Transcript Soil Microorganisms And Their Functions
Happy Critters Healthy Soils
Mark Scarpitti, CCA State Agronomist, Ohio NRCS (740) 653-1500 ext 103 [email protected]
Bacteria
Are single cell organisms (and are therefore microscopic) • There are 100 MILLION to 1 BILLION bacteria in just 1 TEASPOON of soil!!!
What Bacteria Do
• They are decomposers, eating dead plant material and organisms' waste • The bacteria release nutrients Do you know what soil smells like? Actinomycetes, a unique type of bacteria, cause that smell, and it is a good sign of healthy soil.
Actually, people have been smelling soil for many, many years as a way to judge if the land is good for planting.
A Few Important Bacteria
Nitrogen-fixing bacteria form symbiotic associations with the roots of legumes • The plant supplies to the bacteria and the bacteria convert nitrogen (N2) from air into a form the plant can use. • When leaves or roots from the host plant decompose, nitrogen is released in the surrounding soil.
Nodules formed where Rhizobium bacteria infected soybean roots
A Few Important Bacteria
Denitrifying bacteria convert nitrate to nitrogen (N2) or nitrous oxide (N2O) gas Denitrifiers are anaerobic, meaning they are active where oxygen is absent, such as in saturated soils or inside soil aggregates Nitrogen loss can result in yellow looking corn plants
A Few Important Bacteria
Nitrifying bacteria change ammonium (NH4+) to nitrite (NO2-) then to nitrate (NO3-) – a preferred form of nitrogen for grasses and most row crops. Nitrate is leached more easily from the soil, so some farmers use nitrification inhibitors to reduce the activity of one type of nitrifying bacteria.
FUNGI
Fungi are organisms • They are not plants, nor are they animals • They group themselves into strings called hyphae The hyphae then form groups called mycelium. They are less than an 1/32 of an inch wide but can get as long as several meters.
The Good and Bad and the Fungi
They are helpful but can also be harmful
Fungi are helpful
• because they have the ability to break down nutrients that other organisms cannot • Fungi release them into the soil, and other organisms get to use them • Fungi can attach themselves to plant roots. Most plants grow much better when this happens.
• This is a good relationship called mycorriza
The Good and Bad and the Fungi
They are helpful but can also be harmful
Some Fungi can be harmful
• Fungi can get food by being parasites, attaching themselves to plants or other organisms, but offering nothing in return
A Few Important Fungi
Decomposers – saprophytic fungi – convert dead organic material into fungal biomass, carbon dioxide (CO2), and small molecules, such as organic acids. Like bacteria, fungi are important for immobilizing, or retaining, nutrients in the soil.
A Few Important Fungi
Many byproducts of fungi are organic acids:
• Humic-acid rich organic matter is resistant to degradation for hundreds of years Like bacteria, fungi are important for immobilizing, or retaining, nutrients in the soil.
A Few Important Fungi
Mutualists – the mycorrhizal fungi – colonize plant roots. In exchange for carbon from the plant, mycorrhizal fungi bring soil nutrients (phosphorus, nitrogen, micronutrients, and water) to the plant One major group of mycorrhizae, the ectomycorrhizae grow on the surface layers of the roots
MYCORRHIZAL FUNGI IN AGRICULTURE
Mycorrhiza: a symbiotic association between fungi and plant roots
• Fungi can serve as a bridge to bring nutrients to plant roots • Under dry conditions, fungi can bring moisture to the roots they colonize
MYCORRHIZAL FUNGI IN AGRICULTURE
Practices that affect the formation of mycorrhizae: • Mycorrhizal fungi will decline in fallowed fields • Mycorrhizal fungi will increase in fields planted to diverse crops or cover crops • Tillage drastically reduces fungi • Broad spectrum fungicides are toxic to mycorrhizal fungi • High levels of fertilizer reduces inoculation of roots
PROTOZOA
Protozoa are organisms that have only one cell, and are microscopic, but larger than bacteria They are grouped by the ways they move: • • • Amoebae use a psuedo (fake) foot Ciliates have cilia (short hair) and move very fast Flagellates have flagella (whips) and move very fast
PROTOZOA
Protozoa need:
• Bacteria to eat • And water in which to move So moisture determines which types of protozoa will be present and active.
Like bacteria, protozoa are particularly active in the rhizosphere next to roots
Cool Things About
PROTOZOA
Protozoa eat bacteria • • Bacteria have a C:N ratio of about 3:1 Protozoa have a C:N ratio of about 10:1 This means that protozoa do not need all the nitrogen they consume and so excrete nitrogen in a plant available form Bacteria Ingested by an Ameba
WHAT DO PROTOZOA DO?
Protozoa
• Mineralize nutrients • Regulate bacteria populations • Serve as a food source for other organisms • Help suppress disease by feeding on pathogens
NEMATODES
Worms typically 1/500 of an inch (50 µm) in diameter and 1/20 of an inch (1 mm) in length Most nematodes in the soil are not plant parasites. Beneficial nematodes help control disease and cycle nutrients.
Credit: Elaine R. Ingham, Oregon State University, Corvallis
NEMATODES
There are a few species responsible for plant diseases, but not much is known about the majority of nematodes that are beneficial This bacterial-feeding nematode, Elaphonema, has ornate lip structures that distinguish it from other nematodes. Bacterial-feeders release plant-available nitrogen when they consume bacteria.
Credit: Elaine R. Ingham, Oregon State University, Corvallis
WHAT DO NEMATODES DO?
Nutrient cycling:
Like protozoa, nematodes are important in mineralizing, or releasing, nutrients in plant available forms A predatory nematode consumes a smaller nematode.
Credit: Kathy Merrifield, Oregon State University, Corvallis.
Root Feeding Nematode:
WHAT DO NEMATODES DO?
•
Disease suppression:
Some nematodes cause disease • Others consume disease-causing organisms, such as root-feeding nematodes, or prevent their access to roots •
These may be potential biocontrol agents
WHAT DO NEMATODES DO?
Disperse microbes:
Nematodes help distribute bacteria and fungi through the soil by carrying live and dormant microbes on their surfaces and in their digestive systems.
ARTHROPODS
Arthropods are invertebrates - they have no backbone Rather they have a covering called an exoskeleton.
Sowbugs are relatives of crabs and lobsters. Their powerful mouth-parts are used to fragment plant residue and leaf litter.
Credit: Gerhard Eisenbeis and
Wilfried Wichard. 1987. Atlas on the Biology of Soil Arthropods.
Springer-Verlag, New York. P. 111.
WHAT DO ARTHROPODS DO?
Although the plant feeders can become pests, most arthropods perform beneficial functions in the soil plant system
WHAT DO ARTHROPODS DO?
•
Shred organic material
•
Stimulate microbial activity
•
Mineralize plant nutrients
ARTHROPODS IN AGRICULTURE Control pests:
Some arthropods can be damaging to crop yields, but many others eat various root- and foliage-feeders Tillage and insecticide application have enormous effects on non- target species in the food web Intense land use (especially monoculture, tillage, and pesticides) depletes soil diversity As total soil diversity declines, predator populations drop sharply and harmful populations can reproduce exponentially
EARTHWORMS
Of all the members of the soil food web, earthworms need the least introduction Their presence is usually an indicator of a healthy system
Earthworms mix the soil
As they consume organic matter and mineral particles, earthworms excrete casts, a type of soil aggregate.
EARTHWORMS IN AGRICULTURE
Earthworm cocoons
One or two worms will hatch from a cocoon after several weeks. L. terrestris cocoons are about a quarter inch long.
Credit: Clive A. Edwards, The Ohio State University, Columbus
Decomposers
Earthworm pulled plant residue into burrow
EARTHWORMS IN AGRICULTURE
Increase infiltration.
Earthworms enhance porosity as they move through the soil. Some species make permanent burrows deep into the soil.
Bury and shred plant residue
Earthworms eat residue, digest and excrete rich organic matter
Conclusions
• A healthy soil will contain an abundant, diverse population of soil organisms • A diverse healthy population of soil organisms can self regulate and is sustainable
Conclusions
The best tools we have to nurture an abundant, diverse population of soil organisms • No Tillage Cropping Systems (when you till, you kill) • A diverse Conservation Crop Rotation and/or use of diverse Cover Crop mixes • Maintaining a living root year round through the use of a diverse Conservation Crop Rotation and/or use of diverse Cover Crop mixes