Wednesday, June 12, 2013

Soil Health: The Realtionbship Between Soil, Bacteria, and Plants




 

The soil has diverse communities of organisms, as well as complex biological functions, which enable plants and crops to grow. In addition, people use pesticides in order to get rid of pests that can harm the plants’ health. Its use influences the microbes around it, and has consequences for the soil. Herbicides, pesticides, as well as other chemicals and pollutants, can disrupt biological functions in the soil, and can even reduce the amount of CO2 in it (Mishra 2004). Even worse, if these accumulate in the soil, it degrades it at a later point. Currently, scientists believe that using bacteria and nutrients can help plants and soil against damage by chemical pollutants, which would enable plants and crops to keep growing healthy.

According to Upasan Mishra and Dolly Wattal Dhar’s paper, “Biodiversity and Biological Degradation of Soil”, pesticides can reduce the amount of CO2 production, degrade cellulose, and can inhibit biological function in the soil. For example, at normal doses, amitrole, 2,4-DB, and diallate can inhibit nitrification for about eight weeks (Mishra 2004). This is a longer inhibition period than atrazine, bromacil, picloram, and simzine, but that doesn’t mean that these four pollutants are nay better. Damage to the soil can also inhibit the plant from absorbing nutrients available in the soil, if they have not died already. Repairing the damage done to it would take a long time, but even if this were accomplished, the soil would never return to its original and pristine state. 

 

A study from 2012 shows that even dead bacteria is important for the soil because its converted into soil components. It was commonly assumed that organic components in the soil were made from decomposed plants, but the truth is that degraded plant material is initially convereted into microbial biomass, that then provides source for soil organic matter (Arnhold 2012). This is imortant because part of the reason why CO2 escapes unto the atmostphere is because of the degraded organic material in soils. This will affect climate, which will also have an effect on soil fertility. This fact emphasises the imporatnce of keeping soil healthy. To prevent this from happening, its important to ensure that soil is healthy enough for plants and crops to grow in. Since long term effects from soil pollutants inhibit biological processess, and has the potential to damage soil, we must look at other alternatives to keep soil fertile and healthy, even if its no longer in its pure state. However, to accomplish this, soil’s relationship with plants, bacteria, and nutrients needs to be understood.

Strategies for repairing damaged soil, and to grow healthy plants, all have one thing in common; the relationship between plants and the soil. This relationship is basically the plants interacting with the soil whether it’s the plant absorbing nutrients from the soil, or building immunity against bad bacteria in soil. In Amy Coombs’ article, “Fighting Microbes with Microbes”, an experiment with tomato plants is used to explain a mechanism of the relationship in action. During the experiment, tomato plants were covered with plastic bags for a time; part of the tomato plants, inoculated with the Alternaria solani fungus, had their fruit rot. Meanwhile, the rest, that were not inoculated, were able to build a defense against the fungus, and made enzymes that would fight against the fungus. Why?


An interaction between the soil and a different fungus was found to a possible cause; the Glomus Mosseae, which attaches itself to the plant’s root hair, forming a symbiotic relationship with the plant. In other words, this warns the plant, through the roots, to create defenses against Alternaria solani. Researchers are certain that, based on this discovery, “the mycorrhizal network can extend from one set of plant roots to another, it’s possible that the network of fungal mycelia acts like telephone wires, allowing the plants to communicate underground” (Coombs, 2013). However, this is currently just a hypothesis; not much is known to researchers about the specific details of this mechanism, so how this relationship works is not completely understood yet. If this hypothetical communication were understood and proven to be correct, then scientists would use this as a way to prevent disease in plants by cultivating a mix of good microbes into the soil to accomplish this goal. The benefit from this would be that the amount of pesticides, and other chemical products generally used in soil, would be reduced. This in turn would reduce, or prevent, further damage to the soil by pollutants.

A research paper by Richardson et al, “Plant and Microbial Strategies to Improve Phosphorous Efficiency of Agriculture”, suggest a different alternative. It outlines three possible strategies where  plants and microorganisms could possibly improve phosphorous efficiency “(i) Root-foraging strategies that improve P acquisition by lowering the critical P requirement of plant growth and allowing agriculture to operate at lower levels of soil P; (ii) P-mining strategies to enhance the desorption, solubilisation or mineralisation of P from sparingly-available sources in soil using root exudates (organic anions, phosphatases), and (iii) improving internal P-utilisation efficiency through the use of plants that yield more per unit of P uptake” (Richardson et al 2011). The Richardson team believes that P-efficient plants can be created if “architecture” and root growth is modified, through root manipulation or by managing mycorrhizal fungi and microbial inoculants.

However, the success from the genetically manipulated plants to enhance nutrient secretion from roots, is very difficult to repeat outside controlled lab procedures due to variable results given after being evaluated in soil. The team concludes that understanding “trait interactions and the ecophysiology of the rhizosphere is emerging as an important factor in development of improved plants via these P-efficiency routes”, because it’s important to consider this when trying to breed plants for P-efficieny (Richardson et al). According to the article, these novel P-efficiency plants (with better root traits and extracting genotypes) can be beneficial in an agro-ecological and socio-economic system level, as it can lessen P-depletion.

In another study, the “Using soil bacteria to facilitate phytoremediation” paper by Bernard R. Glick, explored the possibility of using soil bacteria with plants to remove pollutants from the soil. Here, pythoremoderation (where plants are used to remove pollutants from soils and water) would be assisted by soil bacteria to work against organic and metallic contaminants. Glick hopes that not only could phytoremediation be facilitated, but future field research studies as well when dealing with toxic contaminants and pollutants (Glick 2010).

Unfortunately, there were complexities during the experiment that made it difficult to create an ideal set of conditions that can work for all phytoremediation experiments. These ranged from plant type, added bacteria, and soil composition, to conditions of contaminants found in soil and temperature range. There was also a drawback; while the method was used against metallic and organic pollutants inside controlled laboratory conditions, it has not been tried for metallic pollutants outside of them (Glick 2010). This is worrisome, because metallic pollutants are far more harmful to the soil than organic ones. Another unfortunate bit; some of the plants could not produce enough biomass inside these controlled conditions to be considered efficient in the field. Glick concludes that; 1.) “obstacles” in the experiment are traceable, 2.) it is important to address problems of metal bioavailability in contaminated soils, and 3.) to use phytoremediation, assisted by bacteria, on a larger scaled environment of organic and/or metal pollutants. All which will require a better understanding of the relationship between, soils, plants, contaminants, and bacteria with each other, as well as with phytoremediation assisted by bacteria.

Scientist researching these alternative methods to improve soil health look very promising, but unfortunately for them to be successful, and for the use in chemicals to be reduced, soil’s relationship with plants needs to be understood. This is because it will be beneficial in the long run, both health wise and economically.


Sources:

Arnhold, Tilo. "Fertile soil doesn't fall from the sky. The contribution of bacterial remnants to soil fertility has been underestimated until now." Research for Environment. Hermholtz Centre for Environmental Research-UFZ, 14 Dec 2012. Web. 29 May 2013. <http://www.ufz.de/index.php?en=31184>.

Coombs, Amy. "Fighting Microbes with Microbes." Scientist. 1 Jan 2013: n. page. Web. 29 May. 2013. <http://www.the-scientist.com/?articles.view/articleNo/33703/title/Fighting-Microbes-with-Microbes/>.
Glick, Bernard R. "Using soil bacteria to facilitate phytoremediation." Biotechnology Advances. 28.3 (2010): 367–374. Web. 29 May. 2013. <http://www.sciencedirect.com/science/article/pii/S0734975010000212>. 
Mishra, Upasana, and Dolly Wattal Dahr. "Biodiversity and Biological Degradation of Soil." Resonance. (2004): n. page. Web. 29 May. 2013. <http://www.ias.ac.in/resonance/Jan2004/pdf/Jan2004p26-33.pdf>. 
Richardson, Allan E., , et al. "Plant and microbial strategies to improve the phosphorus efficiency of agriculture." International Journal on Plant-Soil Relationships. (2011): n. page. Web. 29 May. 2013. <http://link.springer.com/article/10.1007/s11104-011-0950-4/fulltext.html>.

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