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3.1.1 Phosphorus

Phosphorus has a number of indispensable biochemical roles and is an essential element for growth in all organisms, being a component of nucleic acids such as DNA, which hold the code for life. However, phosphorus is a scarce element in the Earth’s crust and natural mobilization of phosphorus from rocks is slow. Its compounds are relatively insoluble, there is no reservoir of gaseous phosphorus compounds available in the atmosphere (as there is for carbon and nitrogen), and phosphorus is also readily and rapidly transformed into insoluble forms that are unavailable to plants. This tends to make phosphorus generally unavailable for plant growth. In natural systems, phosphorus is more likely to be the growth-limiting nutrient than is nitrogen, which has a relatively rapid global cycle and whose compounds tend to be highly soluble.

Human activities, notably the mining of phosphate-rich rocks and their chemical transformation into fertilizer, have increased rates of mobilization of phosphorus enormously. A total of 12 × 1012 g P yr−1 are mined from rock deposits. This is six times the estimated rate at which phosphorus is locked up in the ocean sediments from which the rocks are formed. The global phosphorus cycle is therefore being unbalanced by human activities, with soils and water bodies becoming increasingly phosphorus-rich. Eutrophication produces changes in the concentrations of phosphorus in all compartments of the phosphorus cycle.

The mechanisms of eutrophication caused by phosphorus vary for terrestrial and aquatic systems. In soils, some phosphorus comes out of solution to form insoluble iron and aluminium compounds, which are then immobilized until the soil itself is moved by erosion. Eroded soil entering watercourses may release its phosphorus, especially under anoxic conditions.

SAQ 16

What changes occur to iron(III) compounds (Fe3+) as a result of bacterial respiration in anoxic environments, and how is their solubility affected?


Bacterial respiration can reduce Fe3+ to Fe2+, increasing the solubility of iron salts, including phosphates of iron.

Once in rivers, retention times for phosphorus may be short, as it is carried downstream either in soluble form or as suspended sediment. Algal blooms are therefore less likely to occur in moving waters than in still systems. In the latter, there is more time for the phosphorus in enriched sediments to be released in an ‘available’ form, increasing the concentration of soluble reactive phosphorus (SRP), and thus affecting primary production.

Phosphorus is generally acknowledged to be the nutrient most likely to limit phytoplankton biomass, and therefore also the one most likely to cause phytoplankton blooms if levels increase. However, there do appear to be some systems that are ‘naturally eutrophic’, with high phosphorus loadings. In these systems, nitrogen concentrations may then become limiting and play a dominant role in determining phytoplankton biomass.