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!!
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 + 8H + + 6e - + nitrogenase → 2NH 3 + H 2 NH 3 + 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 rhizobia Rhizobia 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 + 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
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 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.