From data presented in Amundson (2001).

example, 1-20 kg/ha/year of phosphorus input comes from plant litter Whitehead (2000). The situation is exacerbated in regularly harvested agricultural soils, where the annual input of litter is minimized by harvest and removed off-field. Obtaining nitrogen- and phosphorus-containing molecules and soluble ions for cell growth is competitive. Soluble phosphorus compounds are readily scavenged and stored in intracellular pools. The soil solution near the rhizosphere is notoriously poor in K, N and P ions, as plants are effective and most competitive at absorbing these ions, especially in mycorrhizal roots. Typically, soil organisms, particularly eukaryotes, are effective at absorbing dissolved ions through cell membranes by osmotrophy. Even though they consist of 2-6% of the total soil organic carbon, they hold up to 20% of the soil phosphorus. The soil organisms are probably the most important source of soluble and biologically available phosphorus compounds. Most of the available phosphates are as HgPO^, HPO|~ and polyphosphates in cells. The difficulty in tracing phosphorus is that only a small fraction is available to the soil solution, and the larger fraction is bound to clays and inactive fractions of the soil. The fraction that is available for cell membrane transport into cells is rapidly absorbed and immobilized. Much of the phosphorus is insoluble or bound to soil particles and not available for membrane transport. Some is bound to calcium phosphates including apatites, some is clay bound such as to kaolinite, and some is bound to

Fig. 6.4. Global phosphorus pools and flux rates (adapted from Schlesinger, 1997).

organic matter. Apatite consists of 3[Ca3(PO4)2].CaX2, where X can be carbonate, OH-, F- or C-. Sometimes, other P-containing minerals are present, such as strengite (FePO42H2O), variscite (AlPO42H2O) and other iron or aluminium phosphates. A fraction of the organic phosphorus pools may be liberated through secretion of phosphatases by protists and bacteria, releasing soluble inorganic phosphate ions. Phosphorus limitation in the decomposition food web is most limiting to ATP cytoplasmic pools, and to nucleotide synthesis which prevents DNA replication and reduces cellular activity. Reduced phosphorus availability limits cell growth and activity. In turn, reduced cellular activity decreases the rate of decomposition and the cycling of other nutrients. Therefore, the overall rate of nutrient use and uptake decreases in P-poor soils.

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