Soil Fertility Lecture 15 Nitrogen 2

What percentage of total soil N is organic in surface soils?

More than 95%.

What are the main inorganic forms of soil nitrogen?

NH₄⁺, NO₃⁻, NO₂⁻, and nitrogen gases.

Which two nitrogen forms are most important for plant uptake?

Ammonium (NH₄⁺) and nitrate (NO₃⁻).

How does soil organic matter affect total soil N?

Total soil N generally increases as soil organic matter increases.

What are the main organic forms of soil N?

Proteins, amino acids, amino sugars, and complex N compounds.

What are the two main reactions of N mineralization?

Aminization and ammonification.

What is aminization?

The breakdown of proteins into amino acids, amines, and urea.

What is ammonification?

The conversion of organic N compounds into ammonium (NH₄⁺).

What is N mineralization?

The conversion of organic N into inorganic NH₄⁺.

Which organisms carry out N mineralization?

Heterotrophic microorganisms that use organic carbon for energy.

Which microbes dominate mineralization at different pH levels?

Bacteria at neutral and high pH; fungi at low pH.

What are the major fates of NH₄⁺ in soil?

Nitrification, plant uptake, immobilization, clay fixation, or volatilization.

What is N immobilization?

The conversion of inorganic NH₄⁺ or NO₃⁻ into organic N in microbial biomass.

How is immobilization different from mineralization?

Immobilization stores inorganic N in organic forms; mineralization releases inorganic N.

Why do soil microbes immobilize N while decomposing residues?

They need N to build microbial proteins and biomass while using carbon for energy.

How does a high C:N residue affect plant-available N?

Microbes immobilize N, temporarily reducing its availability to plants.

What C:N ratio generally causes net immobilization?

Greater than 30:1.

What C:N ratio generally causes net mineral N release?

Less than 20:1.

What C:N ratio generally causes neither net immobilization nor release?

20:1 to 30:1.

Why can high-C:N residues cause plant N deficiency?

Microbes temporarily take up NH₄⁺ and NO₃⁻ that plants need.

How does residue decomposition change its C:N ratio?

The C:N ratio narrows as carbon is lost during decomposition.

What factors affect residue breakdown and N release?

Residue amount, inorganic N supply, decomposition resistance, temperature, and moisture.

What is nitrification?

The microbial conversion of NH₄⁺ to NO₃⁻ through NO₂⁻.

What are the two steps of nitrification?

NH₄⁺ → NO₂⁻ → NO₃⁻.

Which bacteria are associated with the two nitrification steps?

Nitrosomonas: NH₄⁺ → NO₂⁻; Nitrobacter: NO₂⁻ → NO₃⁻.

Why does NO₂⁻ rarely accumulate in soil?

Its conversion to NO₃⁻ is faster than its production from NH₄⁺.

Why is NO₂⁻ accumulation harmful?

Nitrite is toxic to plant roots.

How does nitrification affect soil acidity?

It increases soil acidity.

Why can nitrification increase N leaching losses?

It produces mobile NO₃⁻, which moves readily with percolating water.

What conditions favor rapid nitrification?

Warm, moist, well-aerated soil with available NH₄⁺.

How does NH₄⁺ supply affect nitrification?

Nitrification requires NH₄⁺, supplied partly by organic matter mineralization.

How does high C:N material affect nitrification?

High C material promotes N immobilization, reducing mineral N availability and potentially limiting NH₄⁺ available for nitrification.

How does the initial nitrifier population affect total nitrification?

The lecture states that total nitrification is not affected by the initial microbial population.

Why might the initial nitrifier population not affect total nitrification?

The number of nitrifying organisms can increase rapidly when conditions and resources support their growth.

What soil pH range allows nitrification?

Approximately pH 4.5–10.

What is the optimum soil pH for nitrification according to the lecture?

Approximately pH 8.5.

What nutrients besides NH₄⁺ do nitrifying organisms require?

Calcium, H₂PO₄⁻, and micronutrients.

Why is oxygen important for nitrification?

Nitrifying bacteria are aerobic and require oxygen.

What soil conditions favor nitrification through aeration?

Coarse-textured or well-structured soils with good oxygen supply.

At what water-filled pore space does nitrification occur well?

Approximately 60%.

What is the optimum temperature range for nitrification?

Approximately 25–35°C.

How does nitrate uptake affect plant pH balance?

Nitrate uptake tends to increase rhizosphere pH by promoting anion-related ion balancing and reducing acidity near the roots.

Why does nitrate uptake promote cation uptake?

Nitrate increases anion synthesis, which is associated with increased uptake of Ca, Mg, and K.

What must happen to NO₃⁻ before plants use it in protein synthesis?

It must be reduced to NH₄⁺ in two steps, requiring energy.

What is one advantage of NH₄⁺ uptake over NO₃⁻ uptake?

Less energy is required for protein synthesis.

Why is NH₄⁺ less subject to leaching than NO₃⁻?

NH₄⁺ is positively charged and can be retained by negatively charged soil particles.

How does NH₄⁺ uptake affect the root-zone pH?

It releases H⁺ and decreases pH near roots.

How can excessive NH₄⁺ affect plants?

Excessive NH₄⁺ can become toxic.

How does NH₄⁺ uptake affect uptake of other cations?

It can reduce uptake of other positively charged nutrients.

What determines whether a plant prefers NH₄⁺ or NO₃⁻?

Plant species, age, environment, and other factors.

Why can some plants, such as blueberries, not tolerate NO₃⁻?

The lecture attributes this to a lack of nitrate reductase.

Which N form do tobacco and tomato prefer in high proportion?

NO₃⁻ relative to NH₄⁺.

What NO₃⁻:NH₄⁺ ratio does the lecture recommend for horticultural plants?

75:25, to help avoid NH₄⁺ toxicity.

What percentage of total N in surface soil is generally inorganic?

Approximately 2–5%.

How much total N is in an acre-furrow slice containing 0.4% N?

8,000 lb N/acre, using 2,000,000 lb soil/acre.

How is plant-available N estimated from mineralization?

Soil mass × organic matter fraction × N fraction of organic matter × mineralization rate.

If soil has 4% OM and 2% mineralization, how much N is released annually?

80 lb N/acre, assuming OM contains 5% N and the soil mass is 2,000,000 lb/acre.

If OM increases from 4% to 8% with the same mineralization rate, what happens to N release?

It doubles from 80 to 160 lb N/acre annually.

If mineralization increases from 2% to 4% at 4% OM, what happens to N release?

It doubles from 80 to 160 lb N/acre annually.

Why is soil N supply difficult to predict accurately?

Mineralization and other N transformations vary with soil and environmental conditions.

What happens to NH₄⁺ during clay fixation?

It becomes trapped within expanding 2:1 clay minerals.

What is N volatilization?

The loss of soil N as gaseous NH₃.

What conditions increase NH₃ volatilization losses?

High soil pH and dry soil conditions.