W7.1, Nitrogen (N) nutrition Flashcards
How is urea transformed within the soil to become plant available?
1. UREA HYDROLYSIS:
(NH2)2CO (urea) + Urease + H2O → 2 NH3 (ammonia) + CO2
A urease enzyme catalyses the hydrolysis of urea, forming 2 ammonia molecules and 1 molecule of carbon dioxide.
2. AMMONIFICATION:
NH3 + H2O → NH4+ + OH-
When ammonia comes into contact with water, it forms an ammonium ion and a hydroxide ion.
3. NITRIFICATION
2 NH4+ + 3 O2 + Nitrosomonas or Nitrococcus → NO2- (nitrite) + H2O + 2H+
2 NO2- + O2 + Nitrobacter → NO3- (nitrate)
Firstly, ammonia is oxidised by Nitrosomonas or Nitrococcus bacteria, forming nitrite, water, and 2 hydrogen ions.
Following this, the nitrate is further oxidised by Nitrobacter, forming nitrate.
An important thing to note about this reaction is the formation of 2 hydrogen ions (acidifying) for every molecule of urea that is converted into (one molecule of) nitrate.
What is the difference between how urea and organic matter are transformed into plant-available nitrate? Explain.
Nothing, except that the urea hydrolysis step is replaced with proteolysis
1. PROTEOLYSIS:
Proteins + Proteinases → Peptides/Polypeptides
Peptides/Polypeptides + Peptidases → Amino acids
Proteolysis happens in two stages. Firstly, proteins are converted into peptides and polypeptides by proteinases.
Then, peptidases further break down those peptides and polypeptides into amino acids.
Both of these stages are accomplished by many microorganisms.
2. AMMONIFICATION:
Amino acids → deamination → NH3 (ammonia)
NH3 + H2O → NH4+ + OH-
Animo acids released during proteolysis undergo deamination, in which nitrogen-containing amino groups are removed, which then form ammonia. This is mediated by several microorganisms.
When ammonia comes into contact with water, it forms an ammonium ion and a hydroxide ion.
3. NITRIFICATION
2 NH4+ + 3 O2 + Nitrosomonas or Nitrococcus → NO2- (nitrite) + H2O + 2H+
2 NO2- + O2 + Nitrobacter → NO3- (nitrate)
Firstly, ammonia is oxidised by Nitrosomonas or Nitrococcus bacteria, forming nitrite, water, and 2 hydrogen ions.
Following this, the nitrate is further oxidised by Nitrobacter, forming nitrate.
True or false?
An agricultural system with a high OM turnover is at a high risk of acidification.
True. For every molecule of nitrate produced by the breakdown of the organic matter, two hydrogen ions are produced, which will eventually accumulate and acidify the soil.
True or false?
Some plants take up more ammonium than nitrate.
True, though they’re not very common. Rice is an example of one, and it’s become that way because rice is grown in paddies which have low oxygen concentrations which inhibit nitrification.
At what C:N ratio will organic nitrogen be mineralised almost immediately rather than being immobilised first?
C:N < 25:1
What is the point/idea behind urease inhibitors, nitrification inhibitors, and slow/controlled-release fertilisers?
By delaying the conversion of nitrogen into easily-lost forms (or the availability of the coated fertiliser), you are better able to match nutrient supply with crop demand whilst spreading fertiliser less often. Often the effects are more strongly associated with reduced N losses than increased yield (but that means needing less fertiliser overall, which increases profits anyway).
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How is nitrogen taken up and transported throughout a plant?
- Most nitrogen is taken up as nitrate
- must be taken up with a cation (K+, Na+, Ca2+, Mg2+), or plant must release an anion (OH-, HCO3-)
- once it’s taken up, it can be metabolised in the root and/or transported to the leaves and stored in vacuoles.
- uptake of ammonium requires the release of a cation.
- must be taken up with a cation (K+, Na+, Ca2+, Mg2+), or plant must release an anion (OH-, HCO3-)
- High concentrations of ammonium within the plant can be toxic, so it’s metabolised quickly in the roots (high energy and carbon demand to do this)
Explain what high affinity transport and low affinity transport systems are.
They allow plants to adjust N uptake to variable N supply.
Low affinity transport systems are often constitutive (i.e. they operate all the time), whereas high affinity transporters can be switched on and off depending on the N concentration. They can also be active in parts of the root where there is localised variation in nutrient concentration..
Finish the following sentences:
With low N availability, protein production comes at the expense of: ________ _____ ______ ___ _____.
With high N availability, protein production comes at the expense of: _______ _____________.
With low N availability, protein production comes at the expense of: nitrogen pools within the plant.
With high N availability, protein production comes at the expense of: storage carbohydrates
What is a possible negative consequence of accumulating nitrate in plants, and how might it happen?
Nitrate accumulation can occur from low rates of photosynthesis (e.g. from dull, overcast days) and high rates of N availability, and can result in nitrate poisoning of livestock or people (ties up O2 in the bloodstream).
True or false?
High shoot protein correlates with high water soluble carbohydrate content.
False. High amounts of protein production (from high rates of N) draws down carbohydrate reserves.
If you want to increase water soluble carbohydrate content, you’re better off starting from a point of N defeciency, then giving an adequate amount.
How does nitrogen fertilisation affect fruit colour?
It affects carbohydrate accumulation, anthocyanin production, shading, and ripening time.
How does N affect the development of perennial fruit crops?
Increasing the rate of N results in:
Increased vegetative growth,
Delayed time to first blossom and maturity,
Increased flower bud initiation (though can be reduced under high shading),
Delayed, and less fruit colour development,
Larger fruit size,
Lower sugar content,
Lower firmness,
Shortened post harvest storage
