Lecture 8: Terrestrial Plant Nutrient Use

Dra. Elisabeth Huber-SannwaldIPICYT

Global Change and Ecology

Summer semester 2010

Belowground resources affect GPP, NPP and decomposition of organic material

What controls the acquisition of belowground resources?

Nutrient supply in less than optimal balance limits plant growth.Tissue nutrient ratios are fairly similar across plants. Thus, the most growth limiting nutrient is responsible for the cycling rates of all nutrients.

What factors control the cycling rates of the most growth limiting mineral nutrient?

TERRESTRIAL PLANT NUTRIENT USE

Availability of mineral nutrients in soil (100%): - 0.2% are dissolved in soil water - rest is bound

* 98% are bound in organic detritus, humus, or incorporated in minerals (~ storage of mineral nutrients)

* 2% are adsorbed on soil colloids

Nutrient Movement to Roots

1) Diffusion (most important) – when nutrient uptake by the root exceeds the supply by mass flow; exceedingly important under low soil fertility conditions

2) Mass flow – driven by plant transpiration, depends on soil mineral concentration and rate of water movement to root

3) Root interception (~< 10% of mineral nutrient uptake) – physical displacement of soil particles

Depletion zones(Diffusion shells)mobile ions: NO3

-, K+ immobile ions: PO4-, NH4

+

Diffusion

1 cm 1 mmBulk Soil

mineralization mineralizationroot uptake root uptake

NO3- > K >> NH4

+ >> P

Monovalent (NO3-) ions move more rapidly than bivalent (Ca2+) ions.

Process of resource supply of macronutrients (i.e. required in large quantities)

Pool of dissolved ions

H+ under high cation uptake

Cation Exchange Capacity (CEC)

Def.: the capacity of a soil to hold exchangeable cations on negatively charged sites on the surfaces of minerals and organic matter.

The clay minerals that dominate the temperate zone soils possess a net negative charge that attracts and holds cations dissolved in the soil solution.

Any element with a positive charge is called a cation and in this case, it refers to the basic cations, calcium (Ca2+), magnesium (Mg2+), potassium (K+) and sodium (Na+) and the acidic cations, hydrogen (H+) and aluminum (Al3+).

The total amount of these positively charged cations a soil can hold is described as the CEC and is expressed in miliequivalents (mEq) per 100 grams (meq/100g) of soil.

Soil organic matter has a very high CEC; they originate from the phenolic (-OH) and organic acid (-COOH) radicals of soil humic materials.

• The larger the CEC, the more cations the soil can hold. • Clay soil has larger CEC than sandy soil. • CEC gives an indication of the soils potential to hold plant nutrients. • Increasing the organic matter content of any soil will help to increase the CEC.

Cations are held and displace one another in the sequence

Al3+ > H+ > Ca2+ > Mg2+ > K+ > NH4+ > Na+

base cations

on cation exchange sites. This sequence assumes equal molar concentration in the initial soil solution. It can be altered by the presence of large quantities of the more weakly held ions.

E.g.: agricultural liming is an attempt to exchange Al3+ ions from the exchange sitesby adding a lot of Ca2+.

Most stronglyheld

Most weaklyheld

The percentage of the total exchangeable cation pool occupied by base cationsis termed base saturation.

Soil buffering

CEC acts to buffer the acidity of many temperate soils. When H+ is added to thesoil solution, it exchanges for cations, especially Ca2+, on clay minerals and organic matter.

As long as there is enough base saturation (>15%), buffering by CEC explains whythe pH of many temperate soils shows relatively little change when they are exposed to acid rain.

Mass flow

Important when:

1) nutrient ions are abundant in soil solution (Ca2+)2) nutrient demand is covered by small ion quantities (micronutrients)

Mechanism:

1) transpiration flow2) gravitational flow after precipitation --- > to replenish diffusion shells

Root interception

Encounter of mineral nutrients via root growth in yet unoccupied soil. Nutrient supply through root interception does not cover plant demand for new root growth. -- > unimportant, no-win situation

What controls nutrient absorption by roots

Root length and nutrient supply - THE major control of nutrient uptake

Phosphate uptake

Nutrient supply controls nutrient uptake by vegetation and NPP of ecosystems

Root elongation

122 x 24 x(without “Crops”)

44 x

Root elongation is the main way plants can increase nutrient uptake in soils; this is highest in soils that are not fully occupied by roots.

nutrient supply!!!

Roots proliferate preferentially in zones of high nutrient accumulation (called nutrient “patches” or “hot spots” )

Root hairs

= tubular extensions of rootepidermis to increase the absorptive root surface; e.g. in 4-months-old rye plants, one plant had 14 bill. root hairs, with 401 m2 absorbing surface; ~ 10,000 km!

Root length

Paul & Clark 1988

Root symbiosis: Mycorrhizae

Symbiotic association (“balanced parasitism”) between fungi and plants for reciprocal carbon and phosphor exchange.

• Mycorrhizal hyphae extendup to 60 cm away fromthe associated plant root;

• 1-15 m hyphae per cm ofroot

• for improved phosphate and ammonium uptake

root

mycorrhizal hyphae

root hair

spore

arbuscule

vesicle

Grasslands, tropical forests

woody speciesmycelial strands

external hyphal mantle

Hartig net

intercellular hyphal net

intercellular hyphal complexes

*

*

*… These mycorrhizae hydrolyze organic compounds, mostly organic nitrogen and transport aminoacids to plant roots

Larcher 2003

Schematic depiction of different mycorrhiza forms

• Diazotrophy: fixation of atmospheric N by microorganisms

• All N2-fixing organisms are prokaryonts, e.g. bacteria, cyanobacteria, and actinomycetes

• Either free-living or symbionts• N2-Fixation:

– reductive splitting of N2 molecule catalysed by nitrogenase systems, which consists of 2 proteins (Fe-S, Mo-Fe-S)

– energy required for reduction is provided by respiration– host plant

• supplies bacteria with carbohydrates • provides enzymes for NH4 assimilation• protects bacteria from excessive concentrations of oxygen

(leghaemoglobin)• ensures a permanently moist environment

– for every gram of N, 4 g of C in form of carbohydrates are required

• N becomes available to plants once the nodules senesce and release the nitrogen to the soil

Root symbiosis: N-fixation

Mechanisms of nutrient uptake• Active transport of nutrients across cell membranes is most

important– Requires energy– Moves against concentration gradient

• The concentration of mineral nutrients in soil solution is very low

• Abundant nutrients may enter by diffusion or mass flow (Ca2+)• Proteins in plasma membrane transport

ions actively across cell membranes

Larcher 2003

Ion transport from soil solution to xylem

… Carrier systems: large protein molecules involved in transporting ions from cell to cell to the xylem

Apoplastic transport: nutrients move with inflowing water in cell walls and in water-filled intercellular spaces of root cortex

Casparian bands (CS) and lignified cells:function as a seal to apoplastic transport, here active ion uptake in cytoplasm (C) and from there ion transport from cell to cell through endoplastmatic reticulum (ER) and plasmodesmata. Vacuoles (V) function as storage place and deposits of excretion products

Ions move passivelyalong concentrationgradient from parenchyma cells to dead water-filled vessel cells.

~

Pools of nutrients and preference of uptake

Species show usually high capacity to absorb the forms of nitrogen that are most abundant in the ecosystems.

Nitrogen uptake and use

• Nitrate must be reduced to ammonium before it can be assimilated– Nitrate reduction often expensive, occurs mostly in leaves where

energy sources are highest– major form of N in basic soils

• Ammonium must be assimilated– Attached to a carbon skeleton– when primary N source, cation uptake exceeds anion uptake,

hence H+ is secreted into rhizosphere decreasing the pH.– major form of N in acidic soils

• Amino acids must be transported through plant– Used for protein synthesis– overall the least energetically expensive form

Overall, in nitrogen limited ecosystems, plants take up all formsof nitrogen, regardless of the costs, as it is essential for survival.

Phosphorus uptake and use

• Phosphatase enzymes released by roots make inorganic phosphate available to plant absorption; mostly in tundra plants

• Chelate secretion into bulk soil: cleave iron from iron phosphate and make phosphate soluble; mostly in dry climates; (“chelating”: when a metal ion combines with a chemical compound to form a heterocyclic ring).

• Proteoid roots (cluster roots) in Proteaceae (produce chelators, siderophores)

Proteoid roots

Hakea spp. Gervilea spp.

Function:- massive increase (up to 25 fold) of surface area- high spatial concentration of root exudates - exuded organic compounds: carboxylate organic anions, acid phosphatases, phenolics, mucilages, and water, - facilitate nutrient mobilization from soil - Organic anions (citrate) mobilize P by chelating Fe, Al, and Ca

Root response to changes in plant demand

Depending on the limiting nutrient, plant roots enhance the production ofnutrient specific transporter proteins in root membranes; e.g., the proteinsare different for ammonium, nitrate, phosphate, potassium, and sulfate

• high nutrient conditions, together with high T and light:increase in transporter proteins in cell membranes and thereby

enhanced ion uptake capacity

• low nutrient conditions: species-specific differences in enhanced uptake capacity; overall, shade and drought reduce uptake capacity

V = Vmax C1 (C1+ Km)

Change in nutrient uptake capacityV flux of ion into the root per unit timeVmax maximum influx rateC1 soil solution concentration at the root surfaceKm soil solution concentration where influx is 50% of Vmax

E.g., A species with more enzymes per roots, thus more carrier proteins per unit root area (Vmax), has a higher ion affinity of enzymes (smaller Km), or with the ability todraw nutrients down to a low level (smaller C1) will be at competitive advantage.

km

Effect of environmental stress on nutrient absorption rates

Redfield ratio

Nutrient ratios, like N:P ratios, define a stoichiometry of nutrient cycles inecosystems. E.g., phytoplankton have a Redfield ratio of 14. Plants with lower ratios (<14) increase the absorption of N and in proportion also the absorption of other nutrients, while plants with higher ratios (> 14) increase the absorption of P and in proportion the absorption of other nutrients.

Hence, following the redfield ratio and the stoichiometry of nutrient cyclesthe most growth limiting nutrient controls the cycling rates of all elementsin an ecosystem.

HOWEVER, keep in mind:1) Ratios are somewhat variable2) Similarity of ratios reflects regulation of uptake3) Differences among ratios reflects storage

Relationship between N and P concentration of leaves in heath plants

Nutrient use

1) Tissue production: N for proteins, P for ATP, K osmotic regulator, Ca for cell walls2) Highest concentrations in leaves and young fine roots3) Enhance plant growth rate4) Storage

in plants in ecosystems

Nutrient use efficiency (NUE)

Differences in tissue nutrient concentration indicate how much biomassplants or ecosystems can produce per unit nutrient.

Plants maximize NUE in infertile systems by decreasing biomass (~nutrient)loss not by increasing nutrient productivity.

At plant level

NUE = an x tr

an = nutrient productivity (photosynthesis/g N)

t = residence time of nutrient in plant

At ecosystem level

NUE = g biomass /g nutrient in litter

NUE is high in infertile sites where production is nutrient limited, in long-lived and old tissue

C… coniferous forestD… deciduous forestT… evergreen tropical forestM… mediterranean ecosystem,N… ecosystems with N2 fixers

NUE is highest in unproductive ecosystems

Nutrient loss from plants

• tissue senescence: most important!• death• leaching of dissolved nutrients• consumption of root and shoot tissue by herbivores• loss to parasites• exudation into soils• catastrophic loss by fire, windthrow

Table 8.6. Nitrogen and Phosphorus Resorption Efficiency of Different Growth Formsa. Growth form Resorption efficiency (% of maximum pool) Nitrogen Phosphorus All data 50.3 + 1.0 (287) 52.2 + 1.5 (226) Evergreen trees and shrubs

46.7 + 1.6 (108)a 51.4 + 2.3 (88)a

Deciduous trees and shrubs

54.0 + 1.5 (115)b 50.4 + 2.0 (98)a

Forbs 41.4 + 3.7 (33)a 42.4 + 7.1 (18)a Graminoids 58.5 + 2.6 (31)b 71.5 + 3.4 (22)b

All species are similar in resorption efficiency, i.e. the transfer of soluble nutrients (Ca and Fe are immobile) out of a senescing tissue through the phloem to flowers, seeds, new leaves or storage organs. There are no major differences in the proportion of nutrients lost.

Senescence and nutrient resorption

Letters indicate statistical difference between growth forms.

High resorption efficiency occurs usually in growth forms where senescence and production of new leaves coincide. Drought reduces resorption efficiency. Resorption efficiency ~ 50%; usually it is fairly high in graminoids!

Table 8.7. Nutrients leached from the canopy (throughfall) as a percentage of the total aboveground nutrient return from plants to the soil. Nutrient Throughfall (% of annual return) Evergreen forests Deciduous forests Nitrogen 14+3 15+3 Phosphorus 15+3 15+3 Potassium 59+6 48+4 Calcium 27+6 24+5 Magnesium 33+6 38+5

All species are similar in the proportion of nutrients leached from leaves and stem to the soil via throughfall (i.e. canopy drip fall; 90%) and stem flow (10%).

Leaching loss

Leacheats from shoots account for ~ 15% of annual nutrient return to soil.

Nutrient loss through herbivory

• Herbivores consume a small fraction of plant production in many ecosystems.

• Consumption by herbivores is more correlated with foliage production than with NPP, since a large fraction of NPP is not edible by herbivores.

• Has largest impact on plant nutrient budget, since herbivory does not allow for reabsorption (- > plants loose 2X as much N and P) and herbivores feed on tissue with highest N and P concentration

Reading:

Chapin et la. (2002) Chapter 8 - Terrestrial Plant Nutrient Use. In: Principlesin Ecosystem Ecology. Springer Verlag, NY, New York