Microorganisms of the Rhizosphere

The microbial rhizosphere structure is a cylinder of soil particles bonded by fungal secreted adhesive polysaccharides that surrounds active perennial roots of grassland plants growing in intact soils and the cylinder is inhabited by symbiotic resident organisms and frequent regular visiting organisms. The numerous types of rhizosphere microorganisms are organized along a trophic hierarchy with a means of “communication” among the microbes and with the plant. The resident organisms are bacteria, protozoa, and endomycorrhizal fungi and the visiting organisms are nematodes, springtails, and mites, and another resident organism is ectomycorrhizal fungi. The numerous types of bacteria have low carbon content. Bacteria are microscopic single celled saprophytic organisms that collectively consume large quantities of soil organic matter and are major primary producers of the rhizosphere. Increases in biomass and activity of the bacteria trophic level elevates the concentration of respiratory carbon dioxide (CO2) resulting in stimulation of activity in the other rhizosphere organisms. Protozoa are single celled microorganisms that are mainly small amoeba in grassland soils and feed primarily on bacteria.

The slightly larger rhizosphere organisms are mobile and move among various rhizosphere structures. Nematodes are a diverse group of small nonsegmented worms. Most nematodes feed primarily on bacteria or fungi, some feed on protozoa, and some eat other nematodes. Springtails are among the most abundant insect in grassland soils that travel among rhizosphere structures. Minute springtails ingest considerable quantities of soil organic matter in order to eat fungi and bacteria. Mites are small eight-legged arachnids that travel among rhizosphere structures and feed on fungi, nematodes, small insects, and other mites. Mites help distribute fungus spores and bacteria through the soil by carrying them on their exoskeleton.

Two types of fungi are resident organisms of the rhizosphere; Endomycorrhizal fungi and Ectomycorrhizal fungi. Endomycorrhizal fungi are also major primary producers of the rhizosphere and are achlorophyllous saprophytes that live on dead organic matter and cannot fix carbon for energy. Endomycorrhizal fungi develop arbuscules, vesicles, and hyphae within root tissue of the host plant and secrete adhesive polysaccharides that bond soil particles around grass roots forming the structural environment for all rhizosphere organisms. The adhesive polysaccharides also bind soil into aggregates resulting in increased soil pore spaces, increased water holding capacity, and increased rooting depth. Endomycorrhizal fungi also move phosphorus, other macro and micro mineral nutrients, and water through the hyphae to the grass roots for absorption.

Ectomycorrhizal fungi develop a sheath around the grass root with hyphae that do not enter the tissue of the host plant and secrete large amounts of adhesive polysaccharides forming water-stable aggregates in soil that are water permeable but not water soluable. The increased soil aggregation improves soil quality, increases soil oxygenation, increases water infiltration, and decreases erodibility.

The bacteria and fungi are the microflora saprotrophic organisms at the bottom of the food chain and makeup the greatest biomass of the rhizosphere. Both bacteria and fungi contain high proportions of nitrogen in relation to their carbon content. The microfauna trophic level organisms with normal ratios of carbon to nitrogen, consume bacteria or fungi and ingest greater quantities of nitrogen than they need for a balanced diet based on energy (carbon); the excess nitrogen is excreted as ammonium (NH4). The endomycorrhizal fungi can nitrify the excreted ammonium into nitrate (NO3) and pass either form of mineral nitrogen into the grass plant through its endophytic vesicles and arbuscules. The elevated rhizosphere organism activity caused by the increase in available short chain carbon energy exudated from the grass lead tillers following partial defoliation by graminivores results in greater quantities of organic nitrogen mineralized into inorganic nitrogen.

The belowground biogeochemical processes are performed by rhizosphere microorganisms. Grassland soil microflora trophic levels cannot produce their own carbon energy; they lack chlorophyll (achlorophyllous). Also, a large biomass of soil microbes cannot be supported on the small quantities of energy remaining in dead grass roots and leaves. However, grass lead tillers produce large quantities of surplus photosynthate containing short chain carbon energy during vegetative growth stages. This source of carbohydrate energy can be used annually between 1 June and 15 July to feed and sustain a large biomass of rhizosphere microbes. The combined weight of the belowground microorganisms on an acre of grassland should be greater than 960 tons/ac.

Biogeochemical Processes

The indispensable rhizosphere microorganisms are responsible for the performance of the ecosystem nutrient flow activities and for the ecosystem biogeochemcial processes that determine grassland ecosystem productivity and functionality.

Biogeochemical processes transform stored essential elements from organic forms or ionic forms into plant usable mineral forms.

Biogeochemical processes capture replacement quantities of lost or removed major essential elements of carbon, hydrogen, nitrogen, and oxygen with assistance from active live plants and transform the replacement essential elements into storage as soil organic matter for later use.

Biogeochemical processes decompose complex unusable organic material into compounds and then into reusable major and minor essential elements.

The quantity of biogeochemical processes conducted in grassland ecosystems is dependent on the rhizosphere volume and microorganism biomass. Rhizosphere volume and microorganism biomass are limited by access to simple carbohydrate energy. Healthy grass plants produce double the quantity of leaf biomass, capture and fix large amounts of carbon during photosynthesis, and produce carbohydrates in quantities greater than the amount needed for normal growth and maintenance. Partial defoliation of grass lead tillers at vegetative phenological growth stages by large grazing graminivores causes greater quantities of exudates containing simple carbohydrates to be released from the grass tillers through the roots into the rhizosphere. With the increase in availability of carbon compounds in the rhizosphere, the biomass and activity of the microorganisms increases. The increase in rhizosphere organism biomass and activity results in greater quantities of biogeochemical cycling of essential elements.

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