Lecture 11: Rethink N fertiliser

Question 1

Why do we need to rethink N fertiliser?

Answer 1

Most limiting nutrient but highly abundant element
GHGe
drives climate change
expensive
generates CO2 via Haber Bosch
and N2O via soil application of fertilisers

Question 2

Role of microbes in N cycling

Answer 2

nitrogen fixation, denitrification, anammox, nitrification, assimilation, leaching, decomposition

Question 3

Nitrogen fixation

Answer 3

di-nitrogen to ammonia and ammonium

Question 4

Denitrification

Answer 4

nitrate to nitrite to NO to nitrous oxide

Question 5

DNRA

Answer 5

dissimilatory nitrate reduction to ammonium

Question 6

DNRA

Answer 6

nitrate to ammonium without reduction to biomass

Question 7

Anammox

Answer 7

ammonium to dinitrogen

Question 8

What types of N is assimilation, leaching and decomposition more likely to occur with?

Answer 8

nitrates and nitrites as mobile and soluble

Question 9

Functions of bacteria in N cycle

Answer 9

generalists and specialists
phylogenetically dispersed
defined by trait not species

Question 10

Diazotrophs

Answer 10

fix nirogen
bacteria, archaea, rhizobia
depend on nif gene to produce nitrogenase
require low or no oxygen

Question 11

How do diazotrophs get low oxygen?

Answer 11

symbiotic microbe: nodules or mucus
heterocyst: cyanobacteria fix nitrogen in heterocysts and pump oxygen out

Question 12

Why do microbes fix N?

Answer 12

selfish only for own benefit NOT CROP
also assimilation
more N produced than required

Question 13

Dentrifiers

Answer 13

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

Question 14

DNRA microbes

Answer 14

compete with denitrification for substrates but maintain soil N
produces some N2O
NrfA strains may also have NosZ gene

Question 15

Using microbes to reduce N loss

Answer 15

promote conditions that favour retention and reduce denitrification such as leys which increase C and bias towards DNRA

Question 16

Using microbes to enhance N fixation

Answer 16

legumes
inoculants
natural diazotrophs favoured by Sierra Mixe maize variety

Question 17

Issues with free living diazotrophs

Answer 17

sensitive to amount of N
only fix what they need
consume own NH3 produced

Question 18

Engineer new N-fixing symbioses

Answer 18

fix the trait in other crops than legumes
complex and many steps required
mycorrhizae may be easier to produce

Question 19

Engineer new N-fixing crops

Answer 19

improve crop N fixation without bacteria
hard as not shared with eukaryotes
need correct environment for each part

Question 20

Ways to overcome GM N-fixing crop issue

Answer 20

build protein outside of mitochondria and assemble inside
use chloroplasts to separate into optimal environments