Lecture 12 - efficient nutrient use Flashcards
why are rice paddy fields highly productive?
- 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
what is the issue with paddy fields?
- 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
how do rice crops contribute to GHG emissions?
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
paddy rice production has increased 130% - what is this down too?
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
why is it hard to compare the greenhouse gas potential of methane and nitrous oxides?
they gradually break down but at different rates - so depends on the time frame you look at
- N20 persists more than methane
what is important when considering the effect of greenhouse gases?
the time frame over which they do these assessments is important - need to expand timeframes beyond 20 yrs to understand long term consequences
what is a redox potential?
measure of the oxidation-reduction status
what is redox driven by?
water saturation of the soil -related to the soil structure, drainage and water inputs
why is redox driven by water?
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
how does reduction affect PH?
removes free H ions and PH increases
how does oxidation affect PH?
releases H ions and PH falls
what is the consequence of most agricultural crops having roots close to the soil surface?
- 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
what is the issue with focusing on breeding crops with maximum grain yields?
there isn’t enough attention on maximising the effectiveness of root systems for capturing water and nutrients
why is capturing water via root systems important?
because climate change is causing increased droughts and reduced reliability of rainfall therefore efforts need to be put into considering root systems
what is another advantage of putting plants with deeper roots back into rotations ?
they may generate more pore space deeper in the soil and make it easier for the next crop to get into those spaces
what is the issue with short crop life cycles?
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
perrenial crops wont produce enough grain but what could be a trade off in growing them?
they may be able to grow in more environments with lower inputs so the cost effectiveness may be greater with yields being lower
how else could you improve soil structure and nutrient status of arable soils?
use of diverse herbal leys (for 3-4 years) in crop rotations including deep rooting species and nitrogen fixing legumes
why is getting carbon back into the soil through roots important?
because it stays there for longer - carbon currency from roots is better than for shoots for persistence in the soil
how could altered root morphology help plants obtain phosphorus and when would the technique not be of use?
- 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
how do plants without mycorrhizal associations uptake P and N?
Root adaptations: very long root hairs and secretion of large amounts of organic acids to facilitate P uptake
what happens to the roots the more P is added?
the more phosphorus is available the less efficient becomes the roots
what happens when roots save tissue by investing less in physical tissue and more in surface area?
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
where does most P end up in crops?
- 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%.
what is the issue with alot of the P in crop grains?
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
what are potential solutions to waste of P?
putting enzymes in pigs food to digest it or breed wheat that puts less phosphorus into grain
5 strategies for improving P efficiency in cropping systems?
- Greater use of crop species with cluster roots in environments with large amounts of total P but low P availability –
- Potential of native species with P-efficient traits for use in crops or pastures? New crops?
- Breeding of greater P-use efficiency in existing crop species.
- Better understanding of the molecular basis of P-efficient traits
- Intercropping and crop rotations to provide benefits of P-efficient crops to less efficient ones.
describe the specialised root adaptation: cluster roots?
- 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
what do plants that have cluster roots also often have but also rarely have?
N fixing symbiosis but rarely have mycorrhiza
describe how root clusters are under control of the plant?
- 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
describe dauciform roots
- 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
describe the benefit of growing legume trees and grasses together?
- 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
name other uses of legume trees and how this is positive?
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
name as many potential ways to breed crops for greater nutrient use efficiency
- 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
