Soil Biology AGSC 215 THE SOIL IS ALIVE!!!

What is Soil Biology? Study of microbial and faunal activity and ecology in soil. These organisms include earthworms, nematodes, protozoa, fungi and bacteria....

Where do Soil Organisms live? Around roots: rhizosphere is the narrow region of soil directly around roots. In litter, particularly fungi. On humus, only fungi can degrade humus. On the surface of soil aggregates biological activity is greater than within aggregates. In spaces between soil aggregates

Metabolic Grouping Carbon Source Energy Source Heterotrophs (use organic C) Autotrophs (fix CO 2 ) PhotoheterotrophsPhotoautotrophs Light (Photo) ChemoheterotrophsChemoautotrophs Chemical (Chemo)

Metabolic Grouping 1. Producers: autotrophic ("self-feed") organisms; produce the carbon and energy they need. Examples: photoautotrophs (plants, plankton) and chemoautotrophs (sulfur bacteria)

Metabolic Grouping 2. Consumers: heterotrophic organisms; obtain energy and carbon by feeding on the tissues of other organisms. Herbivores, carnivores, and parasites are examples.

Herbivores: eat plants Carnivores: eat animals Omnivores: eat 'a variety of edibles' = plants animals, etc Parasites: extract energy from living hosts Decomposers / detrivores: heterotrophs that get energy and carbon from dead or decomposing organic matter (remains, products, or wastes) of other organisms. Helps cycle nutrients back to producers (fungi and bacteria).

Soil Food Web

Earthworms Hermaphrodites (exhibit both male and female characteristics) Major decomposers of dead and decomposing organic matter Derive their nutrition from the bacteria and fungi that grow upon these materials. Fragment organic matter and make major contributions to recycling the nutrients it contains.

WHAT DO EARTHWORMS DO? Stimulate microbial activity (MO in their feces) Mix and aggregate soil Increase infiltration (enhance porosity) Improve water-holding capacity (via porosity) Provide channels for root growth Bury and shred plant residue

Nematodes Non-segmented worms (d=50µ, L=1mm) Feed on plants and algae; others are grazers that feed on bacteria and fungi, and some feed on other nematodes A predatory nematode consumes a smaller nematode

WHAT DO NEMATODES DO? Nutrient cycling (mineralizing or releasing nutrients in plant-available forms such as NH 4 + ) Grazing Dispersal of microbes (Carrying live and dormant bacteria) Food source Disease suppression and development

Bacteria One-celled organisms (1 µm wide ) A ton of microscopic bacteria may be active in one acre of soil. Bacteria dot the surface of strands.of fungal hyphae

WHAT DO BACTERIA DO? Water dynamics, nutrient cycling, and disease suppression Affect water movement by producing substances that help bind soil particles into small stable aggregates  improve water infiltration and the soil’s water- holding ability

A FEW IMPORTANT BACTERIA Nitrogen-fixing bacteria: –form symbiotic associations with the roots of legumes. –infect a growing root hair  creation of visible nodules –Convert nitrogen (N2) from air into a form the plant host can use.  When leaves or roots from the host plant decompose, soil nitrogen increases in the surrounding area.  Nodules formed where Rhizobium bacteria infected soybean roots.

A FEW IMPORTANT BACTERIA Nitrifying bacteria: –Change ammonium (NH4+) to nitrite (NO2-) then to nitrate (NO3-) Denitrifying bacteria: –convert nitrate to nitrogen (N2) or nitrous oxide (N2O) gas.

A FEW IMPORTANT BACTERIA Actinomycetes: –Grow as hyphae like fungi –Degrade chitin and cellulose Actinomycetes, such as this Streptomyces, give soil its "earthy" smell.

Soil Fungi Microscopic cells Grow as long threads called hyphae (push their way between soil particles, roots, and rocks) Hyphae sometimes group into masses called mycelium or thick, cord-like “rhizomorphs” that look like roots. Fungal fruiting structures (mushrooms) are made of hyphal strands, spores, and some special structures like gills on which spores form.

What do Soil Fungi do? Water dynamics, nutrient cycling, and disease suppression Convert hard-to-digest organic material into forms that other organisms can use Fungal hyphae bind soil particles together, creating stable aggregates that help increase water infiltration and soil water holding capacity.

Fungi Groups 1.Decomposers – saprophytic fungi: convert dead organic material into fungal biomass, carbon dioxide (CO2), and small molecules. 2.Mutualists – the mycorrhizal fungi – colonize plant roots. In exchange for carbon from the plant, mycorrhizal fungi help solubolize phosphorus and bring soil nutrients (phosphorus, nitrogen, micronutrients, and perhaps water) to the plant.

Fungi Groups 2. Mutualists – the mycorrhizal fungi – colonize plant roots. In exchange for carbon from the plant, mycorrhizal fungi help solubolize phosphorus and bring soil nutrients (phosphorus, nitrogen, micronutrients, and perhaps water) to the plant.

The mycorrhizal fungi 1.Ectomycorrhizae: grow on the surface layers of the roots 2.Endomycorrhizae: grow within the root cells Ectomycorrhizae Endomycorrhizae

Fungi Groups 3. Pathogens or parasites : cause reduced production or death when they colonize roots and other organisms ex: Verticillium, Pythium, and Rhizoctonia

Soil Protozoa Single-celled animals that feed primarily on bacteria, other protozoa, soluble organic matter, and sometimes fungi. Larger than bacteria (d = 5 to 500 µm) As they eat bacteria, protozoa release excess nitrogen that can then be used by plants. Notice the size of the speck-like bacteria next to the oval protozoa and large, angular sand particle

Protozoa Groups 1.Ciliates are the largest and move by means of hair-like cilia. They eat the other two types of protozoa, as well as bacteria and release plant available nitrogen. Ciliates use the fine cilia along their bodies like oars to move rapidly through soil.

Protozoa Groups 2. Amoebae also can be quite large and move by means of a temporary foot or “pseudopod.” Bacteria ingested by an amoeba.

Protozoa Groups 3. Flagellates are the smallest of the protozoa and use a few whip-like flagella to move. Flagellates have one or two flagella which they use to propel or pull their way through soil. A flagellum can be seen extending from the protozoan on the left. The tiny specks are bacteria.

WHAT DO PROTOZOA DO? Mineralizing nutrients, making them available for use by plants and other soil organisms. Release the excess nitrogen (obtained from eating bacteria) in the form of ammonium (NH4+). Regulating bacteria populations Are also an important food source for other soil organisms Suppress disease by competing with or feeding on pathogens.