Soil Biogeochemical Cycles Carbon, Nitrogen, Phosphorus

24/118 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.

Soil Carbon Cycle

CARBON CYCLE atmosphere photosynthesis respiration biosphere

Soil organic carbon Plant residues Applied organic materials GAINS LOSSES Respiration Plant removal Erosion

Pools (compartments) of soil organic matter: (categorized by susceptibility to microbial respiration) 1. Active/Fast 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

pre-Industrial Revolution: 280 ppm CO2 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

Not in directly accessible form for organisms Atmosphere 78% nitrogen Not in directly accessible form for organisms Made usable by fixation Most terrestrial N is in the 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 2. Industrial 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 Nonsymbiotic, autotrophic: (use solar energy) Some actinomycetes Cyanobacter (formerly known as blue-green algae) Photosynthetic bacteria

B. Symbiotic, in association with legume plants (plants supply energy from photosynthesis) Rhyzobium Infect root hairs and root nodules of legumes

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

B. Mineralization (ammonification) Heterotrophic microorganisms Decomposition Organic N compounds broken down to ammonia; energy released for microorganisms to use 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

NITROGEN applied to soil as fertilizer

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

Soil phosphorus cycle in a grazing system

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

Phosphate crisis

Arbuscular mycorrhizae and N cycle Involve 2/3 of plant species. Unlike most fungi, the AM fungi get their supply of sugars for energy and growth from their plant partner and not from the decomposition of organic matter AM fungi thrive on decomposing organic matter and obtain large amounts of nitrogen from it. The fungus itself is much richer in N than plant roots, and calculations suggest that there is as much nitrogen in AM fungi globally as in roots. Since fungal hyphae (the threads of which the fungus is composed) are much shorter-lived than roots, this finding has implications for the speed with which nitrogen cycles in ecosystems.

Biotic regulation vs. synthetic fetilizers Biota capture and store soil nutrients and return them to plants when they need them. When plants need nutrients, they stimulate soil biota to release the nutrients. In biotic regulation, nutrients are held in resistant forms, not readily lost from soil. Synthetic fertilizers cause physiological changes in plants that make them withhold energy from soil biota.