Nitrogen cycling, fertilizer and pollution

Question 1

Nitrogen
Typically 1-5% of shoot dry matter

Required as a constituent of proteins, amino acids, nucleic acids, some membrane lipids

Answer 1

The largest requirement:
Ribulose Bisphosphate Carboxylase -
The most abundant enzyme in the world -
Allows plants to fix CO2

Question 2

Fundamental relationship between

Field and Mooney 1986

Answer 2

shoot N concentrations, plant functional types, and rates of photosynthesis

Question 3

Nitrogen (N)

Answer 3

The mineral nutrient most in demand by plantsThe 4th most abundant element in plants
(on a mass basis) after C, H, and O
The N content of most rocks and minerals has been considered to be exceedingly low.

Plants have high N requirements relative to supply.

N- most often limits plant growth in natureN- the main constituent of artificial fertilizer

Question 4

Plants have much more N and P than surrounding crustal rock

Answer 4

Plants extract and retain Silica, P and Iron from earth materials

Question 5

19 to 31 teragrams of nitrogen are mobilised from near-surface rocks

Answer 5

annually. Bedrock is a nitrogen source that rivals atmospheric nitrogen inputs.

Question 6

93.5% of nitrogen is inorganic

Answer 6

of that, 96% is dead organic matter and 4% biomass, of this 4%, 94% is plants

Question 7

The N cycle: processes and pathways

Answer 7

1. Microbial depolymerisation and assimilation
2. Microbial organic N release
3. Mineralization (ammonification)
4. Nitrification
5. Denitrification
6. Microbial immobilisation
7. Humification
8. Microbial N fixation

Question 8

Nitrogen is mainly cycled through

Answer 8

organic molecules in litter inputs

Question 9

Soil profile-distribution of N

Answer 9

Nitrogen is mainly stored at the surface, as is S and P

Question 10

Rooting responses to localised nitrogen supply

Answer 10

Proliferation of secondary laterals in zones with more nitrate

Question 11

The N cycle is the biogeochemical cycle most altered by humans - far more than the C cycle.

Answer 11

Globally
Natural N fixation
	120 M tonnes y-1 by organisms naturally
	  10 M tonnes y-1 from lightning
        = 130 M tonnes y-1 

Anthropogenic
110 M tonnes y-1 fertilizer factories (International Fertilizer Association 2017)
~40 M tonnes y-1 Biological N fixation (legume crops (clover, beans etc)
= ~150 M tonnes y-1

Question 12

Global food production is currently strongly dependent on nitrogen fertilizer inputs

Answer 12

(Hafner, 2003) Nature 422, 397-8
Commercial fertilizer is responsible for 40 to
60% of the world’s food production.

Question 13

Increasing care with N fertilizer-

Cost
Precision agriculture
Pollution
Nitrate-vulnerable zone restrictions

Answer 13

Precision agriculture- sensors on the tractor detect the “greeness” of the crop and adjust fertilizer dose to match requirements- and can be linked to historical yield maps of a field via GPS.

Question 14

Annual excess of 90 Kg ha-1 =

Answer 14

9000 Kg ha-1 per century

Question 15

Farmers are driven by short-term economics

Answer 15

employment is low economic contribution

Question 16

Nitrogen use efficiency - Low rates of N fertilizer use in Africa give high efficiency of use, but suboptimal crop yields

Globally more that 50% of N fertilizer is not used by crops

Answer 16

Increasing rates of fertilizer use in China to feed the population, 80% inefficient use
EU Water framework Directive gives 60% efficiency in UK

Question 17

More than half of the environmental impact of producing a loaf of bread arises directly from wheat cultivation

Answer 17

with the use of ammonium nitrate fertiliser alone accounting for around 40%

Question 18

Synthetic nitrogen fertilisers deplete soil nitrogen

Answer 18

synthetic N applied as cost-effective insurance against low yields.

Question 19

Ammonium sources:
ammonification
clay weathering

Answer 19

Sinks of ammonium:
plant uptake
clay fixation
microbial assimilation

Question 20

What nitrifying bacteria do: NH4+ -> NO3-

Answer 20

If the nitrate is taken up by plants or micro-organisms the acidification is neutralised.
If the nitrate is washed out of soil net acidification results.
In acid soils (pH <4.5) bacterial nitrification is inhibited.

Question 21

Denitrification- a process of poorly aerated soil

Answer 21

Reductive nature of denitrification. Organisms take an oxygen from nitrate and leave the nitrite ion which in turn loses and oxygen and produces nitric oxide

Question 22

Nitrification is more temperature sensitive than ammonification.

Answer 22

Nitrification and denitrification can occur simultaneously in different microsites

Question 23

N2O is a major greenhouse gas with tropospheric lifetime of 120 years.
Its global warming potential is 296 X greater than an equal mass of CO2.
Its major source is soils. Of the atmospheric increase, 89% is due to emissions from cultivated soils, especially those high in nitrogen.

Answer 23

Rice paddy fields are contributing
10 to 25% of global methane emissions, and have made increasing contributions to N2O fluxes too. Increasing rice productivity is associated with increasing N2O release

(Zou et al. 2009)

Question 24

Agriculture in the UK is a modest source of nitrogen oxide emissions
DEFRA 2016

Answer 24

Livestock farming is the major source of ammonia emissions in the UK

Question 25

BAD Practice- An open slurry pit not apparently sealed, unfenced – a likely major source of greenhouse gas and ammonia pollution and health hazard.
Illegal in Europe and probably illegal in many US states.

Answer 25

Surface application of slurry results in evaporation/ volatilization
High risks of run-off and wash-off by rain- especially when the soil is compacted (note the wetness in the wheel tracks).

Winter applications with no vegetation present to absorb the nutrients risks contamination of surface water by runoff and groundwater by nitrate leaching.

Question 26

“splash plate slurry application offers only a 20% nutrient efficiency,

Answer 26

Slurry injection/
but the slit Injector delivers slurry down to the ground and to the root zone, so it gives an estimated 85% nutrient efficiency.

Question 27

slurry injectors- directly supplied from a tractor-towed tanker and pump unit can cause serious soil compaction- typical tanks hold upwards of 15,000 litres- i.e. over 15 tonnes.

Answer 27

Umbilical slurry injectors- fed by a tanker / pump unit parked off the field to avoid soil compaction. This is the state-of-the-art. Wide tires of low air-pressure, and very wide injector boom reduces the number of times the tractor has to drive across the field.

Question 28

Nitrogen deposition: 23% species reduction

Answer 28

compared with grasslands receiving the lowest levels of nitrogen deposition

Question 29

Nitrogen is a critical nutrient that limits growth of plants in many ecosystems.

Nitrogen cycling depends upon a complex series of microbial transformations of atmospheric N2 and recycling of organic N.

Answer 29

These processes are highly dependent on environmental conditions: temperature, water, oxygen

Adaptations to conserve N in ecosystems can result in strong accumulation of soil carbon in humus.

The N cycle has been highly disrupted by humans