Environmental and Low-Temperature Geochemistry by Peter Ryan;
Author:Peter Ryan;
Language: eng
Format: epub
ISBN: 9781119568612
Publisher: John Wiley & Sons, Inc.
Published: 2019-10-16T19:50:39+00:00
6.2.2.2 The global phosphorus cycle
The largest reservoir of P at and near the Earth's surface is sedimentary rock and sediments, and the main means by which P is transported at the Earth's surface is by stream flow. Given that P is largely insoluble, the majority is transported as particulate material. Approximately 22 TgP yr−1 are transported by rivers to lakes and to the global ocean, and ∼90% is carried as adsorbed P or as P minerals (e.g. phosphates) – this component is mainly deposited in deltas and other types of marine sediments near continents. To reiterate a point made above, the main particulates responsible for P transport in the solid state are iron hydroxides and particulate organic matter (Ruttenberg 2003). The dissolved P carried by rivers is ultimately transported by currents into the open ocean, where it can be taken up as a nutrient or can circulate at the surface or to deeper levels.
Physical and chemical weathering of rock and sediment plus decomposition and erosion of organic matter yields ∼20 TgP yr−1 to biota and soils, and this flux is matched by erosion of soil P and biologically bound P into streams and, ultimately, the ocean. The cycle is closed by burial of marine P into marine sediments and the eventual transformation of these sediments in rock. In general, the flux of P between the main reservoirs (sediment/rock → soils/biota → streams → ocean → sediment/rock) is ∼20 TgP yr−1, although as is indicated on Figure 6.11, the riverine flux is about 10% greater, reflecting increased erosion due to anthropogenic activities, including road building, construction, logging, agriculture, and application of mined phosphorus as fertilizer. The increased fluvially transported P is responsible for much eutrophication.
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