Soil Biogeochemical Cycles Carbon Nitrogen Phosphorus Refer to

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 LOSSES Respiration 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 CO 2 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 N 2 2. Industrial production of N fertilizer N 2 + H 2 → NH 3 3. Biological (soil organisms) (industrial fixes 85% as much N as organisms)

Biological fixation (soil organisms) Immobilization: microbes convert N 2 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: N 2 + 8 H+ + 6 e- + nitrogenase → 2 NH 3 + H 2 NH 3 + 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 + O 2→CO 2 + H 2 O +NH 3 + energy

C. Nitrification Oxidizes ammonia to nitrate; 2 step oxidation process: 1. Nitrosomonas: NH 3→NO 2 - (nitrite) + energy 2. Nitrobacter: NO 2 -→NO 3 - (nitrate) + energy

D. Denitrification Completes N cycle by returning N 2 to atmosphere (prevents N added as fertilizer from being “locked” in roots and soil) Requires energy; Reduction of nitrate/nitrite NO 2 or NO 3 + energy→N 2 + O 2 (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 H 2 PO 4 - or HPO 4 -2 ion • Microbes constantly consume and release P to soil solution

Unavailable forms of P depend on soil p. H: • High p. H: calcium phosphate Ca. HPO 4 – Stable in high p. H – Soluble in low p. H • E. g. , rhizosphere, so plants can get it – Can be transformed to less-soluble Ca-P form (apatite) • Low p. H: iron and aluminum phosphates – Highly stable – Slightly soluble in low p. H

Role of mycorrhizae in P cycle: Can infect several plants: Hyphae connect plants ; conduits for nutrients Fungi get E from plant ‘s photosynthesis.