________________ and ___________ are the two bioavailable forms of soil N

Inorganic ammonium (NH4+), nitrate (NO3‐)

 • NH^₄+ in anaerobic soils

• NO₃^‐ in aerobic soils

What are 4 Inputs of the soil N cycle

N2 Fixation, OM deposition, Rainfall, Fertilization

What are 3 transformations of the soil n cycle?

Immobilization, Mineralization, Nitrification

What are 3 losses of the soil N cycle?

Denitrification, NO₃ leaching, NH₃ volatilization

What is N₂ Fixation?

What is Immobilization?

What is Mineralization?

 High C:N ratio favors ______________
• C:N  > 25:1
• low N content

immobilization

mineralization

What is Nitrification?

What is Denitrification?

Biological reduction of NO₃‐ (oxidized N) to more reduced forms
NO₃‐ is converted to N₂O or N₂ (Anaerobic process) & NO₃‐ is used (terminal electron acceptor) 

Sequential process: NO₃^‐ → NO₂^‐ → N₂O → N₂
Final product can be N₂O or N₂ gas

This represents a loss of soil N
Both N₂ and N₂O are gases, and volatilize from soil

~30% of all fertilizer N is lost from soils by denitrification

What is Nitrate leaching?

NO₃‐ moves with H₂O, is leached through the soil profile
Nitrate is very susceptible to leaching, because it is an anion and is not adsorbed to soil colloids
– Ammonium, a cation, does not leach significantly

Nitrate leaching: areas with excess N (manures, fertilizers, sludges)

Nitrate leaching degrades water quality

What is Ammonia Volatilization?

NH₃ is ammonia It is a volatile gas at room temperature and pressure

NH₄+ is ammonium An exchangeable or fixed soil cation These two forms of N are closely related:
NH₄+ + OH‐ ↔ H₂O + NH₃ (gas)

↑                                          ↑

Low pH                              High pH

When does volatization occur? How can it be prevented?

NH₃ volatilization occurs when NH₄ + is applied at the surface of high pH soils
To prevent NH4+ volatilization, incorporate NH₄+ fertilizers into soil

Name 3 reason for N pollution.

– Greenhouse gases
• Nitrous oxide emitted from soils

– Water supplies
• Nitrate contamination of groundwater and surface water

– Precipitation
• NOx gases contribute to acid rain
• N in precipitation fertilizes soils, and can affects species diversity in sensitive ecosystems

___________ is Critical for protein synthesis

Sulfur

In anaerobic soils, ________is present in reduced forms, what are the 2?

Sulfur

• Metal sulfides such as FeS₂(pyrite)
• H₂S is a toxic gas

What is the main inorganic form of S in aerobic soils

 Sulfate (SO₄^2‐)

 Oxidized sulfur
– Sulfate is the form of sulfur taken up by plants.
– An anion, sulfate is subject to leaching, similar to nitrate

__________ __________ is an important component of air pollution

Sulfur dioxide (SO₂)

S Deficiencies are most likely in?

S deficiencies are not likely in?

sandy soils
low organic matter soils
highly leached soils 

Not likely:
– irrigated areas
most irrigation water contains enough sulfate salts to satisfy plant S needs  
– desert soils

What causes eutrophication?

Nitrogen and Phosphorus

Plants use the _____________ ions ________ and ________.

(P)

phosphate ions HPO‐ and HPO₂‐

Only about ______ of total soil P is bioavailable because of low solubility

0.01%

Factors Affecting P Availability?

Soil pH – P availability is maximized at pH 6 ‐ 6.5
– Lower availability at higher or lower pH
Presence of Fe oxides, Al oxides, or amorphous silicates in acid soils (strongly sorb or bind P)
– Highly weathered soils usually have low available P
• Fe, Al oxides present in Oxisols, Ultisols– Amorphous silicates in Andisols
• Presence of CaCO3 (sorbs P in alkaline soils)

• Soil Temperature
– Lower P availability in colder soils (slower P ion movement)
• Soil Moisture
– Higher P availability in flooded soils (more P minerals dissolve)

The original form of P is __________

apatite