Chapter 8 Plant Nutrient Use

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Chapter 8Plant Nutrient Use

• Part II Mechanisms
• Chapin, Matson, Mooney
• Principles of Terrestrial Ecosystem Ecology

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Belowground resources control GPP, NPP and
decompositionWhat controls acquisition of
belowground resources?
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Nutrient supply is a critical control over NPP
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Most nutrients absorbed by plants are recycled
from detritus
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How do nutrients get to roots?

• 1. Diffusion (most important)
• 2. Mass flow (sometimes important)
• 3. Root interception (unimportant)

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How do nutrients get to roots? 1. Diffusion

• Driving forces
• Nutrient uptake
• Mineralization
• Consequence
• Diffusion shell
• Zone of nutrient depletion around each root
• Large for mobile ions

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How do nutrients get to roots? 2. Mass flow

• Nutrient movement in flowing water
• Transpirational water
• Gravitational water after rain
• Insufficient for growth-limiting nutrients
• Replenishes diffusion shells

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How do nutrients get to roots?3. Root
interception

• Concentration in root higher than soil it
  displaces
• No-win situation
• Unimportant

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What controls nutrient absorption by roots?
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Nutrient supply is THE major control over
nutrient uptake
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Root elongation is main way plants can increase
nutrient uptake

• Increased rootshoot ratio
• Increased investment in roots
• Root proliferation in nutrient hot spots
• Root growth occurs where it does the most good
• Longer root hairs

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Mycorrhizae increase soil volume used by plants

• Trade carbohydrates for nutrients
• Balanced parasitism
• Most advantageous for immobile nutrients
• e.g., phosphate

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Types of mycorrhizae

• Ectomycorrhizae
• Form sheath around root
• Common in woody plants
• Arbuscular mycorrhizae
• Proliferate around root
• Common in grasses, herbs, and tropical trees

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Mechanisms of nutrient uptake

• Active transport most important
• Requires energy
• Moves against concentration gradient
• Abundant nutrients may enter by diffusion or mass
  flow

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Some plants tap nutrients unavailable to other
plants High-latitude plants absorb amino
acids Prefer ammonium over nitrate Take
whatever they can get
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N availability in Alaskan boreal forests
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Nitrogen uptake and use

• Nitrate must be reduced to ammonium
• Nitrate reduction often expensive
• Ammonium must be assimilated
• Attached to a carbon skeleton
• Amino acids must be transported through plant
• Used for protein synthesis

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Tapping Phosphorus

• Phosphatase enzymes
• Cleaves P from SOM
• Siderophores
• Solubilize mineral P
• Chelate-P complex diffuses to root

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Plants compensate for specific nutrient stresses
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Strong correlation between nutrient uptake and
production
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Nutrient uptake correlates with growth

• Sometimes nutrient uptake drives growth
• Sometimes growth drives nutrient uptake

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Nutrient uptake influences rhizosphere

• Reduces nutrient concentrations
• Enhances decomposition
• Changes pH
• Excretes H when absorb cations

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Summary

• Nutrient supply constrains uptake
• Diffusion is main mechanism of supply
• Plants adjust uptake rate to meet demands
• Root growth
• Uptake capacity
• Mycorrhizae
• Enhance supply

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Nutrient ratios dont vary too much

• Means availability of growth-limiting nutrient
  governs uptake of all nutrients, BUT
• Ratios are somewhat variable
• Similarity of ratios reflects regulation of
  uptake
• Differences among ratios reflects storage

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Nutrient use efficiency
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Nutrient use efficiency (NUE)(two definitions)

• Physiological approach (plant level)
• NUE a t
• a nutrient productivity (photosynthesis/g N)
• t residence time of nutrient in plant
• Ecosystem approach (stand-level)
• NUE g biomass/g nutrient in litter

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A long residence time is the major adaptation to
low-nutrient Habitats Plants dont increase
their carbon gain per unit nutrient
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All species are similar in resorption
efficiency No major difference in proportion of
nutrients lost
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All species are similar in the proportion of
nutrients leached
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