-1235.  doi:10.1016/j.soilbio.2010.04.014

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One Response to “Biofertilizers for Biofuels”

  1. Murk September 9, 2010 at 11:34 am #

    I had been arguing with my close friend on this issue for quite a while, base on your ideas prove that I am right, let me show him your webpage then I am sure it must make him buy me a drink, lol, thanks.

    -Murk

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