Plant-soil-microbial interactions in agri-ecosystems Flashcards
What have forest FACE sites shown as a result of elevated CO2?
23% increase in net primary production
Soil CO2 efflux, 12-16% increase (flowing out)
14-96% increase in fine root/external mycorrhizal mycelium
Define agri-ecosystem
Spatially and functionally coherent unit of agricultural activity
What do climate shocks cause?
50% of crop failures
How much GHG emissions is agriculture responsible for?
25% of all GHG emissions from humans
What is important to control GHG emissions from agriculture and why?
Soil management because some GHG are mainly soil derived (e.g. nitrous oxide)
How is nitrogen supplied to nitrification and denitrification?
N mineralisation (only for nitrification), fertiliser, manure, deposition
Define denitrification
Nitrate reduction
Can be carried out by a large diversity of microorganisms, including many bacteria
Describe denitrification
High energy yields, efficient metabolism
The principal mechanism by which N is returned to the atmosphere
Name four enzymes used in the process to release N2O and N2
Nitrate reductase gene (nar), nitrite reductase gene (nirS), nitric oxide reductase gene (norB), nitrous oxide reductase gene (nosZ)
What are five environmental controls on nitrate reduction (denitrification)?
Temperature, soil moisture, soil redox (denitrification is anaerobic), substrate availability, pH
How does substrate availability control denitrification?
Low C or NO3^- availability surprises denitrification
High labile C favours N2O production (not N2)
How does pH control denitrification?
Low pH surpresses denitrification enzyme activity and inhibits complete denitrification to N2 (results in more N2O)
Describe the relationship between soil moisture and nitrification/denitrification
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
How does cultivation impact denitrifying soil microbe?
Their activity and population decreases, reducing N2O emissions
Describe the family Fabaceae
Legumes
Form associations with mycorrhizal fungi and N-fixing bacteria
Fix N from atmosphere
How does the family Fabaceae fix bacteria?
Mutualistic bacteria that colonise their roots
Includes alpha and beta proteobacteria
Fixation is catalysed by nitrogenase
Give three types of non-biological nitrogen fixation, in order of most N fixation to least
Industrial, combustion, lightning
Give three types of biological nitrogen fixation, in order of most N fixation to least
Agricultural land, forest and non-agri land, sea
Which fixes more N2, non-biological or biological pathways?
Biological (175 10^6 metric tons per year, compared to 80 10^6m metric tons per year)
What is meant by a bacteria ‘cheating’?
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
How do host plants reward bacteria for N fixation?
Controlling oxygen supply to nodules
Define mycorrhiza
A mutualistic association between plant roots and soil fungi
Over 90% of all land plants form these associations
Describe mycorrhiza
Mutualism based on the transfer of soil-derived nutrients to the plant from the fungus and the reciprocal transfer of photosynthate
What are the two types of mycorrhizal fungi?
Arbuscular (AM) and ectomycorrhizal (ECM)
Describe arbuscular mycorrhizal (AM) fungi
Form arbuscules in root cells, this is where nutrient exchange between the fungus and the plant occurs
Describe ectomycorrhizal (ECM) fungi
Forms a mantle, a thick sheath of fungal tissue that covers the root tips
Often produces fruiting bodies, e.g. mushrooms, truffles
What type of mycorrhizal fungi do grasses associate with?
Arbuscular
What did Cavagnaro et al (2005) conclude about AM fungi?
Different species have different foraging strategies
What can sometimes be determined by AM fungal diversity?
Plant productivity
Including biomass, hyphae length, and P content
Describe how consensus on the N cycle has changed
Relies much less on mineralisation by free-living microorganisms that previously thought, much more on ECM
What have trials on mycorrhizal fungi and crop production found?
In most trials, AM fungi positively affected grain yield
What does mycorrhizal phenotype depend on?
Plant genotype, fungal genotype, and environment
Give three examples of challenges with taking advantage of mycorrhizal fungi
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
What can enhance phosphorus availability?
Phosphorus solubilising microorganisms (PSB)
What do plants have an inherent capacity to do?
Acquire phosphorus from different forms
Describe how mycorrhizal fungi can enhance agricultural systems
Stabilise soil, enhance soil fertility, establish perennial of functional symbiotic networks that lead to distribution of resources and transfer of pest-alert signals
What else may be able to be transferred by mycorrhizal fungi networks?
Signalling compounds produced by plants in response to an aphid attack, leading to aerial release of defence VOCs
