Transcript Nitrogen and Phosphorous Cycles

Soil Biogeochemical Cycles
Carbon, Nitrogen, Phosphorus
• Refer to BIOTIC REGULATION in Farm as
Natural Habitat book, pp 156-7
24/103 required by organisms
Macronutrients: C,H,N,O,P,S
Micronutrients
BIOGEOCHEMICAL CYCLES
The complete pathway that a chemical
element takes through the biosphere,
hydrosphere, atmosphere and lithosphere.
Elements transferred between
compartments (pools)
Active: accessible to
living things
Storage: inaccessible
Soil Carbon Cycle
CARBON CYCLE
atmosphere
respiration
photosynthesis
biosphere
Gains?
Soil Organic Carbon
Losses?
Plant residues
Applied organic materials
GAINS
Soil organic carbon
Respiration
LOSSES
Plant removal
Erosion
Pools (compartments) of soil organic matter:
(categorized by susceptibility to microbial respiration)
1. Active
C:N 15:1 – 30:1
1-2 years
readily accessible to microbes; most of mineralizable N
10 – 20% of total
2. Slow
C:N 10:1 – 25:1
15-100 yrs
food for autochthonous microbes ; some mineralizable N
3. Passive
C:N 7:1 – 10:1
500-5000 yrs
colloidal; good for nutrient and water-holding
60 -90% of total
Soil management may help curb greenhouse
effect due to carbon dioxide emissions
pre-Industrial Revolution: 280 ppm CO2
post: 370 ppm
0.5% increase per year
Causes:
1. Fossil fuel burning
2. Net loss of soil organic matter
By changing balance between gains
and losses, may limit loss of OM…how?
How?
1. Restore passive fraction in soils that are
degraded.
-sequesters carbon for long time
2. Switch to no-till practices
3. Convert to perennial vegetation
• Cornfield in warm, temperate climate
Net loss of carbon!!
Soil Nitrogen Cycle
• Atmosphere 78% nitrogen
• Not in directly accessible form for
organisms
– Made usable by fixation
• Most terrestrial N in soil
– 95-99% in organic compounds
– Made usable by mineralization
Let’s look at all components and processes
in nitrogen cycle…..
A. Nitrogen fixation
1. Atmospheric: lightning
– Oxidation of N2
2. Industrial
production of N fertilizer
N2 + H2 → NH3
3. Biological (soil organisms)
(industrial fixes 85% as much N as organisms)
Biological fixation
(soil organisms)
Immobilization: microbes convert N2 to
N-containing organic compounds
Nitrogenase
2 groups of N-fixing
microorganisms
A. Nonsymbiotic, autotrophic:
(use solar energy)
Cyanobacter (formerly known as blue-green
algae) in anaerobic;
Azotobacter in aerobic
5-50 lbs....../acre/year
B. Symbiotic, in association with legume
plants
(plants supply energy from photosynthesis)
1. Rhyzobium
2. Bradyrhizobium
Infect root hairs and
root nodules of legumes
peas, clover, alfalfa, cowpeas, peanuts,
beans, soybeans
Alfalfa - 200 lbs....../acre/year
Soybeans - 100 lbs......./acre/year
Beans - 40 lbs...../acre/year
* Green manure is live plant material added to
soil to increase N content and SOM.
Symbiosis: mutualistic: plants provide
energy, bacteria provide ammonia for
synthesis of tissue
Energy-demanding process:
N2 + 8H+ + 6e- + nitrogenase → 2NH3 + H2
NH3 + organic acids → amino acids → proteins
Infection and
nodule
formation
Rhizobium
Alfalfa root nodule
Dazzo & Wopereis, 2000
Root hair curling around rhizobia
Rhizobia reproduce
in infection threads
M. Barnett
Bacteroids filling a single cell
Dazzo & Wopereis, 2000
Michael Russelle - USDA-ARS
Plant Science Research Unit
Gage and Margolin, 2000
Vance et al., 1980
B. Mineralization
(ammonification)
Heterotrophic
microorganisms
Decomposition
Organic N compounds
broken down to
ammonia; energy
released for
microorganisms to
use
ammonification
Organic N + O2→CO2 + H2O +NH3 + energy
C. Nitrification
Oxidizes ammonia to
nitrate; 2 step
oxidation process:
1. Nitrosomonas:
NH3→NO2- (nitrite) + energy
2. Nitrobacter:
NO2-→NO3- (nitrate) + energy
D. Denitrification
Completes N cycle by returning N2 to atmosphere
(prevents N added as fertilizer from being
“locked” in roots and soil)
Requires energy; Reduction of nitrate/nitrite
NO2 or NO3 + energy→N2 + O2
(many steps)
Denitrifying bacteria and fungi in anaerobic
conditions
Phosphorus Cycle
Phosphorous Cycle
 P often limiting factor for plants:
 low in parent materials
 inclination to form low-soluble inorganic
compounds
 After N, P is most abundant nutrient in
microbial tissue
Differs from N cycle
1. No gaseous component
2. N goes into solution as nitrate
– Stable, plant-available
But P reacts quickly with other ions and
converts to unavailable forms
Available P in soil solution:
• as H2PO4- or HPO4-2 ion
• Microbes constantly consume and release
P to soil solution
Unavailable forms of P depend on
soil pH:
• High pH: calcium phosphate CaHPO4
– Stable in high pH
– Soluble in low pH
• E.g., rhizosphere, so plants can get it
– Can be transformed to less-soluble Ca-P form
(apatite)
• Low pH: iron and aluminum phosphates
– Highly stable
– Slightly soluble in low pH
Role of mycorrhizae in P cycle:
Can infect several plants:
Hyphae connect plants ; conduits for
nutrients
Fungi get E from plant ‘s photosynthesis.