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Sustainable agroecosystems - year 3 > Lecture 9 - nitrogen > Flashcards

Lecture 9 - nitrogen Flashcards

1
Q

what is nitrogen required as a constituent of?

A

proteins, amino acids, nucleic acids, some membrane lipids

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2
Q

what requires the largest amount of nitrgoen?

A

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

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3
Q

what is there a fundamental relationship between - shoot N and what?

A

plant functional types, and rates of photosynthesis

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4
Q

describe plants grown in low N concentrations

A

typically everygreen shrubs and trees grown in nitrogen poor environments) have inherently slow rates of growth and photosynthesis

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5
Q

5 facts about N

A
  • The 4th most abundant element in plants (on a mass basis) after C, H, and O
  • Plants have high N requirements relative to supply
  • N- most often limits plant growth in nature
  • N- the main constituent of artificial fertilizer
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6
Q

because N is primarily being cycled back through from sources from the atmosphere rather than the ground - what have we ignored for years?

A
  • the amounts of nitrogen that might be cycled back from the earth’s crust itself (from rocks)
  • quite a lot of N is released back from crustal rocks and clay rocks
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7
Q

describe what might happen to N being put into the oceans?

A

so much N put into the oceans these days its becoming a geological sink for excess N in the ocean that it may get bound onto organic matter and clays then settle on the ocean and be buried into the rocks and then get bought back onto the land by plate tectonic or into the atmosphere by volcanic activities and degassing

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8
Q

when you look at the budget of N in the surface layers of terrestrial ecosystems most of the nitrogen is present in what?

A

organic matter and a small amount is present in inorganic matter

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9
Q

where is most of the organic component of N present in?

A
  • most of the organic component is dead material - i.e. soil
  • small percent is in living biomass - majority in plants small amount in animals
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10
Q

what other component is nitrogen interlinked too?

A

carbon - dead organic matter is a interlinked store

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11
Q

what can plants do as they take up nitrogen?

A
  • convert it into biomass
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12
Q

8 processes in the N cycle

A
  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
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13
Q

what is the N cycles primarily driven by?

A

microbial activity

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14
Q

what are inputs into the N cycle via the biological cycling activities strongly dependent on?

A

inputs into the system via the biological cycling activities are strongly dependent upon inputs coming as organic materials

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15
Q

what are outputs from the N cycle and how can they vary?

A

leaching - nitrate is the most mobile form of nitrogen - the extent of leaching losses depends on the form of nitrogen - ammonium = low leaching

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16
Q

in what form is nitrogen mainly taken up as and then returned to the soil as?

A
  • Nitrogen is mainly taken up as mineral nitrogen and then returns to the soil as organic nitrogen
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17
Q

what do plants transform nitrogen into?

A

plants are transforming the nitrogen from ammonium and nitrate into amino acids and lignin and various other carbon compounds which contain nitrogen but bound to carbon

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18
Q

why do you tend to see a deeper distribution of nitrogen and less peaking at the surface like P?

A
  • More mobile stages of nitrogen than phosphorous
19
Q

how do roots respond to nitrogen?

A

Plants roots respond to localised nitrogen supply - useful when pellets of fertilisers are added - once roots find it they will just proliferate around it

20
Q

what effects the N cycle the most?

A

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

  • natural N fixation = 130 m tonnes
  • anthropogenic = 150 m tonnes
21
Q

what causes natural nitrogen fixation? (2)

A

1) organisms

2) lightening

22
Q

2 examples of anthropogenic nitrogen fixation?

A

1) fertiliser factories

2) biological N fixation through the use of legume crops etc

23
Q

how could we reduce our reliance on N fertiliser?

A
  • Keeping fertiliser use slightly below optimum plant yield might be better than maximising yield rather than using a lot of fertiliser and wasting it and causing environmental pollution as a result
  • May be potential to breed for higher nitrogen use efficiency - needs to be more of a goal
24
Q

what is the worry about biogeochemical cycles?

A
  • Are we changing biogeochemical cycles to a point where it’s not going to be habitable for humans in the future?
  • In terms of climate change, we are in a state of uncertainty
  • For P and N we are already in a high risk zone - past the point of uncertainty
25
Q

describe the issue of excess consumption

A
  • Producing an awful lot more protein than we need - need more efficient management
  • As grain yield goes up the percentage of protein goes down - nutrition of food diminishes
  • UK N fertilizer use has peaked
26
Q

4 ways to increase care with N fertiliser?

A
  • Cost
  • Precision agriculture
  • Pollution
  • Nitrate-vulnerable zone restrictions
27
Q

what is precision agriculture?

A

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

28
Q

describe fertiliser use in Africa?

A
  • Low rates of N fertilizer use in Africa give high efficiency of use, but suboptimal crop yields
29
Q

describe fertiliser use in china?

A

Increasing rates of fertilizer use in China to feed the population- inefficient use

30
Q

what is the issue with how our nitrogen fertiliser is generated?

A
  • All of our nitrogen fertiliser is generated by fossil fuels/ natural gas = implications in terms of co2 release
31
Q

what happens to organic carbon and nitrogen stocks in the soil when you add mineral nitrogen fertiliser?

A

when you add a lot of nitrogen (mineral fertiliser) to the soil you tend to deplete the organic carbon stocks of the soil and deplete the organic nitrogen stocks of the soil

32
Q

nitrogen exists in multiple states- why is the conversion between these states important?

A
  • The conversion between these states is very important - some components are greenhouse gases e.g. nitrous oxide
33
Q

how is ammonium generated?

A

by ammonification - organic nitrogen is broken down by a microorganism and the carbon is removed - nitrogen is released as ammonium into the environment - then taken up again by plants

34
Q

what do nitrifying bacteria do?

A

Convert ammonium to nitrate

  • They cause acidification - release hydrogen ions
  • Conversion of ammonium to nitrate releases nitric acid plus 4 hydrogen ions
  • Contributes to soil acidity
35
Q

what happens if the nitrate is taken up by plants or micro-organisms?

A

the acidification is neutralised

36
Q

when does nitrification become inhibited?

A

When the soil becomes very acidic the microorganisms involved in nitrification tend to become inhibited
In acid soils (pH <4.5) nitrification is inhibited
-The fate of the nitrogen has major implications for long term soil PH

37
Q

describe the process of denitrification

A
  • Occurs in poorly aerated soils
  • Process releases amino acids
  • ammonia tends to occur in very cold or very warm environments
  • Nitrification and denitrification can occur simultaneously in different microsites - these pathways in aerobic and anaerobic zones can be very important
38
Q

what is the issue of N20?

A
  • 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 - cultivated soils are the major culprit
39
Q

why are rice paddys problematic?

A
  • 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
40
Q

3 contributions of different types of nitrogen into the atmosphere from agriculture sources

A
  • NOx release is dominated by vehicle pollution
  • Agriculture in the UK is a modest source of nitrogen oxide emissions
  • In terms of ammonia - pollutant comes mainly from livestock farming
41
Q

what is the issue with slurry pits?

A

a likely major source of greenhouse gas and ammonia pollution and health hazard.
Illegal in Europe and probably illegal in many US states.
Inefficient, environmentally damaging poor practice

42
Q

3 examples of bad slurry practice

A

1) Surface application of slurry results in evaporation/ volatilization- High risks of run-off and wash-off by rain- especially when the soil is compacted
2) Winter applications with no vegetation present to absorb the nutrients risks contamination of surface water by runoff and groundwater by nitrate leaching
3) Avoiding driving on soft soil by keeping to roads and spraying slurry over the hedge- but such surface applications and aspirating into the air results in major losses by volatilization contributing to diffuse air pollution

43
Q

what is the best way to get slurry into the soil?

A

injecting it - it gives an estimated 85% nutrient efficiency

44
Q

2 types of slurry injection

A

1) 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.
2) 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 tyres of low air-pressure, and very wide injector boom reduces the number of times the tractor has to drive across the field

Sustainable agroecosystems - year 3 (17 decks)

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