Soil and Plant Nutrition

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5

Slide 37

Bacteria in the Nitrogen Cycle

Nitrogen can be an important limiting nutrient for plant growth.

The nitrogen cycle transforms nitrogen and nitrogen-containing compounds. Most soil nitrogen comes from actions of soil bacteria.

Plants absorb nitrogen as either NO3– or NH4+

Bacteria break down organic compounds or use N2 to produce NH3, which is converted to NH4+

Nitrification is carried out by bacteria that convert NH3 into NO3– .

Slide 38

The role of soil bacteria in the nitrogen nutrition of plants

Atmosphere

Atmosphere

N2

N2

Soil

Nitrogen-fixing bacteria

Soil

H+

(from soil)

Ammonifying

bacteria

NH3

(ammonia)

NH4+

(ammonium)

Nitrifying

bacteria

Organic material (humus)

N2

NO3–

(nitrate)

Denitrifying

bacteria

NH4+

Nitrate and

nitrogenous

organic

compounds

exported in

xylem to

shoot system

Root

Slide 39

Nitrogen-Fixing Bacteria: A Closer Look

N2 is abundant in the atmosphere, but unavailable to plants.

Nitrogen fixation is the conversion of nitrogen from N2 to NH3.

Symbiotic relationships with nitrogen-fixing bacteria provide some plant species with a built-in source of fixed nitrogen.

Key symbioses occur between nitrogen-fixing bacteria and plants, including those in the legume family (peas, beans, and other similar plants)

Slide 40

Along a legume’s roots are swellings called nodules, composed of plant cells “infected” by nitrogen-fixing Rhizobium bacteria.

Inside the root nodule, Rhizobium bacteria assume a form called bacteroids, which are contained within vesicles formed by the root cell.

The bacteria of a root nodule obtain sugar from the plant and supply the plant with fixed nitrogen ++ mutualism.

Slide 41

Root nodules on legume plants

Nodules

Roots

(a) Pea plant root

(b) Bacteroids in a soybean root

nodule

5 µm

Bacteroids

within

vesicle

Slide 42

Each legume species is associated with a particular strain of Rhizobium.

The development of a nitrogen-fixing root nodule depends on chemical dialogue between Rhizobium bacteria and root cells of their specific plant hosts.

Slide 43

Development of a soybean root nodule

Rhizobium

bacteria

Infection thread