Mechanisms of Nitrogen Loss From Soil

• Loss of nitrate (NO₃-) that is carried by soil water downward below the crop root zone.

Situations at Risk of N Loss

• Leaching risk is typically greatest during April to June when intense rainfall events are most common and applied N is most susceptible to loss.

Processes and Reactions

• Nitrate is a negatively charged ion that is not attracted to soil particles and can move with soil water.
• Ammonium ions are converted to the nitrate form by the action of soil bacteria in a process known as nitrification.
• Nitrification is a two-step process: 1) oxidation of ammonia (NH₃) into nitrite (NO₂-), and 2) oxidation of nitrite into nitrate (NO₃-). Both steps are carried out by bacteria in the soil that use oxidation of chemical compounds as a source of energy.

Important Factors

• Rainfall – the more water that moves through the root zone, the greater the potential for leaching.
• Soil texture – Coarse-textured soils have a lower water-holding capacity and more potential for leaching.

• Rate of nitrification – Multiple factors influence the rate of nitrification, which affects the susceptibility of N to leaching.
  • Soil temperature – Nitrification is a biological process, so it is highly affected by temperature. It is maximized at soil temperatures above 75°F, (24°C), slows at cooler temperatures, and essentially ceases below 40°F (4°C).
  • Soil moisture – Both water and oxygen are required for nitrification, so it is favored by adequate but not excessive soil moisture. Nitrification is limited when saturation of soil pore space with water exceeds 60%.
  • Soil pH – The optimal pH range for nitrification is between 6.5 and 8.8. Nitrification rates are reduced in more acidic soils.

Tactics to Reduce N Loss

• Application timing – Applying a portion of nitrogen in-season closer to the time of greatest crop demand reduces the opportunity for it to leach before being taken up.
• Nitrification inhibitors – Products such as N-Serve^® and Instinct NXTGEN^® slow the conversion of ammonium to nitrate, prolonging the period of time that nitrogen is in the ammonium form and reducing nitrogen loss from the soil.

• Loss of N to the atmosphere as N₂O or N₂ gas.

Situations at Risk of N Loss

• Denitrification occurs in saturated soils. The greatest nitrogen losses through denitrification generally occur in the spring when rainfall events are most frequent and crop uptake of nitrogen from the soil is relatively low.

Processes and Reactions

• Denitrification is a microbially facilitated process where nitrate (NO3−) is reduced and converted to N₂ gas through a series of intermediate steps.
• When oxygen in the soil is limited, a variety of bacteria will use the oxygen atoms from nitrate molecules for respiration.
• When nitrate is not completely converted to N₂ gas, the resulting byproduct is nitrous oxide (N₂O), a greenhouse gas.

Important Factors

• Soil water – Denitrification is triggered by rainfall events of sufficient volume to saturate at least 60% of soil pore space.
  2-3 days of saturation are required to begin denitrification.
• Nitrogen in the nitrate form – Nitrogen in the ammonium form (NH4+) isn’t subject to denitrification.
• Soil temperature – Denitrification is a biological process, so it increases with temperature.

Tactics to Reduce N Loss

• Keep nitrogen in the ammonium form – The use of nitrification inhibitors has been shown to be highly effective at reducing N losses through denitrification.
• Reduce the potential for soil saturation – Improving soil drainage can lower the risk of N loss from denitrification.

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• Loss of surface-applied urea to the atmosphere as ammonia gas (NH3).

Situations at Risk of N Loss

• Manure and fertilizer products containing urea that are not incorporated into the soil by tillage or rainfall within a short time following application.

Processes and Reactions

• Urea hydrolysis is catalyzed by urease, an enzyme that is produced by many types of common soil bacteria.
• Urea is hydrolyzed into one CO₂ and two NH₃ molecules.
• If this reaction occurs within the soil, the NH₃ reacts with soil water to form NH₄⁺, which is then bound to the soil.
• If it occurs on the soil surface, the NH₃ can be lost into the air.
• Most volatilization occurs within 2-3 weeks after application.

Important Factors

• Air temperature - Urease activity increases as temperature increases. Hydrolysis is normally completed within ten days at 40°F (4°C) and within two days at 85°F (29°C).
• Soil surface moisture – The hydrolysis reaction requires water, so volatilization increases with soil water content.
• Soil characteristics – High soil pH, clay content, CEC, and/or organic matter are all associated with lower volatilization.
• Crop residue – High levels of surface residue can increase
  soil surface moisture, prevent urea from reaching the soil,
  and harbor bacteria that hydrolyze urea.

Tactics to Reduce N Loss

• Incorporation by tillage.
• Incorporation by rainfall or irrigation.
• Urease inhibitors - Urease inhibitors such as PinnitMax^® TG slow down urea hydrolysis, which can reduce volatilization.