SoilMicrobial activity

and nitrogen

• 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 – pH and its effect on

nutrients (primary, secondary and micro-elements); water; oxygen ( SOIL {3})

SOIL

NITROgen• Nitrate (NO3

-) 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 mineralization• Nitrifying microbes• Denitrifying microbes• Nitrogen immobilization

Nitrogen Fixation

The conversion of atmospheric nitrogen into

ammonium (NH4+).

Nitrogen Fixation• 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 nodesRhizobia nodule

on soybeanRhizobia

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:–Bacteria–Fungi–Other nematodes including rootknot• Feed on them directly or• Prevent the root-feeding nematodes from

entering the roots.

(www.ext/ 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.

LeghemoglobinClover roots

www.etd.ohiolink.edu AND www.sccs.swarthmore.edu

Leghemoglobin is a protein that binds oxygen and is red like hemoglobin.

Enzymes that produce the conversion of N2 to nitrates (N03

−) are inhibited by oxygen. Leghemoglobin binds oxygen and allows the N fixation to proceed.

LeghemoglobinClover roots

Symbiotic relationClover 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 MineralizationConversion of organic N {which

the plants can’t use} into inorganic forms

(mainly nitrate (NO3-)and

ammonium (NH4+) – 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.

• 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 (NH4) is released to the plants• 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 (NH4) 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.

Nitrogen Mineralization

Denitrifying microbesThese are microorganisms that convert nitrogen in the soil, back into atmospheric nitrogen (N2).

Nitrogen immobilizationHigh 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;– Supplies a steady source of sugars to 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

www.uta.edu/biology

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.

Mycorrhizae - fungi

Water and microbesMicrobes 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.

If the soil dries out, the microbial activity naturally declines.