Plant-soil-microbial interactions in agri-ecosystems

Question 1

What have forest FACE sites shown as a result of elevated CO2?

Answer 1

23% increase in net primary production
Soil CO2 efflux, 12-16% increase (flowing out)
14-96% increase in fine root/external mycorrhizal mycelium

Question 2

Define agri-ecosystem

Answer 2

Spatially and functionally coherent unit of agricultural activity

Question 3

What do climate shocks cause?

Answer 3

50% of crop failures

Question 4

How much GHG emissions is agriculture responsible for?

Answer 4

25% of all GHG emissions from humans

Question 5

What is important to control GHG emissions from agriculture and why?

Answer 5

Soil management because some GHG are mainly soil derived (e.g. nitrous oxide)

Question 6

How is nitrogen supplied to nitrification and denitrification?

Answer 6

N mineralisation (only for nitrification), fertiliser, manure, deposition

Question 7

Define denitrification

Answer 7

Nitrate reduction
Can be carried out by a large diversity of microorganisms, including many bacteria

Question 8

Describe denitrification

Answer 8

High energy yields, efficient metabolism
The principal mechanism by which N is returned to the atmosphere

Question 9

Name four enzymes used in the process to release N2O and N2

Answer 9

Nitrate reductase gene (nar), nitrite reductase gene (nirS), nitric oxide reductase gene (norB), nitrous oxide reductase gene (nosZ)

Question 10

What are five environmental controls on nitrate reduction (denitrification)?

Answer 10

Temperature, soil moisture, soil redox (denitrification is anaerobic), substrate availability, pH

Question 11

How does substrate availability control denitrification?

Answer 11

Low C or NO3^- availability surprises denitrification
High labile C favours N2O production (not N2)

Question 12

How does pH control denitrification?

Answer 12

Low pH surpresses denitrification enzyme activity and inhibits complete denitrification to N2 (results in more N2O)

Question 13

Describe the relationship between soil moisture and nitrification/denitrification

Answer 13

Lower % of water-filled pore space favours nitrification and NO is more in flux
N2O and then N2 flux increases with more water-filled pore space

Question 14

How does cultivation impact denitrifying soil microbe?

Answer 14

Their activity and population decreases, reducing N2O emissions

Question 15

Describe the family Fabaceae

Answer 15

Legumes
Form associations with mycorrhizal fungi and N-fixing bacteria
Fix N from atmosphere

Question 16

How does the family Fabaceae fix bacteria?

Answer 16

Mutualistic bacteria that colonise their roots
Includes alpha and beta proteobacteria
Fixation is catalysed by nitrogenase

Question 17

Give three types of non-biological nitrogen fixation, in order of most N fixation to least

Answer 17

Industrial, combustion, lightning

Question 18

Give three types of biological nitrogen fixation, in order of most N fixation to least

Answer 18

Agricultural land, forest and non-agri land, sea

Question 19

Which fixes more N2, non-biological or biological pathways?

Answer 19

Biological (175 10^6 metric tons per year, compared to 80 10^6m metric tons per year)

Question 20

What is meant by a bacteria ‘cheating’?

Answer 20

If it colonises a host plant that is already colonised by a different genotype/species and benefits from the plant without doing the proportional amount of N fixation
This is a mutualism

Question 21

How do host plants reward bacteria for N fixation?

Answer 21

Controlling oxygen supply to nodules

Question 22

Define mycorrhiza

Answer 22

A mutualistic association between plant roots and soil fungi
Over 90% of all land plants form these associations

Question 23

Describe mycorrhiza

Answer 23

Mutualism based on the transfer of soil-derived nutrients to the plant from the fungus and the reciprocal transfer of photosynthate

Question 24

What are the two types of mycorrhizal fungi?

Answer 24

Arbuscular (AM) and ectomycorrhizal (ECM)

Question 25

Describe arbuscular mycorrhizal (AM) fungi

Answer 25

Form arbuscules in root cells, this is where nutrient exchange between the fungus and the plant occurs

Question 26

Describe ectomycorrhizal (ECM) fungi

Answer 26

Forms a mantle, a thick sheath of fungal tissue that covers the root tips
Often produces fruiting bodies, e.g. mushrooms, truffles

Question 27

What type of mycorrhizal fungi do grasses associate with?

Answer 27

Arbuscular

Question 28

What did Cavagnaro et al (2005) conclude about AM fungi?

Answer 28

Different species have different foraging strategies

Question 29

What can sometimes be determined by AM fungal diversity?

Answer 29

Plant productivity
Including biomass, hyphae length, and P content

Question 30

Describe how consensus on the N cycle has changed

Answer 30

Relies much less on mineralisation by free-living microorganisms that previously thought, much more on ECM

Question 31

What have trials on mycorrhizal fungi and crop production found?

Answer 31

In most trials, AM fungi positively affected grain yield

Question 32

What does mycorrhizal phenotype depend on?

Answer 32

Plant genotype, fungal genotype, and environment

Question 33

Give three examples of challenges with taking advantage of mycorrhizal fungi

Answer 33

Controlling role of native soil biodiversity, carry-over effects from previous rotations, symbioses evolved for plants to survive poor conditions rather that largest/nutritious yield

Question 34

What can enhance phosphorus availability?

Answer 34

Phosphorus solubilising microorganisms (PSB)

Question 35

What do plants have an inherent capacity to do?

Answer 35

Acquire phosphorus from different forms

Question 36

Describe how mycorrhizal fungi can enhance agricultural systems

Answer 36

Stabilise soil, enhance soil fertility, establish perennial of functional symbiotic networks that lead to distribution of resources and transfer of pest-alert signals

Question 37

What else may be able to be transferred by mycorrhizal fungi networks?

Answer 37

Signalling compounds produced by plants in response to an aphid attack, leading to aerial release of defence VOCs