BSC 314 Lecture Notes - Lecture 65: Evaporite, Sea Spray, Deep Time
28 Jun 2018
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The Phosphorus Cycle
The phosphorus cycle is a sedimentary cycle (unlike carbon, oxygen, and nitrogen), the
atmosphere is nota reservoir for phosphorous nor do microorganisms fix phosphorus as
they do nitrogen. Phosphorus enters the biosphere almost entirely from the soil through
absorption by plant roots. Weathering of rocks containing phosphate minerals,
chiefly apatite [Ca 5(PO 4) 3OH], results in the relatively small pool of inorganic
phosphorus available for organismal use. In most soils the major amount of phosphorus
absorbed by plants comes from organic molecules that undergo decomposition
releasing phosphorus in plant available inorganic forms. The release of organically ‐
bound nutrients to plant available forms is termed‐ mineralization, a process important
in the release to the soil of sulfur and nitrogen as well as phosphorus. Phosphorus is
used by organisms in energy transfers (ATP, NAPD), as a component of nucleic acids
(RNA, DNA), and as a structural element of membranes (phospholipids).
The phosphorus cycle has fewer compartments than the other major nutrient cycles and
also has a significant “leak” of phosphorus back to its lithosphere reservoir from which it
is returned to active cycling only after long intervals of geologic time. The combination of
three factors makes phosphorus a nutrient of concern in most ecosystems:
Most soils have only small amounts from the weathering of disjunctly distributed
rocks.
Phosphorus is more insoluble than other nutrients and less mobile, hence less
phosphorus travels in the soil solution; roots generally must grow into a zone of
phosphorus availability.
Phosphorus that drains from the land to the ocean is used by organisms in the
surface waters, but a considerable amount is lost to the sediments in the shells
and bones of marine organisms and by precipitation and settling of phosphates.
Human activities alter the phosphorus cycle chiefly by adding more available
phosphorus where little was available previously. Phosphate containing detergents ‐
used in the 1960s were carried by sewage systems into rivers and lakes and were a
boon to algae and microorganisms, which responded with exuberant flushes of growth.
Widespread eutrophication resulted and detergent makers were obliged to remove ‐
phosphates from their products. Eutrophication—the enrichment of fresh waters with
nutrients—results in blooms of plankton and algae. Death of these organisms
increases the populations of aerobic bacteria of decay which, in turn, deplete the
dissolved oxygen in the waters, thereby killing fish and other aerobic organisms. The
anaerobic microorganisms move in, and the fresh water becomes an unpleasant, smelly
soup of decay.
Agricultural use of phosphate containing fertilizers has increased as the acreage of ‐
farmlands has expanded over time. At first, guano (the dung of seabirds) was collected
from deposits on seashore rocks and added to the fields, but demand by inland farmers
for phosphate fertilizer stimulated the mining of phosphate deposits (ocean sediments of
past geologic ages). These applications, too, wash out of the fields into the world's
waters and also can cause eutrophication.
The Sulfur Cycle
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Document Summary
The phosphorus cycle is a sedimentary cycle (unlike carbon, oxygen, and nitrogen), the atmosphere is nota reservoir for phosphorous nor do microorganisms fix phosphorus as they do nitrogen. Phosphorus enters the biosphere almost entirely from the soil through absorption by plant roots. Weathering of rocks containing phosphate minerals, chiefly apatite [ca 5(po 4) 3oh], results in the relatively small pool of inorganic phosphorus available for organismal use. In most soils the major amount of phosphorus absorbed by plants comes from organic molecules that undergo decomposition releasing phosphorus in plant available inorganic forms. The release of organically bound nutrients to plant available forms is termed in the release to the soil of sulfur and nitrogen as well as phosphorus. Phosphorus is used by organisms in energy transfers (atp, napd), as a component of nucleic acids (rna, dna), and as a structural element of membranes (phospholipids). mineralization, a process important.
