Soil Fertility Lecture 16 Nitrogen 3

What are the major pathways of nitrogen loss from soil?

Nitrate (NO₃⁻) leaching, gaseous loss through denitrification, and ammonia (NH₃) volatilization. Ammonium (NH₄⁺) fixation is also covered because it affects nitrogen availability and movement.

Why is nitrate (NO₃⁻) highly susceptible to leaching?

Nitrate is very soluble in water and is not strongly adsorbed to clay or organic matter, which are generally negatively charged. Therefore, nitrate is mobile and can move downward with percolating water.

When is nitrate leaching a major pathway of nitrogen loss?

In humid conditions and irrigated systems, where substantial amounts of water move through the soil.

What are the major problems caused by nitrate leaching?

It removes valuable nitrogen needed for crop production; approximately 10–40% of applied fertilizer N may be lost this way; it contributes to soil acidification and the loss of Ca²⁺, Mg²⁺, and K⁺; and nitrate entering groundwater can pollute drinking water and stimulate unwanted plant and algal growth in lakes, reservoirs, and ponds.

How does nitrate leaching contribute to soil acidification and the loss of Ca²⁺, Mg²⁺, and K⁺?

Nitrate can leach alongside positively charged base cations such as Ca²⁺, Mg²⁺, and K⁺. Losing these cations reduces the soil's resistance to acidity. Nitrification, which produces nitrate, also releases H⁺ and contributes to acidification.

What factors influence nitrate leaching?

The volume of leaching water; the amount and pattern of precipitation or irrigation; soil characteristics; nitrogen concentration; fertilizer rate, timing, source, and application method; and crop growth and nitrate uptake.

Why are sandy soils more prone to nitrate leaching?

Sandy soils generally drain rapidly, allowing dissolved nitrate to move downward with water.

A farmer applies nitrate fertilizer to a sandy field, followed by 3 inches of rain the next day. What is the greatest concern?

Nitrate leaching, because nitrate is water-soluble and sandy soil drains rapidly.

How can nitrogen leaching be reduced?

Avoid excessive N fertilization; apply N when crop demand is high; split fertilizer into multiple applications; use nitrification inhibitors; and plant N-scavenging cover crops to take up unused nitrate between crop cycles.

Why does applying nitrogen fertilizer in split applications reduce nitrogen loss?

It matches nitrogen availability more closely to crop demand, reducing the amount of unused nitrogen that can leach or be lost through other processes.

How do N-scavenging cover crops reduce nitrate leaching?

They absorb unused nitrate from the soil between crop cycles, reducing the amount available to leach.

What is ammonium (NH₄⁺) fixation?

The trapping of NH₄⁺ within certain clay-mineral layers, making it less readily available for immediate plant uptake or microbial processes.

Which clay minerals are associated with ammonium fixation?

Vermiculite and mica, which are 2:1 clay minerals.

How does ammonium become fixed in clay minerals?

NH₄⁺ can replace other cations in the clay-mineral lattice and become trapped between clay layers.

How does potassium (K⁺) affect ammonium fixation?

The presence of K⁺ can restrict NH₄⁺ fixation. Applying potassium fertilizer before ammonium fertilizer can reduce the amount of NH₄⁺ that becomes fixed.

What soil conditions can increase ammonium fixation?

Alternating cycles of wetting and drying, and freezing and thawing, can increase NH₄⁺ fixation.

How does ammonium fixation prevent rapid nitrification and leaching?

Fixing NH₄⁺ temporarily reduces its availability for nitrifying microorganisms. This can slow its conversion into mobile NO₃⁻, reducing the amount of nitrate potentially available for leaching.

Can fixed ammonium become available again to plants?

Yes. Some fixed NH₄⁺ can be released, but release may be too slow to meet the needs of fast-growing annual plants.

Why might the percentage of ammonium fixation be higher in the B horizon than in the A horizon?

The B horizon may contain accumulated clay and more ammonium-fixing minerals. However, this depends on the soil's actual mineral composition; not every B horizon has greater fixation.

A soil contains considerable vermiculite and mica. What may happen after ammonium fertilizer is applied?

Some NH₄⁺ may become fixed between the clay-mineral layers.

What are the two primary pathways of gaseous nitrogen loss from soil?

Denitrification and ammonia (NH₃) volatilization.

What is denitrification?

A microbial process in which microorganisms use nitrogen oxides, including NO₂⁻ and NO₃⁻, under low-oxygen conditions, releasing gaseous nitrogen products such as N₂ and N₂O.

Under what soil conditions does denitrification commonly occur?

In saturated or waterlogged soils with low oxygen availability, where anaerobic microorganisms can use NO₂⁻ and NO₃⁻.

Which microorganisms involved in denitrification are identified in the lecture?

Pseudomonas, Bacillus, Paracoccus, and Thiobacillus denitrificans.

Why can denitrification occur in agricultural soils that are normally well aerated?

Small saturated or oxygen-poor pockets can develop within the soil, allowing microorganisms to switch from aerobic to denitrifying metabolism.

What three conditions together indicate high denitrification potential?

Waterlogged or anaerobic soil conditions; available inorganic nitrogen, particularly nitrate; and an ample supply of decomposable or soluble carbon.

What factors influence the rate of denitrification?

The amount and nature of organic matter, soil moisture, aeration, pH, temperature, and the presence and type of inorganic nitrogen.

How does decomposable organic matter affect denitrification?

It provides carbon that microorganisms can use, supporting denitrification when oxygen is limited and nitrate is available.

How does soil water content affect denitrification?

High water content can reduce oxygen availability and promote denitrification. Rainfall and snowmelt can contribute to nitrogen losses by creating saturated conditions.

According to the lecture, what nitrogen loss was observed per inch of rain during a four-week period?

An experiment found that 8% of nitrogen was lost per 1 inch of rain during the four-week period.

How do N₂ and N₂O losses differ under different soil conditions?

N₂ loss predominates under saturated conditions, while N₂O loss is greater under less-reduced conditions.

Why is N₂O loss environmentally important?

Nitrous oxide (N₂O) is a potent greenhouse gas.

Why can nitrate fertilizer be ineffective in rice paddy soils?

Saturated, low-oxygen conditions favor denitrification, which can convert nitrate into gaseous nitrogen products before crops use it.

At what oxygen level does the lecture indicate that denitrification losses can become appreciable?

When oxygen levels are approximately 10% or less.

How does soil pH affect denitrification?

Denitrification is negligible in soils below pH 5 because the microorganisms are sensitive to acidic conditions. It occurs rapidly in high-pH soils.

How does temperature affect denitrification?

The rate increases rapidly from 2°C to 25°C, increases more slowly from 25°C to 60°C, and is inhibited above 60°C.

Why may denitrification losses be high in spring?

Soil temperatures can rise rapidly in spring, increasing denitrification rates when other favorable conditions are present.

How can fertilizer management reduce denitrification losses?

Split nitrogen applications and time them to match crop demand, reducing the amount of nitrate present in the soil before crops can use it.

A corn producer wants to reduce nitrate available for denitrification early in the growing season. What practice is most effective?

Split nitrogen fertilizer into multiple applications based on crop demand.

A turfgrass field frequently develops waterlogged areas after irrigation. What management practice would most directly reduce denitrification?

Improve field drainage and avoid excessive irrigation to reduce saturated, oxygen-poor conditions.

What is ammonia (NH₃) volatilization?

The loss of nitrogen to the atmosphere as gaseous ammonia.

How does soil pH affect ammonia volatilization?

Ammonia volatilization is favored at high pH, particularly above pH 7.5, or when soil pH temporarily increases.

In what soils can ammonia volatilization occur readily?

It can occur in high-pH soils, including calcareous soils.

Why do highly buffered soils resist ammonia volatilization?

They resist changes in soil pH, making it harder for conditions to shift toward those that favor NH₃ formation and volatilization.

A farmer broadcasts urea on warm, high-pH soil without incorporating it or receiving rainfall. Which nitrogen loss is most likely?

Ammonia (NH₃) volatilization.

How do nitrate leaching, denitrification, and ammonia volatilization differ?

Leaching moves dissolved NO₃⁻ downward with water; denitrification converts nitrogen oxides into gaseous products such as N₂ and N₂O under low-oxygen conditions; ammonia volatilization releases gaseous NH₃, especially under high-pH conditions.