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NC State Soil Science: SSC 341, Soil Fertility and Fertilizers

Denitrification and Volatilization
Assignments
Reading: Text, Chapter 4, pages 131-141.
Homework:9 Problems at the end of this page.

Notes
Topic Outline

1. Overview

2. Learning Objectives

3. Discussion

4. Summary

5. Homework



Unit Overview

Learning Objectives

  • Distinguish between denitrification and volatilization. Write the general reduction reactions involved in denitrification and the soil, environment, and management factors affecting gaseous N loss. Assess the positive and negative impacts of denitrification on plant available N and contribution of N to the degradation of water quality.
  • Write the reactions involved in urea hydrolysis and the soil, environment, crop residue, and management effects on NH3 volatilization.

Discussion

Gaseous losses of N

Inorganic N in the soil solution (NH4+ and NO3- ) can be converted to gasses and lost to the atmosphere (Figure 4.1) through denitrification and NH3 volatilization (text Table 4.17)

Denitrification

Anaerobic conditions in soils reduce O2 and encouraging growth of anaerobic organisms responsible for transformation of NO3 to N2, N2O, and NOx (where x = 1 or 2)

  • anaerobic organisms obtain O2 from NO2 and NO3
  • most organisms are bacteria
  • increased C near plant roots (rhizosphere) enhances growth of anaerobic bacteria
  • N2 usually > N2O, except when soil O2 supplies improve
  • Denitrification potential is high in most fields, but actual denitrification is usually less
    • Actual denitrification is highly variable (2 to 20 lbs N/a unfertilized) due to variation in environment between years

Denitrification reduces the oxidation state of N:

Reduction of oxidation state diagram

Progress of N reduction during denitrification is shown in text Figure 4.29

Factors Affecting Denitrification

  1. Decomposable soil OM or C —> crop residues added to soil stimulate denitrification (text Figure 4.30).
  2. Soil water content —> water saturated soil enhances denitrification by reduced O2 diffusion (text Figure 4.33)
  3. Aeration —> low O2 enhances denitrification (text Figure 4.33). Denitrification can occur in well-aerated soil in anaerobic microsites (Figure 4.30).
  4. Soil pH —> low pH reduces activity of denitrifying bacteria (text figure 4.34)
  5. Temperature —> denitrification increases with increasing temperature between 2 to 25°C, and decreases above 60°C.
  6. NO3 levels —> increasing NO3 concentrations increase denitrification
  7. Presence of plants —> plant growth increases denitrification through increasing C and reduced O2 through root activity

Agricultural and Environmental Significance of Denitrification

  • Denitrification returns N2 to the atmosphere by rapid N2 loss following heavy rainfall, irrigation, or snowmelt, and by continuous small losses from anaerobic microsites. N2 losses can range between 0 to 70% of applied N, depending on N rate, time of application, and factors identified above.
  • Emissions of N2O have been identified as a potential cause of ozone degradation. Contributions from soil and fertilizer N are being studied.
  • Denitrification is important in removing NO3 from surface and ground water (Chapter 13).

Agronomic Significance of NO2

  • NO2 does not usually accumulate in soil, although small amounts exist in calcareous soils and in soil zones high in NH4+ from fertilizers (usually high pH).
  • Above 7.5 pH NH4+ to NO2 conversion > NO2 to NO3 conversion, and breakdown of NO2 to N2O and N2 is reduced (text Figure 4.35), thus NO2 accumulation is favored
  • Elevated levels of NO2 are toxic to plants and microorganisms

Volatilization

Volatilization is the loss of NH3 gas from the soil (Figure 4.1). The reaction is:

NH4+ —> NH3 + H+

NH3volatilization from N generated from N mineralization is relatively small compared to NH3 loss from NH4+ in fertilizer and manure.

Factors affecting NH3 Volatilization

  • NH3 loss favored by high pH (Figure 4.35) and high NH4+ concentrations
  • NH3 loss decreases with increasing buffer capacity (text Figure 4.38)
  • Volatilization increases with increasing temperature
  • NH3 volatilization is maximized with wet soils in warm drying conditions
  • Surface crop residue cover enhances NH3 loss (text Figure 4.39)
  • Surface broadcast NH4+ fertilizer or manure enhances NH3 loss.

NH3 Exchange by Plants

Plant leaves can absorb NH3 from the air, as well as release NH3 to the atmosphere. NH3 volatilization from plant leaves occurs mainly during ripening and senescence.

Summary

  1. Denitrification and volatilization represent mechanisms of gaseous N loss from soils. Reduction of NO3- to NO, N2, N2O, and NOx gases is greatly enhanced under conditions of high moisture and low aeration. Denitrification is an important mechanism that reduces the impact of NO3- on water quality.
  2. Volatilization of NH3 is enhanced by high pH, low BC, high OM, and moist soil surface conditions. Surface application of urea fertilizer, especially to high pH soils with surface residue cover, can result in increased NH3 volatilization.

Homework

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