Ecological Interactions and Microorganisms in the Soil Foodweb

Soil microorganisms are involved in a variety of ecological interactions with other microorganisms as well as with plants and soil fauna (Table 36.2). Biological nitrogen fixation—one of the most important natural processes for turning inert atmospheric N2 into N available to crops—results from symbiosis between bacteria in the genera Rhizobium and Bradyrhizobium and legumes, which results in a species-specific infection of the roots. The bacterium resides in nodules found in the roots and oxidizes N, to NH4, which can then be used by the plant. In turn, the plant provides organic C and protection for the bacterium.

Most plants form a mutualistic relationship with filamentous fungi known as mycorrhizae. The fungus infects the roots and transports nutrients and water from the surrounding soil to the plant. The plant, in turn, provides organic C for the fungus. Unlike the symbiotic relationship of N-fixing bacteria, mycor- rhizal associations are much less stringent in terms of the particular plant and fungal species involved. Over 80% of plant species studied have been shown to have a mycorrhizal association (Wang and Qiu, 2006).

There may be competition among heterotrophic microorganisms for carbon substrates which they need for energy and biomass. Most microorganisms are capable of utilizing only a limited number of substrates, such that they should be direct competition among them. Ecological theory predicts that

TABLE 36.2 Example Ecological Interactions among Soil Microorganisms, Plants, and Soil Fauna

Type of Interaction

Example

Organisms Involved

Resources Involved

Symbiosis

N2 fixation

Bacteria and plants

NHj and organic C

Mutualism

Mycorrhizae

Fungi and plants

Nutrients, water, and organic carbon

Competition

Carbon limitations

Microorganisms

Organic C substrates

Predation

Grazing

Protozoa, nematodes, and bacteria

Bacterial C and N

direct competition will eventually result in dominance by a single species. However, the large diversity of microorganisms that carry out the same function in soil, such as starch-degrading bacteria or cellulose-degrading fungi, runs counter to this prediction. This apparent paradox is resolved when we realize that direct competition among soil microorganisms is lessened by the fact that they are separated in space. The variably porous nature of soil allows for microorganisms that carry out the same function to live in different soil pores. This results in greater functional redundancy and ecosystem stability, and underscores the importance of physical soil properties like texture in shaping soil ecosystems.

As we have indicated earlier, fungi and bacteria are subject to predation by microbivorous nematodes and protozoa. Predation affects the size and age distribution of the prey population. In addition, predators excrete part of the N and P found in microbial prey in mineral forms, increasing the availability of nutrients to plants and other microorganisms.

The soil foodweb illustrates the interactions of microorganisms with other microorganisms and with plants and soil fauna (Figure 36.2). Unlike a food chain, where a single interaction takes place among organisms, there is considerable redundancy in the ecological interactions in a foodweb, which results in greater stability in ecosystem function. These interactions result in the cycling of carbon, nutrients, and energy that is essential for the functioning of natural and engineered soil ecosystems (Scheu, 2002). For example, the carbon fixed by plants, and the nitrogen fixed by biological nitrogen fixation flows through the foodweb as a result of interactions among soil organisms that result in decomposition and release of nutrients, allowing for growth and energy of microflora and fauna. Carbon processing by microorganisms results in the formation of stable soil organic matter, which physically stabilizes the soil and improves habitat for plants and microorganisms.

 
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