Download L8. Terrestrial introduction and N/P cycle

L8. Terrestrial introduction and N/P cycle
•
Terrestrial biogeochemistry usually focuses on the role of plants
and soil microbes
•
Organisms such as mycorrhizal fungi and nitrogen-fixing
bacteria often help plants to acquire mineral nutrients from soil
Twenty-five percent of the Earth’s surface is covered with plants
The green parts of the blue planet. Image courtesy of NASA/NOAA.
Biogeochemical cycling in the terrestrial environment
•
Terrestrial biogeochemistry usually focuses on the role of plants
and soil microbes
•
Organisms such as mycorrhizal fungi and nitrogen-fixing
bacteria often help plants to acquire nutrients from soil
Clover roots with nodules containing N-fixing bacteria.
The terrestrial and aquatic biogeochemical cycles are linked
40% of the Earth’s land is now used for agriculture.
Image of Bac Son Valley, Vietnam.
Lecture outline
1.
Nutrients essential for terrestrial plant life
2.
Nutrient uptake by plants and factors affecting soil
nutrient availability
3.
The terrestrial nitrogen cycle and anthropogenic changes
as a result of agriculture
4.
The terrestrial phosphorus cycle and anthropogenic
changes as a result of agriculture
Lecture outline
1.
Nutrients essential for terrestrial plant life
2.
Nutrient uptake by plants and factors affecting soil
nutrient availability
3.
The terrestrial nitrogen cycle and anthropogenic changes
as a result of agriculture
4.
The terrestrial phosphorus cycle and anthropogenic
changes as a result of agriculture
Essential nutrient elements
1.
In its absence a plant cannot complete the normal life
cycle
2.
The element is part of an essential plant constituent or
metabolite
The essential nutrient elements of higher plants and
their concentrations considered adequate for growth
Macronutrients are
required in large
quantities and usually
involved in structure
of materials
Micronutrients have
catalytic and
regulatory roles such
as enzyme activators
Taiz & Zeiger (2010) Plant Physiology, Fifth Edition. Sinauer Associates, Inc.
Nutrient classification according to biochemical function
Amino acids
Phospholipid bilayer
Nutrient classification according to biochemical function
Enzyme cofactors
Chlorophyll
Nutrient classification according to biochemical function
Photosynthesis
Lecture outline
1.
Nutrients essential for terrestrial plant life
2.
Nutrient uptake by plants and factors affecting soil
nutrient availability
3.
The terrestrial nitrogen cycle and anthropogenic changes
as a result of agriculture
4.
The terrestrial phosphorus cycle and anthropogenic
changes as a result of agriculture
Mineral ions cross the root apoplast and symplast
• Mineral ions enter through the root and may immediately enter the
symplast of an epidermal cell, or diffuse through the apoplast of the
epidermis and cortex. All ions enter the symplast at the Casparian
band via membrane transport proteins.
Taiz & Zeiger (2010) Plant Physiology, Fifth Edition. Sinauer Associates, Inc.
Three classes of membrane transport proteins
• Channels are transmembrane proteins that function as selective
pores. Carrier proteins do not have pores that extend across the
membrane, rather, they bind ions on one side of the membrane and
release them on the other side (passive transport)
• Primary active transport is carried out by pumps and uses energy
directly. In plant plasma membranes the most prominent pumps are
those that pump H+ or Ca2+ ions
Taiz & Zeiger (2010) Plant Physiology, Fifth Edition. Sinauer Associates, Inc.
Secondary active transport
• Coupling the energetically uphill transport of one solute with the
energetically downhill transport of another
Taiz & Zeiger (2010) Plant Physiology, Fifth Edition. Sinauer Associates, Inc.
Parent material affects nutrient availability
•
Rocks or sediments that give rise to soil determine many
aspects of soil fertility. Granite gives rise to lower P and
other cations compared to limestone; serpentine rocks have
high concentrations of heavy metals.
•
In relatively young landscapes (following recent volcanic
activity or glaciation), P availability is relatively high and N
tends to be the key nutrient limiting plant productivity
•
In ancient, highly weathered soils (like Australia) P is the keylimiting nutrient
Soil pH affects the solubility and availability of
nutrient elements in organic soils
Mo deficiency in peanut by low pH
Fe deficiency in palms by high pH
Some plants can change the rhizosphere pH
•
The solubility of Fe decreases 1000-fold for each unit
increase in soil pH in the range 4-9; that of Mn, Cu, and Zn
decreases 100-fold
•
Plants can strongly reduce rhizosphere pH by excreting
organic acids or protons
•
When Fe supply is insufficient, Helianthus annuus
(sunflower) plants lower the pH of the root solution from
approximately 7 to 4. Similar seen for corn and soybean.
•
Mediated by a proton-pumping ATPase at the plasma
membrane of roots, with cations being exchanged for H+
Cation exchange and soil fertility
• Soil particles, both organic and inorganic, have predominately negative
charges on their surfaces
• Mineral cations (NH4+ , K+, etc.) adsorb to the negative surface and are
not easily leached out by water, providing a nutrient reserve. The degree
to which a soil can adsorb and exchange ions is termed its cation
exchange capacity (CEC)
• Mineral anions (NO3-, Cl-), on the other hand, tend to be repelled by the
negative surface and remain dissolved in soil solution. Anion exchange
therefore small compared to cation exchange
Lecture outline
1.
Nutrients essential for terrestrial plant life
2.
Nutrient uptake by plants and factors affecting soil
nutrient availability
3.
The terrestrial nitrogen cycle and anthropogenic changes
as a result of agriculture
4.
The terrestrial phosphorus cycle and anthropogenic
changes as a result of agriculture
Nitrogen (N)
•
N is a key constituent of many important molecules including proteins,
nucleic acids, chlorophyll and certain hormones
•
Although the Earth’s atmosphere is 80% N, only certain prokaryote
organisms – bacteria and cyanobacteria – can utilize gaseous N directly
•
Natural processes fix about 190  1012 g y-1 of N:
•
Lightning – accounts for 8% of N fixation; occurs when lightning
converts water vapor and oxygen into highly reactive free radicals,
free hydrogen atoms, and free oxygen atoms that attack molecular
nitrogen (N2) to form nitric acid (HNO3) which falls to Earth with rain
•
Photochemical reactions – accounts for 2% of N fixation; reactions
between nitric oxide (NO) and ozone (O3) that produce HNO3
•
Biological N fixation – accounts for 90% of N fixation; occurs when
bacteria fix N2 into ammonia (NH3)
Biological N fixation
•
Accounts for most of the conversion of atmospheric N2 into
ammonium
•
Most bacteria that fix N are free-living in the soil while a few form
symbiotic associations with higher plants such as beans and legumes
The cereal-legume intercrop systems are one of the most
important intercropping systems
The terrestrial nitrogen cycle
Humans have significantly altered the N cycle with
the Haber-Bosch process
•
Discovered by Fritz Haber in 1909 and scaled up by Carl Bosch in 1913
•
Under high temperatures (about 200°C) and high pressure (about 200
atmospheres) and in the presence of a metal catalyst (usually Fe), N2
combines with hydrogen to form ammonia
•
Worldwide industrial production of N fertilizer now amounts to more
than 100  1012 g y-1 (compared to 190  1012 g y-1 of N fixed naturally)
•
Projected to increase to 165  1012 g y-1 by 2050
N fertilizer production and application also produces
greenhouse gases (nitrous oxide)
N2O has become the third most important greenhouse gas after carbon dioxide and methane
Nitrogen Deposition (mg N/m2/yr)
5000
2000
1000
750
500
250
100
50
25
5
1993
1860
(Galloway et al. 2003)
USDFertilizers
$500 million
endworth
up in our
of nitrogen
rivers and
fertilizer
oceansgoes
down the Mississippi river each year
Excess N can cause hypoxic dead zones to occur
Excess N can cause hypoxic dead zones to occur
One of the world’s largest dead zones develops each spring/summer in the Gulf
of Mexico, where the Mississippi River dumps high-nutrient runoff from its vast
drainage basin. In 2002 the Gulf’s dead zone swelled to 22,000 square kilometers.
Researchers have identified more than 200 dead
zones around the world
Increased nitrous oxide (N2O) production also occurs in dead zones. In suboxic water at
depths of less than 300 feet, microbial denitrification rates can be 10,000 times higher than
the average for the open ocean (‘Interesting times for nitrous oxide’, Science, 12 March 2010)
Lecture outline
1.
Nutrients essential for terrestrial plant life
2.
Nutrient uptake by plants and factors affecting soil
nutrient availability
3.
The terrestrial nitrogen cycle and anthropogenic changes
as a result of agriculture
4.
The terrestrial phosphorus cycle and anthropogenic
changes as a result of agriculture
Phosphorus (P)
•
Found largely as phosphate esters which play key roles in
photosynthesis and metabolism. Other phosphate esters are
the nucleotides that make up DNA/RNA and phospholipids in
cell membranes
•
The P cycle differs from the other major biogeochemical
cycles (N, C, S, H2O) in that it does not include a gas phase; it
is a sedimentary cycle
•
When it rains, phosphates are removed from rocks (via
weathering) and are distributed throughout both soils and
water, plants take up the phosphate ions from the soil
•
Unlike N, P cycling includes significant inorganic (mineral)
reactions
The terrestrial phosphorus cycle
Plant acquisition of P from soil
• In agricultural soils, 30-70% of all P is present in organic form.
In nutrient-poor grasslands, peat soils and forest soils this
may be as much as 80-95%
• Plants can only take up inorganic phosphate which is usually
released by microbial enzymes (phosphatases).
• Some plant species can use organic forms of P (nucleic acids,
phospholipids, inositol phosphate, glycerophosphates) by
releasing phosphatases that hydrolyze organic P-containing
compounds, releasing Pi that is absorbed by roots.
BOTA30003
Arbuscular mycorrhizas occur in 82% of all angiosperms
• Vast majority of plant species
form symbiotic associations
with mycorrhizal fungi
• Arbuscular mycorrhizas (AMs)
are the most widespread. A
large fraction of the fungal
tissue is within the root cortical
cells
• The mycorrhizal associations
enhance the plant’s belowground absorbing surface
Many Australian agricultural soil are acutely P deficient
Profitable crop production has only been possible through widespread
applications of P fertilisers.
Cluster roots (dense aggregations of lateral roots with long root hairs)
are plant adaptations to the very low P soils of WA and South Africa
Cluster roots promote nutrient uptake by their large surface area and production of
exudates that increase nutrient solubility. Cluster roots often form a dense mat near
the soil surface in contact with decomposing litter, support populations of Psolubilising bacteria , secrete organic acids and are very efficient at P adsorption.
Effect of organic acid (carboxylates) secretion on
inorganic and organic P mobilization in soil
Cluster root morphologies induced in Proteaceae, Restionaceae,
Cyperaceae and Fabaceae by low P supply
(Lambers et al. 2006 page 700)
Humans have significantly altered the P cycle with
widespread application of P fertilizers
•
Phosphate fertilizer is usually
applied as “phosphate rock”
composed primarily of
phosphorus pentoxide (P2O5).
•
Main commercial reserves of
phosphate rock are in just
three countries – China, USA
and Morocco
•
Currently known phosphate
rock reserves will last for the
next 300-400 years
Marocco &
Western Sahara
The fate of P added to soil
80-90% of the P applied to soils is sorbed by soil particles or combines with other elements through
complex geochemical processes to form insoluble complexes. Most plants are unable to access the P.
Soil P surpluses are building up in many agricultural areas
•
P fertilizer application to croplands is around 14.2  1012 g y-1 , of which more
than half is applied to cereal crops
•
An additional 9.6  1012 g y-1 of P is applied to crops as livestock manure
•
P removed by crops is approximately 12.3  1012 g y-1
•
Results in a global P agronomic surplus of 11.5  1012 g y-1
(MacDonald et al. 2011)
Improving the nutrient-use efficiency of our major food crops
will have enormous environmental and economic benefits