Lecture 11: Rethink N fertiliser Flashcards
Why do we need to rethink N fertiliser?
Most limiting nutrient but highly abundant element
GHGe
drives climate change
expensive
generates CO2 via Haber Bosch
and N2O via soil application of fertilisers
Role of microbes in N cycling
nitrogen fixation, denitrification, anammox, nitrification, assimilation, leaching, decomposition
Nitrogen fixation
di-nitrogen to ammonia and ammonium
Denitrification
nitrate to nitrite to NO to nitrous oxide
DNRA
dissimilatory nitrate reduction to ammonium
DNRA
nitrate to ammonium without reduction to biomass
Anammox
ammonium to dinitrogen
What types of N is assimilation, leaching and decomposition more likely to occur with?
nitrates and nitrites as mobile and soluble
Functions of bacteria in N cycle
generalists and specialists
phylogenetically dispersed
defined by trait not species
Diazotrophs
fix nirogen
bacteria, archaea, rhizobia
depend on nif gene to produce nitrogenase
require low or no oxygen
How do diazotrophs get low oxygen?
symbiotic microbe: nodules or mucus
heterocyst: cyanobacteria fix nitrogen in heterocysts and pump oxygen out
Why do microbes fix N?
selfish only for own benefit NOT CROP
also assimilation
more N produced than required
Dentrifiers
bacteria, fungi, archaea
respire using N oxides not oxygen
modular system
NosZ function which can be lost/inhibited
convert NO2 to harmless N2
effected by context
DNRA microbes
compete with denitrification for substrates but maintain soil N
produces some N2O
NrfA strains may also have NosZ gene
Using microbes to reduce N loss
promote conditions that favour retention and reduce denitrification such as leys which increase C and bias towards DNRA
Using microbes to enhance N fixation
legumes
inoculants
natural diazotrophs favoured by Sierra Mixe maize variety
Issues with free living diazotrophs
sensitive to amount of N
only fix what they need
consume own NH3 produced
Engineer new N-fixing symbioses
fix the trait in other crops than legumes
complex and many steps required
mycorrhizae may be easier to produce
Engineer new N-fixing crops
improve crop N fixation without bacteria
hard as not shared with eukaryotes
need correct environment for each part
Ways to overcome GM N-fixing crop issue
build protein outside of mitochondria and assemble inside
use chloroplasts to separate into optimal environments
