Microbes in Agricultural Soil

Section

Magnified 20,000X, this colorized scanning electron micrograph (SEM) depicts a grouping of methicillin resistant Staphylococcus aureus (MRSA) bacteria. See PHIL 617 for a black and white view of this image. Phoro credit: CDC.

Magnified 20,000X, this colorized scanning electron micrograph (SEM) depicts a grouping of methicillin resistant Staphylococcus aureus (MRSA) bacteria SOIL. See PHIL 617 for a black and white view of this image. Phoro credit: CDC.

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Introduction

Many agroecologists have said that in order to feed an ecosystem you need to feed the soil.Microbes can be found across a variety of environments, including in the soils. In fact, it’s been said that there are more microbes in a teaspoon of soil than there are people on Earth [1]. These microbes serve a multitude of ecosystem functions. Soil microbes are beneficial in determining the nutrient content of food, and this is often done through the transformation of degradable organic compounds to an inorganic, readily available form for crops [2]. Some examples of these microbes include Bacillus, Azotobacter, Microbacterium, Erwinia, Beijerinckia, Enterobacter, Flavobacterium, Pseudomonas, and Rhizobium bacteria, which are all examples of phosphate solubilizers [2]. A phosphate solubilizer turns phosphate, a crucial nutrient, from an inaccessible form to one that is easily taken in and stored by plants. Some other examples include cyanobacteria (Anabaena, Nostoc, Calothrix), Azotobacter, Azospirillum, and Gluconacetobacter, which are nitrogen-fixing endophytes, a type of symbiote [2]. Similar to phosphate solubilizers, nitrogen-fixers convert nitrogen to a form more easily accessible to plants. Many farmers have even turned to the application of microbes to promote and maintain soil health [2]. In addition to microorganisms, earthworms also play a role in soil health. Both microorganisms and earthworms leave castings, the end product of digestion, and residuals that serve to increase plant nutrients [6]. When compared to soil devoid of these, soil that had been shaped by microorganism and earthworm activity showed a significant increase in nutrient levels [6]. The presence and activity of such organisms is crucial not just for plant health but also human and animal health.

The abundance and diversity of species as well as their activity vary drastically depending on the soil environment. The optimal environment for soil organisms is that of a natural and healthy soil, in which the biomass of microbes can amount to 4 to 5 tonnes per hectare [6]. However, soil health and fertility is declining in places as a result of increasing fertilizer use, tillage, and crop protectants [6]. As a result of the soil disruption, populations of soil organisms are subject to decline. Many farmers have begun utilizing sustainable farming techniques that limit fertilizer and soil disruption as well as actively introducing healthy microbes to the soil.

Soil organic Matter

One of the most nutrient rich portions of soil is the soil organic matter (SOM). Not only is SOM beneficial for microbes, but it also acts as a buffer for high acidity and increases water availability within the soil, acting as a sponge and releasing it for plant use [11]. Furthermore, SOM helps decompose excess pesticides and acts as a carbon sink, sequestering atmospheric carbon [5, 11]. SOM is composed of organic substances (those with carbon) in the soil. This includes plants, algae, microorganisms, and decomposing matter left from both plants and microorganisms [1]. The three main components of the SOM are the “living” (microorganisms), the “dead” (fresh residue) and the “very dead” (humus- long decomposed plant and animal substances) [1,11]. These components are then broken down into further classification. The SOM is composed of active (35%) and passive (65%) [1]. Active SOM consists of the “living” and “dead” plant and animal materials. These are composed of easily digested sugars and proteins, acting and food for the microbes within the soil [1]. The active layer contains the microbes that convert nutrients into a form more readily available for plants The passive SOM is unable to be decomposed by microbes and is higher in lignin [1]. Passive SOM is the portion that acts as a carbon sink.

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Section 2

Many of the benefits of soil microbes, including the conversion of nutrients into a readily available form, are a result of a symbiotic relationship. One example is the relationship between the hyphae of soil fungus and plants. The fungi possess a root-like structure, hyphae, that forms a network of structure that helps soil particles bind together, thereby improving both the stability and the structure of the soil for plants [6]. This is not the only example of symbiotic relationship within the soil, many of which involve essential soil microbes.

Mycorrhizal fungi

Another example of a symbiotic relationship within the soil results in nitrogen fixation. Nitrogen fixation is a crucial process that is essential for life and is carried out primarily by nitrogen-fixing bacteria and archaea within the soil [3]. However, this process would not be possible within the symbiotic relationship between soil microbes and arbuscular mycorrhizal fungi. Arbuscular mycorrhizal fungi are also beneficial in getting nutrients to plants. Nitrogen fixing-bacteria and phosphate-solubilizing bacteria work with the mycorrhizal fungi to increase nutrient availability for plants. The bacteria fix the nitrogen, converting it from molecular form to a usable form like ammonia, as well as solubilize phosphorus ions, making them more soluble for plant uptake [2]. Finally, the arbuscular mycorrhizal fungi, which are spread through the soil, transport these nutrients directly to the plants [2]. Without this symbiotic relationship, plant nutrient uptake would not be as successful

Rhizosphere

Similar to the SOM, the rhizosphere is a crucial component of the soil, serving as the nutrient-rich portion surrounding the plant roots. A variety of symbiotic relationships within this portion of the soil aids in plant health. In fact, the aforementioned mycorrhizal fungi extended from within the rhizosphere and throughout the bulk soil. Microbial activity within the rhizosphere performs a number of activities including nitrogen-fixation, nutrient solubilization, suppression of pests and pathogens, tolerance of plant stress, decomposition of organic residue, and the recycling of soil nutrients [4]. Microbes in the rhizosphere establish a number of symbiotic relationships with host plants, including beneficial (like arbuscular mycorrhizal fungi), neutral, and detrimental [2,4]. The symbiotic relationship between rhizobia and legumes is a mutualistic form of symbiosis, benefitting both [3]. Rhizobia, a type of nitrogen-fixing bacteria, can live in the soil on decaying matter or on the root nodules of their host legumes as a symbiont [3]. The rhizobia fix atmospheric nitrogen for their host, converting it to a usable form, while the host supplies rhizobia with the carbon from photosynthesis [3]. Though this is one example of many, the microbes within the soil are crucial components in promoting plant growth. Unfortunately, the microbes are susceptible to disturbance in response to aggressive environmental conditions and farming practices.

Section 3

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References

Authored for BIOL 238 Microbiology, taught by Joan Slonczewski,at Kenyon College,2024