Soil Microbial activity and nitrogen

Physical elements {TILTH} Physical elements {TILTH} – e.g. sand, silt, clay, organic material and aggregates (SOIL {1}) Living elements (other than plants) – e.g. nematodes, earthworms, fungi, protozoa, bacteria, insects, spiders, mites… Chemical elements Chemical elements – pH and its effect on nutrients (primary, secondary and micro-elements); water; oxygen ( SOIL {3}) SOIL

NITROgen Nitrate (NO 3 - ) is the form of nitrogen most often utilized by the plants. It has a negative charge so it is not held by clay particles or organic particles, both of which also have a negative charge. It is readily leached out of the soil by rain and irrigation. Microbial activity in a soil with good tilth, keeps a ready supply of nitrogen available.

Microbial activity and nitrogen Nitrogen fixation Nitrogen fixation Nitrogen mineralization Nitrogen mineralization Nitrifying microbes Nitrifying microbes Denitrifying microbes Denitrifying microbes Nitrogen immobilization Nitrogen immobilization

Nitrogen Fixation The conversion of atmospheric nitrogen into ammonium (NH 4 + ).

Nitrogen Fixation Rhizobia are bacteria present in the soil that can take nitrogen from the air and make it available to the plants. Rhizobia are bacteria present in the soil that can take nitrogen from the air and make it available to the plants. – They form nodules on the plants roots and in exchange for providing nitrogen to the plant, they receive essential minerals and sugars.

Rhizobia ~~symbiotic relation~~

Root nodes Rhizobia nodule on soybean Rhizobia nodule on clover

Rootknot nematode ~~parasitic relation~~ Nematodes entering tomato root tip Nematode propped on human hair

Rootknot nematode on tomato plant in St. Tammany.

Good nematodes!! Many live in the large pore spaces in the soil and feed on: Many live in the large pore spaces in the soil and feed on: – Bacteria – Fungi – Other nematodes including rootknot Feed on them directly or Feed on them directly or Prevent the root-feeding nematodes from entering the roots. Prevent the root-feeding nematodes from entering the roots. ( colostate.edu/mg/gardennotes/212.html)

Pink-colored contents of slightly crushed nodules is the result of the protein leghemoglobin. It is only in nodules and is not produced by either the bacteria or plant when grown alone. Leghemoglobin Clover roots AND

Leghemoglobin is a protein that binds oxygen and is red like hemoglobin. Enzymes that produce the conversion of N 2 to nitrates (N0 3 − ) are inhibited by oxygen. Leghemoglobin binds oxygen and allows the N fixation to proceed. Leghemoglobin Clover roots

Symbiotic relation Clover roots Vascular tissue connects the nodule to the xylem and phloem providing nutrients to bacteria in the nodule and carrying away nitrogen for plant use.

Nitrogen Mineralization Conversion of organic N {which the plants can’t use} into inorganic forms (mainly nitrate (NO 3 - )and ammonium (NH 4 + ) – both of which plants can absorb via their root hairs).

Nitrogen Mineralization The plants cannot directly use the nutrients in organic matter because they are bound in complex organic molecules. The plants cannot directly use the nutrients in organic matter because they are bound in complex organic molecules. To function efficiently and convert these complex molecules, the microbes need: To function efficiently and convert these complex molecules, the microbes need: – food (organic matter), – good aeration and a – warm, moist environment.

Nitrogen Mineralization Protozoa are microbes that eat bacteria. The bacteria contain more nitrogen than the protozoa can use, so some ammonium (NH 4 ) is released to the plants Protozoa are microbes that eat bacteria. The bacteria contain more nitrogen than the protozoa can use, so some ammonium (NH 4 ) is released to the plants Classes of microbes include – bacteria, fungi, viruses, protozoa. Classes of microbes include – bacteria, fungi, viruses, protozoa.

Protozoa

Protozoa are microbes that eat bacteria. The bacteria contain more nitrogen than the protozoa can use, so some ammonium (NH 4 ) is released to the plants Other soil microbes convert the organic forms of nitrogen in the soil (which plants can’t use) into inorganic forms that the plants can use. Other soil microbes convert the organic forms of nitrogen in the soil (which plants can’t use) into inorganic forms that the plants can use. Nitrogen Mineralization

Denitrifying microbes These are microorganisms that convert nitrogen in the soil, back into atmospheric nitrogen (N 2 ).

Nitrogen immobilization High carbon content of organic matter requires increased microbial activity. This increased activity uses up N and less is available for the plants. Upside is that the N is not lost to leaching and can become available with death of microbe.

Mycorrhizae ~~symbiotic relation~~ Mycorrhizae are fungi that enlarge the surface area of the roots up to 1,000 times. This makes the roots much more efficient in the uptake of water and nutrients

The plant; The plant; – Supplies a steady source of sugars to the fungus. The fungus; The fungus; – Increases surface area for water uptake and selectively absorbs minerals in the soil and supplies them to the plant – Secretes growth factors that stimulate root growth and branching Mycorrhizae ~~symbiotic relation~~

Mycorrhizae The mycorrhizae enable the plants to better tolerate environmental stress like drought. These plants may need less fertilizer and have fewer soil born diseases. The mycorrhizae enable the plants to better tolerate environmental stress like drought. These plants may need less fertilizer and have fewer soil born diseases.

Mycorrhizae - fungi

Water and microbes Microbes in the soil compete with plants for available oxygen, which is used up very quickly when excess water is present. Microbes in the soil compete with plants for available oxygen, which is used up very quickly when excess water is present. Beneficial microbes don’t do well in saturated soils. Beneficial microbes don’t do well in saturated soils. If the soil dries out, the microbial activity naturally declines. If the soil dries out, the microbial activity naturally declines.