Soil Food Webs
Introduction
Food webs are used to know different energy interactions in a given ecosystem. The soil's food web is very dynamic and the living part of soil and it is very complex and interchanging depending on it's ecosystem. The interactions found in the food web are soil organisms living all or part of their lives in the soil while producing energy and working together with plants to survive. These interactions carried out in the soil by soil organisms and plants are vital to life on Earth [4].
The organisms found in the soil food web carry out a large amount of microbial processes such as decomposition, mineralization, immobilization, respiration, and fixation along with many others. These processes help support above and below-ground plant growth and their processes make nourishment for plants possible.
Biological Interaction
The start of all biological interactions within any soil food web would start with organic matter. Organic matter is a key component when it comes to fueling any interaction in the food web, such as decomposition, and is made up of humus and active organic material. Without any organic matter, microbes will not be present to fuel the plant with essential nutrients. Soil organic matter is the storehouse for the energy and nutrients used by plants and other organisms. Bacteria, fungi, and other soil dwellers transform and release nutrients from organic matter. These microshredders, immature oribatid mites, skeletonize plant leaves. This starts the nutrient cycling of carbon, nitrogen, and other elements [4]. Organic matter is recycled back into the food web when organisms of higher trophic levels die off and are decomposed to recover the nutrients that were used to create food for them in the very beginning.
Environment
There are many different ecosystems in the world and the soil food web is affected by climate, rainfall, management factors, etc. Various chemical factors like pH, CEC, and mineral composition affect how the soil food web responds to certain environments as well. Soil food webs are found in both terrestrial and freshwater/marine sediment soils whether it be grasslands, production fields, wetlands or prairie soils. The interactions carried out in the soil food web are highly important in both systems [3].
Terrestrial
Terrestrial soil food webs can consist of but are not limited to many soil microbe/plant interactions. Terrestrial soil food webs are more adaptive to having many plant exudates and very rich organic matter coming from mainly alive primary producers. Given that the primary producers do their roles, terrestrial soils will have a more diverse community of soil microorganisms that can contribute to each different trophic level in the soil [5].
Freshwater & Marine Sediment
In more aquatic habitats different composition of soil microorganisms are found. Physical and chemical characteristics also differ from terrestrial habitats. Physically, marine sediments will have less bulk density and more soil porosity. Chemically, marine sediments will differ in mineral composition depending on their sediment source, and that will also affect their pH levels. Organic matter and other materials are introduced in a different way and in different concentrations when looking at deep marine sediment and shallow marine sediment.
Interactions
Interactions among soil microorganisms with each other and also their interactions with primary producers can be very different. Host type and diversity of microbial populations can differentiate these activities. These interactions also affect the composition of microbial communities and also the composition of above-ground plant diversity.
Positive
Some interactions found in the soil food web that are both positive to the host plant/microorganism and another microorganism can include mutualism and symbiosis. An example of this would be how the plant gives soil microorganisms nutrients to survive through root exudates, in return the microorganism fix and mineralize vital nutrients that the plant needs in order to survive.
Negative
Interactions can also have a negative effect on either plant/microorganism or a negative effect on both. These would include predation, competition, parasitism, commensalism, synergism or ammensalism [2]. An example of an interaction having both a negative and positive effect would be parasitism. A microorganism could be attached to a plant's roots and be taking nutrients that are vital to the plants health and the plant could be dying which would be having a positive effect for the microorganism, but a negative one for the plant.
Niche
The characteristics that require the soil food web to work well depends on a variety of physical and chemical factors. Some of these factors include temperature, soil water, pH, porosity, bulk density, etc. Neutral pH, moderate temperatures, and soil water at field capacity would have a much higher rate of decomposition and production than a dry desert soil.
Microbial processes
Many processes are conducted within the soil food web in order to keep a proper balance of nutrients available to plants and microbial communities. These processes also contribute to global cycles of carbon and nitrogen and can have ecological effects depending on how much each element is present in a given environment. Each process found in the soil food web is very important when looking at the big picture of global fluxes, and the soil food web and life on Earth would not be possible without each process present.
A depiction of the soil food web would be the soil microbial loop. These microbial processes affect plant/soil interactions and free up nutrients for plants and other microbial nutrient-cycling bacteria. These processes are very important because they are involved in the cycling of nutrients and production of biomass. Microbial loop
Respiration & Photosynthesis
Respiration includes both plant and soil microorganisms. Some plants need to conduct photosynthesis in order to respire oxygen into the air. Photosynthesis in plants also helps take carbon out of the air and stored in the soil, which is a process that is carried out by soil microorganisms called carbon sequestration. Other soil microorganisms like algae, lichens, and photosynthetic bacteria are also important in the sequestration of carbon in the soil.
Decomposition
Decomposition is also a vital component of the soil food web. Decomposition of organic matter, both living and dead, helps make nutrients available for plants and other soil microorganisms to carry out other processes to survive. Also, decomposition is one of the first steps to energy transformation in the soil food web.
Fixation
Fixation of carbon, nitrogen, phosphorus, potassium and other important elements is another important process in the soil food web. Fixation of these minerals can either make them plant-available for uptake and energy use to carry out other processes, or fixation can make an element immobile in order to not hurt the plant/microorganism or to sequester nutrients for later use.
Key Microorganisms
Many microorganisms are present in soil food webs. Different microorganisms like soil fauna, bacteria, archaea, and fungi carry out all of the processes in soil. Also, these microorganisms differentiate plant-soil interactions and the makeup of microbial communities in the soil.
Bacteria
Bradyrhizobium[1] Rhizobia[2] Soil Bacteria[3]
Soil Fauna
Fungi
Current Research
Linking Soil Food Webs to Environmental Change
Current research of soil food webs interaction with environmental change is starting to be understood by agronomists, ecologists, and various other researchers. They are studying different responses of soil microorganisms to environmental changes because of their roles in various ecosystem processes and the sensitivity of various environments. We could use this information to link global carbon fluxes and the detrimental effects of global climate change that could be directly linked to soil organism interactions [6].
Relationships Between Plant Genotype and Soil Microorganisms
This research is beginning to be used in order to demonstrate how microorganisms respond to plant genetic variation. Using a model developed by soil microbiologists; we can evaluate microbial biomass and community composition in relationship to plant genetic variation. This could be used to see if producers could enhance genetic traits so that the microbial community could interact with plants more efficiently. Also, if different genetic variations in plants require different microbial interactions we can examine that effect on plant and soil life [7].
Land Use and Microbial Community Diversity
With this research scientists can study how land management practices can effect soil microbial community structure and its interactions. This can be used to determine if different cultivation practices or fertilizer treatments could effect the microbial community's biomass, structure, and potential function [1].
References
1. Bissett, Andrew, Alan E. Richardson, Geoff Baker, and Peter H. Thrall. "Long-term Land Use Effects on Soil Microbial Community Structure and Function." Applied Sol Ecology 51 (2011): 66-78. Print.
2. Bonkowski, Michael. "Protozoa and Plant Growth: The Microbial Loop in Soil Revisited." New Phytologist 162.3 (2004): 617-31. Print.
3. Coleman, D. C. "The Microbial Loop Concept as Used in Terrestrial Soil Ecology Studies." Microbial Ecology 28.2 (1994): 245-50. Print.
4. Ingham, Elaine R. "Soil Biology." NRCS - Soil Quality / Soil Health. USDA, n.d. Web. 07 Apr. 2013.
5. Krumins, Jennifer A., Dick Van Oevelen, and Martinjn T. Bezemer. "Soil and Freshwater and Marine Sediment Food Webs: Their Structure and Function." BioScience 63.1 (2013): 35-42. Print.
6. Powell, Jeff R. "Linking Soil Organisms within Food Webs to Ecosystem Functioning and Environmental Change." Advances in Agronomy 96 (2007): 307-50. AGRICOLA. Web. 01 Apr. 2013. <http://web.ebscohost.com.proxy2.library.illinois.edu/ehost/detail?sid=3cec86d3-23bd-40e4-b25e-27a48eb44f50%40sessionmgr104&vid=1&hid=126&bdata=JnNpdGU9ZWhvc3QtbGl2ZQ%3d%3d#db=aph&AN=27594669>.
7. Schweitzer, Jennifer A., Joseph K. Bailey, Dylan G. Fischer, and Carri J. Leroy. "Plant-Soil-Microorganism Interactions: Heritable Relationship Between Plant Genotype and Associated Soil Microorganisms." Ecology 89.3 (2008): 773-81. Academic Search Premier. Web. 01 Apr. 2013. <http://web.ebscohost.com.proxy2.library.illinois.edu/ehost/detail?sid=82bf7fe0-89af-4964-bfe4-8e59e9b00956%40sessionmgr111&vid=1&hid=126&bdata=JnNpdGU9ZWhvc3QtbGl2ZQ%3d%3d#db=aph&AN=31877703>.
Edited by Brian Kirk, a student of Angela Kent at the University of Illinois at Urbana-Champaign.
