Soil Biogeochemical Cycles Carbon, Nitrogen, Phosphorus

Recall BIOTIC REGULATION –in excerpt from Farm as Natural Habitat book Partnership between plants and soil biota soil biota plants Litter, root exudates Nutrients, structure, tilth

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.

Elements transferred between compartments (pools) Active: accessible to living things Storage: inaccessible

Soil Carbon Cycle

CARBON CYCLE atmosphere biosphere respiration photosynthesis

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

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: yrs food for autochthonous microbes ; some mineralizable N 3. Passive C:N 7:1 – 10: yrs colloidal; good for nutrient and water-holding % of total

Soil management may help curb greenhouse effect due to carbon dioxide emissions “soil carbon sequestration” 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 Plants can use nitrogen in 2 forms: –Ammonium ions NH 4 + –Nitrate ions NO 3 - Most terrestrial N is 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 Non-Biological 1. Atmospheric: lightning –Oxidation of N 2 N > NO 3 - Rainfall additions from lightning 2-5 lbs....../acre/year

Non-Biological 2. Industrial production of N fertilizer (Haber process) N 2 + H 2 → NH 3

Non-Biological 3. Air Pollution fuel combustion from cars & stationary fuel combustion sources (electric utilities and industrial boilers) put NO (nitrous oxide) & NO 2 (nitrogen dioxide) in atmosphere may remain in the atmosphere for several days and during this time chemical processes may generate nitric acid, and nitrates and nitrites as particles.

Biological (soil organisms) 1. Nonsymbiotic, autotrophic: (use solar energy) a) Some actinomycetes b) Cyanobacter (formerly known as blue-green algae) c) Photosynthetic bacteria Azotobacter (aerobic) & Clostridium (anaerobic) about 5-50 lbs....../acre/year d) Archeae

Biological B. Symbiotic soil organisms, in association with legume plants ( plants supply energy from photosynthesis) 1. Rhyzobium 2. Bradyrhizobium Infect root hairs and root nodules of legumes

Legumes : peas, clover, alfalfa, cowpeas, peanuts, beans, soybeans Alfalfa lbs....../acre/year Soybeans lbs /acre/year Beans - 40 lbs...../acre/year * Green manure is live plant material added to soil to increase N content and SOM.

Bacteria invades host plant root Response of host plant root is to grow a nodule for the bacteria to live in. Rhizobium Alfalfa root nodule Root hair encircling bacteria

Bacteria takes N 2 from the air and converts it into NH 3 which resides in bacteria in nodule Energy-demanding process: N 2 + 8H + + 6e - + nitrogenase → 2NH 3 + H 2 NH 3 + organic acids → amino acids → proteins

Fate of N Fixed by Rhizobium: 1)used by host plant, 2) leaks out of root to become available to surrounding plants, 3) as roots and nodules are sloughed-off, heterotrophic organisms immobilize the N and it eventually becomes part of the SOM.

Grass Legume Soil N Manure N Grass Legume Fixed N Legumes buffer the N supply and fix what they need from the air Michael Russelle - USDA-ARS Plant Science Research Unit

Grass Legume Soil N Manure N Grass Legume Fixed N We need to fertilize non-legumes and can easily guess wrong Fert N Loss 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 Organic N + O 2 →CO 2 + H 2 O +NH 3 + energy

B. Fates of NH 4 + 1) fixed by clay minerals, 2) lost by soil erosion, 3) used by plants (NH 4 + ), 4) volatilization NH > NH 3 High pH Soils > 7.5

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

Fates of Nitrate *Immobilization ---> Plant uptake of NO3- *NO 3 - is not held by soil particles and is easily leached when ppm NO 3 - is > 10 ppm the water is considered to be contaminated * Denitrification - stimulated by anaerobic conditions.

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

Through nitrification and denitrification % of the applied N is lost. Nitrification inhibitors can be applied like N-Serve. This chemical inhibits the growth of nitrosomonas and nitrobacter or slows conversion of NH4+ conversion to NO3-

Duxbury, 1997, Wm. C. Brown Publishers Excess nitrate gets into water supply:

Nitrate in drinking water supplies Nitrate has been detected in surface- and ground-water supplies in various parts of the state. Low levels of nitrate can be found in most of the surface waters of the state.

In cases where the concentration of nitrate- nitrogen exceeds the maximum contaminant level of 10 mg/L, as set forth by the U.S. EPA - water suppliers are required to issue a nitrate alert to users. The health of infants, the elderly and others, and certain livestock may be affected by the ingestion of high levels of nitrate. Risk of Groundwater Contamination by Nitrate USGS, 1998

Some implications for organic farming/gardening with the soil food web in mind… Some plants prefer their N as ammonium: –Trees, shrubs, perennials Some plants prefer their N as nitrate: –Vegetables, annuals, grasses

When nematodes and protozoa consume fungi and bacteria, they release N in ammonium form IF nitrifying bacteria are present…. –N-fixing bacteria quickly convert ammonium to nitrate –(Bacteria produce slime with a pH >7 to make the environment favorable to bacteria) Fungi produce organic acids to decay OM –Makes environment more favorable to fungi

The key is to foster a bacterial-dominated soil or a fungal-dominated soil –Can do this by managing the C:N ratio of compost

Inorganic, soluble nitrogen fertilizers are great for getting plants to grow but are detrimental to the soil food web! –Water soluble nitrates are readily available to roots They don’t attach to humus or clays Therefore deplete organisms in soil and require constant application

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 pH: High pH: calcium phosphate CaHPO 4 –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.