BIOG 1440 L9 Nitrogen Cycle Diagram

Draw the soil nitrogen-cycle diagram from memory: start at atmospheric N₂ and end at the root. Label every bacterial group and every arrow.

N₂ → (nitrogen-fixing bacteria) → NH₃ (ammonia) + H⁺ from soil → NH₄⁺ (ammonium). Proteins from humus → (microbial decomposition) → amino acids → (ammonifying bacteria) → NH₄⁺. NH₄⁺ → (nitrifying bacteria) → NO₂⁻ (nitrite) → (nitrifying bacteria) → NO₃⁻ (nitrate). NO₃⁻ → (denitrifying bacteria) → N₂ back to atmosphere. Root absorbs NO₃⁻ (main arrow) and NH₄⁺; exports nitrate + nitrogenous organic compounds in xylem to the shoot.

In what forms can plants absorb nitrogen? Can they use N₂?

Nitrate (NO₃⁻) or ammonium (NH₄⁺). NOT N₂.

Where does most of the nitrogen available to plants come from?

The actions of soil bacteria.

From a plant's point of view, what does the nitrogen cycle accomplish overall?

It transforms nitrogen and nitrogen-containing compounds into NH₄⁺ and NO₃⁻, the two forms plants can absorb.

Nitrogen-fixing bacteria: what is the input and what is the direct product?

Input: atmospheric N₂. Direct product: NH₃ (ammonia), which becomes NH₄⁺.

On the nitrogen-cycle diagram, how does NH₃ from nitrogen fixation become NH₄⁺?

NH₃ picks up H⁺ from the soil: NH₃ + H⁺ → NH₄⁺ (ammonium).

Name the two bacterial groups that generate NH₄⁺ in soil, and what each starts from.

Nitrogen-fixing bacteria: from N₂. Ammonifying bacteria: from organic matter (amino acids from decomposed humus proteins).

Fill in the ammonification path: proteins from humus → ___ (by ___) → ___ (by ___).

Proteins from humus → amino acids (by microbial decomposition) → NH₄⁺ (by ammonifying bacteria).

What is humus, and what role does it play in the nitrogen cycle diagram?

Dead organic material. Its proteins are decomposed to amino acids, which ammonifying bacteria convert to NH₄⁺.

Nitrification: fill in NH₄⁺ → ___ → ___. Which bacteria do it?

NH₄⁺ → NO₂⁻ (nitrite) → NO₃⁻ (nitrate). Nitrifying bacteria do both steps.

Denitrifying bacteria: what do they convert, and what does that mean for plants?

NO₃⁻ → N₂, released back to the atmosphere. Removes plant-available nitrogen from the soil.

On the diagram, which nitrogen forms enter the root, and which is drawn as the main (bold) input?

NO₃⁻ (nitrate, the bold main arrow) and NH₄⁺ (ammonium, taken up directly from soil).

After roots take up nitrogen, what is sent to the shoot and through which tissue?

Nitrate and nitrogenous organic compounds, exported in the xylem to the shoot system.

Nitrite vs nitrate: give each formula, and say which is the intermediate and which roots absorb.

Nitrite = NO₂⁻, the intermediate of nitrification. Nitrate = NO₃⁻, the end product that roots absorb.

Name each: NH₃, NH₄⁺, NO₂⁻, NO₃⁻, N₂.

NH₃ = ammonia; NH₄⁺ = ammonium; NO₂⁻ = nitrite; NO₃⁻ = nitrate; N₂ = atmospheric nitrogen gas.

On the diagram, what are the two fates of soil NO₃⁻?

1) Absorbed by roots. 2) Converted to N₂ by denitrifying bacteria and lost to the atmosphere.