Phosphorus geological time

The sediment reservoir (1) represents all phosphorus in particulate form on the Earth s crust that is (1) not in the upper 60 cm of the soil and (2) not mineable. This includes unconsolidated marine and fresh water sediments and all sedimentary, metamorphic and volcanic rocks. The reason for this choice of compartmentalization has already been discussed. In particulate form, P is not readily available for utilization by plants. The upper 60 cm of the soil system represents the portion of the particulate P that can be transported relatively quickly to other reservoirs or solubilized by biological uptake. The sediment reservoir, on the other hand, represents the particulate P that is transported primarily on geologic time scales. 

There is no particular a priori reason to suspect that oxygen production was any different from that in the modern ocean, except that there may have been constraints imposed by different availabilities of key nutrients such as phosphorus (Bjerrum Canfield 2002). Today, the availability of fixed nitrogen may constrain productivity on geological time scales (Falkowski 1997), but in the Archaean phosphorus removal by adsorption on iron oxides could have reduced P availability, significantly reducing productivity compared with today. 

That such soils. should be phosphorus deficient is a prediction from pedogenic theory (Walker and Syers, 1976). This is because, in contra.st to carbon, nitrogen, and sulfur, P is cycled mainly on geological time scales. That is to say, the only substantial primary source of P for plants is from the weathering of parent material at the base of the soil. As soil development proceeds, there is a loss of this weathered P as a consequence of leaching. The rate of leaching is quite small on an annual basis, even in the tropics (typically 0.1-1 mmol P m year Bruijnzeel, 1991) but it occurs over. several thousand years. Moreover, as soils become older, not only does the total amount of phosphorus decline, but there is also a transfer of phosphate from labile pools to nonlabOe pools (Walker and Syers, 1976). 

It required about three billion years for all the oceans of Earth to become saturated with phosphates for the first time. Until then oceans acted as a giant sink for phosphorus, holding it in solution too dilute to be of much use as a nutrient to life forms. When Earth s oceans became saturated, any new dissolved phosphates entering the seas caused precipitation of exactly the same amount of phosphate someplace else. This action became faster and faster until today there is a complete replacement of phosphate in oceans every 4.9 x 10 years, a very short time on a geological time scale. This estimate is based upon the estimate of McKelvey et al. that 2x10 tons of phosphorus are delivered to the seas each year, and Horn and Adams estimate of the phosphorus in the seas. " ... 

Ruttenberg KC (1993) Reassessment of the oceanic residence time of phosphorus. Chemical Geology 107 405-409. 

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