Revistes Catalanes amb Accés Obert (RACO)

Elements limitants, explotabilitat i diversitat. Homenatge a Bolòs i al fòsfor

Ramon Margalef


Phosphorus is essential in all major functions of life, and a first-line candidate for a distinguished role in biogenesis. It becomes a most important limiting element because of its distribution and the relatively high probability to become immobilized in the form of highly insoluble minerals. Redfield ratio derived from the study of plankton is of general application, once shifts are recognized, especially in these cases where materials of eventual organic origin, but phosphorus-poor, provide support, persistence and competitive power, like wood in vascular plants -and concrete and metals, besides wood, in human civilizations-. The lack of assimilable phosphorus and the difficulty in braking photosynthesis leads indeed phytoplankton to an excess production of mucilages or inorganic hard materials (silica, calcium carbonate, etc.).
EIDT (1977) highlights the accumulation of insoluble phosphates in soils that have spent rnil1enia under cultivation. On the other hand, chemical and biochemical erosion of rocks and soils on the continents provides a continuous, if relatively small, supply of phosphorus usable by plants. A large fraction of sea, freshwater and land phosphorus becomes immobilized in the form of hydroxi-fluor-apatites. The local presence of fluor increases phosphorus insolubility, as observed below the major upwelling areas in the oceans, and, eventually, in agricultural soils, more in some particular areas than in others. The consequences are reflected on the local productivity: soils with available phosphorus give more complete crops, like grain and bananas. Under conditions of low availability of phosphorus, agriculture yields relatively more oil and wine (which could be considered as junk food), and also wood. In general, ecosystems under low phosphorus availability are characterized by a slow turnover and an associated high density, as observed over large extensions of the Amazonas and other South American basins.
This also applies to aquatic environments, the oceans being less conductive to high productivity due to their relatively high pH and fluoride contents. Un- fortunately, quite recently, biological and chemical oceanologists have very much neglected the study of the distribution in the sea of available phosphorus, a fact that might have misled them to a wild chase after other possible 1imiting factors. The urgent practical problem of eutrophication in freshwaters has stimulated a more realistic and close examination of phosphorus cycling in them.

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