Lecture 12 - efficient nutrient use

Question 1

why are rice paddy fields highly productive?

Answer 1

• no limitation by water
• little competition from weeds
• nutrients are made available to rice by waterlogging
• up to three rice crops can be planted in a year

Question 2

what is the issue with paddy fields?

Answer 2

• contributing >10 % of global methane emissions, and have made increasing contributions to N2O fluxes too
• Is argued that eating rice is worse for the environment than most other cereal crops

Question 3

how do rice crops contribute to GHG emissions?

Answer 3

the root anatomy - air channels that run through the roots allowing gases to escape out the soil via the plants up into the atmosphere through the stomata
- Enable gases to bypass the microbial communities within the soil that normally process the greenhouse gases and convert them to products that are less damaging to the environment

Question 4

paddy rice production has increased 130% - what is this down too?

Answer 4

due to semi-dwarf, early-maturing rice varieties that can be planted up to three times per year and are responsive to nitrogen fertilizers
- These new rice varieties grown in irrigated land contribute to nearly three-quarters of the world’s total rice production

Question 5

why is it hard to compare the greenhouse gas potential of methane and nitrous oxides?

Answer 5

they gradually break down but at different rates - so depends on the time frame you look at
- N20 persists more than methane

Question 6

what is important when considering the effect of greenhouse gases?

Answer 6

the time frame over which they do these assessments is important - need to expand timeframes beyond 20 yrs to understand long term consequences

Question 7

what is a redox potential?

Answer 7

measure of the oxidation-reduction status

Question 8

what is redox driven by?

Answer 8

water saturation of the soil -related to the soil structure, drainage and water inputs

Question 9

why is redox driven by water?

Answer 9

because co2 and o2 diffuse much more slowly in water than in air however the solubility of co2 in water is much higher than oxygen therefore as oxygen gets used up by microorganisms living in water the rate at which it can diffuse back in is very slow however co2 is much more mobile within a waterlogged system
Soil saturated with water quickly runs out of available O2

Question 10

how does reduction affect PH?

Answer 10

removes free H ions and PH increases

Question 11

how does oxidation affect PH?

Answer 11

releases H ions and PH falls

Question 12

what is the consequence of most agricultural crops having roots close to the soil surface?

Answer 12

• A lot of deep agricultural soils are potentially losing nutrients below the depth at which the plants are rooting
• In dry conditions they can’t access some of the water content in the depths of the soil because of the shallow root systems

Question 13

what is the issue with focusing on breeding crops with maximum grain yields?

Answer 13

there isn’t enough attention on maximising the effectiveness of root systems for capturing water and nutrients

Question 14

why is capturing water via root systems important?

Answer 14

because climate change is causing increased droughts and reduced reliability of rainfall therefore efforts need to be put into considering root systems

Question 15

what is another advantage of putting plants with deeper roots back into rotations ?

Answer 15

they may generate more pore space deeper in the soil and make it easier for the next crop to get into those spaces

Question 16

what is the issue with short crop life cycles?

Answer 16

most crop systems have to grow in relatively short parts of the year and then the whole plant dies as its harvested including roots
- perennial crops generate deeper root systems

Question 17

perrenial crops wont produce enough grain but what could be a trade off in growing them?

Answer 17

they may be able to grow in more environments with lower inputs so the cost effectiveness may be greater with yields being lower

Question 18

how else could you improve soil structure and nutrient status of arable soils?

Answer 18

use of diverse herbal leys (for 3-4 years) in crop rotations including deep rooting species and nitrogen fixing legumes

Question 19

why is getting carbon back into the soil through roots important?

Answer 19

because it stays there for longer - carbon currency from roots is better than for shoots for persistence in the soil

Question 20

how could altered root morphology help plants obtain phosphorus and when would the technique not be of use?

Answer 20

• Root hairs can improve efficiency of capture - root hairs more important in plants that aren’t so heavily mycorrhizal
• Therefore, breeding for root hair length might be beneficial - however if they have a strong mycorrhizal partnership, they may not need this

Question 21

how do plants without mycorrhizal associations uptake P and N?

Answer 21

Root adaptations: very long root hairs and secretion of large amounts of organic acids to facilitate P uptake

Question 22

what happens to the roots the more P is added?

Answer 22

the more phosphorus is available the less efficient becomes the roots

Question 23

what happens when roots save tissue by investing less in physical tissue and more in surface area?

Answer 23

result in the generation of root porosity
- potential consequences of this trait for greenhouse gas emissions from crops grown on waterlogged soils as mentioned in rice paddys

Question 24

where does most P end up in crops?

Answer 24

• most of the P in above ground biomass into the grain

- The proportion of the total P ending up in grain is normally >50%, and often as high as 90%.

Question 25

what is the issue with alot of the P in crop grains?

Answer 25

much of the P in grain (e.g. 88% in wheat) can be stored in phytate or phytic acid which is not readily digested by humans and pigs (fed largely on cereals) and ends up in manure - potentially an environmental contaminant

Question 26

what are potential solutions to waste of P?

Answer 26

putting enzymes in pigs food to digest it or breed wheat that puts less phosphorus into grain

Question 27

5 strategies for improving P efficiency in cropping systems?

Answer 27

1. Greater use of crop species with cluster roots in environments with large amounts of total P but low P availability –
2. Potential of native species with P-efficient traits for use in crops or pastures? New crops?
3. Breeding of greater P-use efficiency in existing crop species.
4. Better understanding of the molecular basis of P-efficient traits
5. Intercropping and crop rotations to provide benefits of P-efficient crops to less efficient ones.

Question 28

describe the specialised root adaptation: cluster roots?

Answer 28

• Increased surface for nutrient absorption- absorptive surface is enormous, but far greater than functional: overlapping depletion zones!
• cluster roots release organic compounds into the soil
• Increased surface is ideal to locally enhance the concentration of the exudates

Question 29

what do plants that have cluster roots also often have but also rarely have?

Answer 29

N fixing symbiosis but rarely have mycorrhiza

Question 30

describe how root clusters are under control of the plant?

Answer 30

• roots are suppressed at a high P supply - highly regulated by the plants
• The cluster root systems develop very quickly - High RGR - whole system is formed in less than 20 days
• Root organic acid exudation as a ‘burst’ coincident with attainment of maximum of root surface area - overwhelms capacity of bacteria to compete with it for the nutrients that are released - once its done the resource is depleted so the root system dies and isn’t needed

Question 31

describe dauciform roots

Answer 31

• Dauciform roots are characterised by exceptionally long root hairs produced at a very high frequency per unit root length
• Dauciform roots release exudates, in an pulse coupled to root maturity, just like cluster roots

Question 32

describe the benefit of growing legume trees and grasses together?

Answer 32

• grasses are helping to build soil aggregation - helping carbon to be stored and the legume trees are putting nitrogen into the system

build up nutrient fund and restructure the soil

Question 33

name other uses of legume trees and how this is positive?

Answer 33

timber, fuel, honey (nectar rich flowers that bees use to make honey) - multi functionality component - balance food production and other environmental goods and services in a sustainable way

Question 34

name as many potential ways to breed crops for greater nutrient use efficiency

Answer 34

• Root hairs
• Root branching
• Root fineness
• Specialized roots for “mining” P and Fe from soils
• Crop rotations e.g. intercropping plants with different rooting traits- especially N fixing
• Use of deep rooted perennial intercrops
• Breeding to promote symbiotic associations with N fixing organisms and mycorrhizas.
  0 Management to promote mycorrhizas and N fixers
• Reduced nutrient demand- selection for reduced phytate accumulation in grains