W6, Nitrogen (N)

Question 1

T/F: Nitrogen is the most commonly deficient nutrient in agriculture

Answer 1

True

Question 2

What happens when too much N makes it into water?

Answer 2

Algal blooms

Eutrophication

Question 3

List some N reservoirs

Answer 3

→ Atmosphere (N₂)
→ Organic N (biology)
→ Soil

Question 4

List some forms of N that are important for management

Answer 4

→ Ammonium (NH₄⁺)
→ Ammonia (NH₃⁺)
→ Nitrite (NO₂⁻)
→ Nitrate (NO₃⁻)
→ Nitrogen dioxide (NO₂)
→ Nitric oxide (NO)
→ Nitrous oxide (N₂O)

Question 5

Explain the processes and products of denitrification

Answer 5

2NO₃⁻ (nitrate) → 2NO₂⁻ (nitrite) → 2NO (nitric oxide) → N₂O (nitrous oxide) → N₂ (nitrogen gas)

→ Nitrate replaces oxygen as the electron acceptor in soil microbial respiration
*** Denitrification occurs under anaerobic conditions
▪︎ waterlogged soils
▪︎ poorly aerated soils
▪︎ anaerobic microsites
▫︎ inside soil aggregates
▫︎ rhizosphere (root and microbial respiration)
* There will be small niches of denitrification in even the best soils.

Question 6

T/F: Volatilisation is higher in acid soils than alkaline soils.

Answer 6

False. Volatilisation is higher in ALKALINE soils.

Question 7

How does inorganic N fertilisation affect the soil?

Answer 7

↑ Plant growth
↑ Requirement for other nutrients = ↓ [nutrient]
↑ Microbial activity
↑ OM decomposition rate
↓ SOC
↓ Potentially mineralisable N

Question 8

Roughly how much of the energy consumed during crop production comes from industrial N fertiliser production?

Answer 8

30-70%

Question 9

List some ways that N fluxes in the soil

Answer 9

→ N fixation
     ▪︎ biological
     ▪︎ industrial
     ▪︎ lightning
→ Assimilation (plant uptake)
→ Immobilisation (microbial uptake)
→ Mineralisation
→ Denitrification
→ Volatilisation

Question 10

Describe the mobility of N in soil.

Answer 10

NO₃⁻ (nitrate) = very mobile due to negative charge

NH₄⁺ (ammonium) = less mobile, can be adsorbed to soil particles. Rapidly converted to NO₃⁻.

Question 11

T/F: Nitrification is an aerobic process

Answer 11

True

Question 12

T/F: Denitrification is an aerobic process

Answer 12

False.

Question 13

T/F: N concentration in soil will differ before and after rainfall

Answer 13

True.

Question 14

How is N lost from the soil, and in what proportions?

Answer 14

→ Plant uptake: 55%
→ Leaching: 16%
→ Soil erosion: 15%
→ Gaseous N losses: 14%

Question 15

How much of the world’s energy consumption is devoted to industrial N fixation?

Answer 15

≈ 1%

Question 16

What is it that makes root (N) nodules pink?

Answer 16

Leghaemoglobin

Question 17

How are plant-rhizobia symbiosis (nodules) formed?

Answer 17

→ Signalling between rhizobia and plant root
→ Rhizobia attach to plant root, root curls around rhizobia
→ Rhizobia degrade cell wall of root and cause the root plasma membrane
→ Rhizobia create infection thread which penetrates the cortex
→ Rhizobia enter cortical cell, stimulate cell division and become bacteroids (nondividing endosymbiotic organelles)
→ Bacteriod-containing cells form a nodule

Question 18

How much N can lucerne fix /ha /yr?

Answer 18

300-350

Question 19

How much N can be fixed by:
• Symbiosis (Rhizobium, Actinomycetes)
• Associations (Azospirillum, Azotobacter)
• Free-living (Azotobacter, Klebsiella)

Answer 19

→ Symbiosis: 20-400 kg N /ha /yr
→ Associations: 10-200 kg N /ha /yr
→ Free-living: 10-80 kg N /ha /yr

Question 20

If you could only know one soil property to give you an idea about that soil’s likelihood to leach nutrients, what would it be?

Answer 20

It’s texture. Sand = strong likelihood of leaching, clay = low leaching risk.

Question 21

What effect does nitrate addition and the presence or absence or mycorrhiza have on plant biomass production and nitrate leaching?

Answer 21

+ N = incr biomass, incr leaching

+ myc = incr biomass, decr leaching (by ~40x)

Question 22

T/F: N2O has a global warming potential (GWP) much greater than CO2.

Answer 22

True

Question 23

T/F: N2O production is generally lower in Australian agriculture.

Answer 23

False, it’s ≈65% higher in Aust.

Question 24

What proportion of nitrous oxide (N₂O) emissions in Australia come from agriculture?

Answer 24

≈ 80%

Question 25

How can farmers and land managers minimise N losses?

Answer 25

→ Use applied N more effectively and efficiently
▪︎ Right amount
▪︎ Right time of year
▪︎ Right method
→ Maintain soil cover (cover crops)
▪︎ Captures nutrients that would otherwise be lost
▪︎ ↓ erosion
→ Improve soil structure
▪︎ Conservation tillage
→ Increase SOM
▪︎ Adsorb more N
▪︎ ↓ runoff and erosion
→ Decrease erosion
→ Livestock (and their waste) management
▪︎ Keep livestock and their waste away from water bodies
▪︎ Maintain effluent ponds and monitor for overflow risk
→ Avoid overly alkaline conditions
→ Promote mychorriza
→ Intercept N before it is leached beyond the root zone
→ Apply management strategies that control N speciation (↓ NO₃⁻ losses by maintaining NH₄⁺)