Chemical cycling includes reservoirs, exchange pools, and the biotic communityReservoir is a source normally unavailable toproducers, such as the carbon present incalcium carbonate shells on ocean bottomsExchange pool is a source from which organismsdo generally take chemicals, such as theatmosphere or soil Chemicals moving along food chains in a biotic community, perhaps never enter an exchange pool
The phosphorus cycle is the biogeochemicalcycle that describes the movement ofphosphorus through the lithosphere,hydrosphere, and biosphere. Unlike many otherbiogeochemical cycles, the atmosphere doesnot play a significant role in the movement ofphosphorus, because phosphorus andphosphorus-based compounds are usuallysolids at the typical ranges of temperature andpressure found on Earth.
Second most commonly limiting nutrient afternitrogen(N)No significant gaseous componentAtmospheric transport as dust and particles issmall (an estimate is 1 x 1012 g/yr)Major source of available P is not created bymicrobial reactions, but by chemical reactionsParticulate loss (sediment in streams, erosion) ismajor pathway for loss from terrestrialecosystems
Apatite(Soil Mineral) in parent material is themajor primary source (in contrast to N, wherethere is no major mineral source)Taken up in anion form H2PO4-, H(PO4)2- at higherpHDeposition and leaching are unimportant in thiscycleForms in plants and animals: ADP(Adenosine Di-Phosphate), ATP(Adenosine Tri Phosphate) whichare the energy currency of plant. DNA and RNAalso contain phosphates as base components.
Duration of available P in soil depends largelyon P fixation: Long-term in non-P-fixing soils Short-term in P fixing soilsSoil adsorbing Fe, Al hydrous oxides do it sostrongly that P is unavailableThis is common in Andisols (volcanics) andOxisols (tropical highly oxidized soils)Human beings boost the supply of phosphate bymining phosphate ores for producing fertilizerand detergents. This results in eutrophication(overenrichment) of waterways.
Main source of P are rivers and streams around21 x 1012 gm P/yrOnly 10% of this is ortho-PWhat ortho-P enters oceans is precipitatedmostly as apatite:Ca5(PO4)3OH --> 5Ca 2+ + 3HPO 2- + 4HCO - +H O 4 3 2 Ksp = 10-58 So ortho-P concentrations are approx. 3 x 10-6 M90% of P taken up is recycled
Some goes deep and is entrained in sediments(2 x 1012 g P/yr). This forms the long term P sink.This P is recycled to the terrestrial system onlyover very long-terms with upliftTherefore, we have what agronomists call the“phosphorus problem”: A finite supply, inessence, which is being slowly sequestered inthe oceanAccelerated erosion increases this problem