22: Biogeochemistry - Soil N and P

Question 1

Nitrogen (N) cycle

Answer 1

Nitrogen is the most limiting nutrient for plant growth

Is disrupted at the global level by industrial fixation of atmospheric N, leading to environmental pollution

This cycle is also ‘leaky’—it exchanges with other compartments - environmental pollution

Question 2

Chemical forms of N

Answer 2

Takes 9 different forms in soil, transformations catalyzed by enzymes (these 9 different forms have different oxidation states)

Question 3

Major processes in the N cycle

Answer 3

Biological N fixation
Mineralization/immobilization
Nitrification
Denitrification

Question 4

Biological N fixation

Answer 4

Conversion of nitrogen gas in air to plant available N
Soil prokaryotes use nitrogenase to break the triple bond in N2

Question 5

Mineralization/immobilization

Answer 5

Mineralization (ammonification) - organic N converted to mineral form (NH4+); common among microbes (organoheterotrophs)

Immobilization (assimilation) - mineral N taken up by microbes, converted to organic forms; common among all microbes

• Can limit plant growth and productivity depending on the C:N ratio of the organic matter
• Leads to competition between plants and microorganisms for soil inorganic N

Question 6

Nitrification

Answer 6

The oxidation of ammonia into nitrate. Proceeds in 3 steps carried out by microorganisms

Question 7

Conditions leading to nitrification

Answer 7

Oxidation (needs good soil aeration)
High ammonium availability
Warm, moist, but well-aerated soils

Question 8

Nitrate leaching

Answer 8

NO3- is ‘washed out’ of the root zone and slowly moves out of the soil
Can lead to eutrophication of water courses

Question 9

Denitrification

Answer 9

Reduction of nitrate back to nitrogen gasses
Performed by microorganisms

Question 10

What soil conditions lead to higher denitrification rates?

Answer 10

High soil NO3- concentrations
Water saturation
High soil organic matter contents

Question 11

Terrestrial phosphorus (P) cycle

Answer 11

P is one of the 3 primary plant macronutrients - regulates photosynthesis and plant primary productivity
Also a nutrient for microbes and soil biota, C:P ratios regulate mineralization
Can cause eutrophication of water systems when leached out of the soil

Question 12

Sorption of phosphates

Answer 12

Phosphate is adsorbed into positively charged colloids in the soil (like aluminum and iron oxides, organic matter, clays with pH-dependent charges)

Question 13

Precipitation of phosphate

Answer 13

Tend to react with cations to form secondary soil minerals: calcium, iron, and aluminum phosphates. Precipitation reactions depend largely on soil pH. There is only a very narrow range of soil pH where phosphates are soluble.

Precipitation is a chemical process that removes phosphate from water to form solid precipitates