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

biosphere

respiration

photosynthesis

Soil Organic Carbon

Gains?

Losses?

Soil organic carbon

Plant residuesApplied organic materials

GAINS

Respiration Plant removal ErosionLOSSES

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. BradyrhizobiumInfect 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

Dazzo & Wopereis, 2000

Vance et al., 1980

Infection and nodule

formation

Rhizobium

Dazzo & Wopereis, 2000

Gage and Margolin, 2000

Root hair curling around rhizobiaRhizobia reproduce in infection threads

Bacteroids filling a single cell

Alfalfa root nodule

M. Barnett

Michael Russelle - USDA-ARS Plant Science Research Unit

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/nitriteNO2 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.