Nitrogen Cycle Flashcards
What does soil N decrease with?
Average soil temperature increase, decreasing by 2-3x every 10C rise due to increasing microbial activity
Why does permanent grass vegetation have higher N content than forests?
Dense rhizosphere promoting humus formation thus N immobilization
What are organic forms of nitrogen in soils?
Proteins, nucleic acids, chitin, peptidoglycan and amino acids
When might organic nitrogen be unavaialble?
Restrictired to inaccessible micropores or binding to clay
Urea decomposers…
Urease secreted as an extracellular enzyme, being 32-69% of bacterial production being urea-decomposers
Why might Urea be bad for soils?
Hydrolysis consuming H and increasing pH, where ammonia can volatise at high pH
How might NH4 be made available to plants?
Involved in CEC or immobilised by interlayers of clays
What is nitrification performed by?
Chemoautotrophs and heterotrophs, being oxidation of NH4 and organic N into NO2 and NO3
Which chemoautotrophs perform nitrification?
Nitrobacter, nitrosomonas, nitrosolobus
What is the process of chemoautotroph nitrification?
O2, H and NH4 required for NO2 formation with NH4 deprotonation to ammonia then oxidation to hydroxylamine, then nitroxyl formation then nitrite formation
Requirements of nitrification…
Obligatory aerobic, requires O2 supply, optimal temperature range of 5-40, neutral-alklaline soils
What can abundant nitrification lead to?
Eutrophication, nitrosamine formation and methemoglobnemia in animals
How can NO3 be limited?
Slow-release fertilisers, like sulfur-coated urea: limits NH4 availabilit rate to nitrifiers
How might nitrogen be immobilised?
Microorganism fixation or NH4 ion assimilation into glutamate
How can Inorganic N be assimilated as NO3?
Reduction into NO2, NH4 depending on Mb cofactor
What can inorganic N assimlation as NO3 be inhibited by?
NH4, glutamine, glutamate due to preferential assimilation
What does C:N ratio determine?
Mineralisation/Immobilisation equilibrium balance of N and NH4
What does C:N ratio tell you?
How much N must be assimilated for every gram of C converted into biomass
What is the typical C:N ratio for bacteria?
5:1 or 8:1
Dissimilatory Nitrate Reduction
A form of aerobic respiraiton reducing nitrate via nitrite, NO2 and nitrous oxide as an electron acceptor forming N2
How can nitrate be lost?
Runoff, assimilary reduction to organic N, dissimlatory reduction to NH4, ntirate respiration to NO2,, reduction to N2
Nitrate Assimilation
NO3 reduction to NH4, then assimilation into aa
How does DNR differ from denitrification?
DNR reduces nitrate to ammonia whilst nitrification to gas
How does N fate depend on carbon concentraiton
If high, NO3 used as electron acceptor, favouring NH4 formation, or used as a terminal electron acceptor in metabolism when C is low
Example of how C and NO3 balance influences microorganism community
Simultaneously Klebsiella dissimilates nitrate to ammonia whilse Pseudomonas NO3 to N2, Klebsella outcompeted with low C, thus denitrif dominant process
What is a consequence of ammonium production?
Source of N in aerobic cnoditions, generates pyridine nucleotides increasing soil pH, NO2 is removed.
Denitrification
Nitrogenous oxides like NO3 and NO2 are generated and used as terminal electron acceptors in absence of O2, reducing them to N2
What do denitrification rates depend on?
O2, moisture, temperature, OM, C and NO3 ratio/content, especially O2 inhbiiting dentrifiying enzyme synthesis and electron flow
Why does N2 oxidation to NO3 require energy?
Due to triple bond
Industrial production of NO3
Haber-Bosch Method requires CH4 and extreme temperatures/pressure
Biological nitrogen fixation…
Performed by legumes/rhizobium and water ferns as well as Azotobacter and Clostridium
Azotobacter
GNB motile heterotroph being 4-7 micrometres forming resting body cysts growing in alkaline mesophylic soils
