Soil Inorganic Phosphorus

Barrow (1983) has pointed out the complexities of the sorption process onto and within soil particles. He suggests that the relatively rapid adsorption of P onto the soil surface is followed by a 

The nonlabile fraction of inorganic phosphorus not available to plants is sometimes divided into the occluded and reductant soluble forms. Occluded phosphorus consists of aluminum- and/or iron-bonded phosphates surrounded by an inert coat of another material such as oxides or hydrous oxides of iron or aluminum. Reductant soluble forms are covered by a coat that may be partially or totally dissolved under anaerobic conditions (Uehara and Gillman, 1981). The opportunities for occlusions to occur increase dramatically with soil age (Walker and Syers, 1976). This is because substantial amounts of Fe and Al oxides tend to be present only in heavily weathered soils in which the secondary silicate minerals have already dissolved (Fox et al., 1991). Data from tropical forest chronosequence studies in Hawaii are more or less in accordance with this view the fraction of P present in the occluded form increases with soil age (Crews et al, 1995). Nevertheless, that study also showed high amounts of nonoc-cluded (i.e., labile and accessible) inorganic phosphorus to be present, even in forests growing on the oldest soils. 

Tropical agronomists have long realized the importance of organic phosphorus as the main source of phosphorus in nonfertilizer agriculture, such as that occurs in traditional systems (Nye and Bertheux, 1957 Sanchez, 1976). In addition to being a source of phosphorus for plant uptake after mineralization, the importance 

States and Fluxes of Phosphorus in Moist Tropical Forests 

From Sec. 2 it can be concluded that, due to the highly weathered state and high phosphorus sorption capacity of many moist tropical forests soils, the level of readily plant available phosphorus is low. Discussion on whether this means that phosphorus availability actually limits productivity of moist tropical forests is reserved until Sec. 4.1. Here we limit our concerns to a discussion of the phosphorus cycle in moist tropical forests and methods by which plant phosphorus acciuisition can occur in environments characterized by low levels of available P. The main aim of this section is to cjuantify the amounts and annual input/output fluxes of P for leaves, branches, boles, and roots of moist tropical vegetation. The inputs of phosphorus into moist tropical forests from rock weathering and wet and dry deposition, as well as from leaching losses, are also considered. This information is then used for model simulations in Sec. 4.3. 

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Williams, J.D.H., Syers, J.K. and Walker, T.W. (1976b) Fractionation of soil inorganic phosphorus by a modification of Chang and Jackson s procedure. Soil Science Society of America Proceedings 31, 735-739. 

Readily available phosphorus This form is present in soil pore water and the exchangeable pool. Phosphorus in this pool is continuously replenished from other stable pools at various rates, depending on the solubility of phosphate minerals and the physicochemical properties of soils. Inorganic phosphorus is extracted with neutral salts such as NaCl, KCl, NH4CI, and NaHCOj. 

Phosphorus (P) is one of the major limiting factors for plant growth in many soils. Plant availability of inorganic phosphorus (Pi) can be limited by formation of sparingly soluble Ca phosphates, particularly in alkaline and calcareous soils by adsorption to Fe- and Al-oxide surfaces in acid soils and by formation of Fe/ Al-P complexes with humic acids (94). Phosphorus deficiency can significantly alter the composition of root exudates in a way that is, at least in some plant species, related to an increased ability for mobilization of sparingly soluble P sources (29,31,71). 

Many more papers deal with rhizosphere phosphatase activity (63-83) in the presence of a number of different plant species this will partly be due to the simplicity of the enzyme activity assay (85,86) and the generally reported, well-correlated variation trends among organic and inorganic phosphorus content and phosphatase activity. More precisely, closer to the roots, the inorganic P depletion zone in comparison with bulk soil is more pronounced in addition, organic and inorganic P contents are inversely correlated, and the mineralization rate of or-... 

Procedure (extraction). Transfer 2.5 g air-dry soil, 2 mm mesh size, into a 250 ml polypropylene screw-cap centrifuge bottle/tube and add 100 ml acetic acid - 8-hydroxyquinoline reagent. Cap the tube and shake overnight (17 h) on a reciprocating shaker, at approximately 275 strokes of 25 mm length per minute at a constant temperature (20°C). Centrifuge for 15 min at 2800 rpm and remove an aliquot for the determination of acid extractable inorganic phosphorus (a). 

To illustrate the effect of outliers, consider the following simple example presented in Figure 6.1, which depicts the concentration of inorganic phosphorus and organic phosphorus in the soil [6], On this plot the classical tolerance ellipse is superimposed, defined as the set of p-dimcnsional points x whose Mahalanobis distance... 

This picture of a substantial phosphorus constraint is drastically altered when the presence of the inorganic labile (i.e., sorbed) phosphorus pool is taken into account (Scenario C). Desorption of phosphate occurs in response to increased rates of removal from the soil solution. Consequently, the reduction in soil solution phosphorus concentration over 1730 levels is only 9% for 1981-1990. This contrasts with the 25% reduction in Scenario B. Consequently, the enhancements of Gp and Np are more similar to the no-P-constraint case, though a full expression of the COi-in-duced growth response is still not possible. Accordingly, the rate of net carbon accumulation by the ecosystem is 6.5 mol m - year", substantially more than Scenario B, but about 20% less than what is modeled to be the case if no phosphorus limitations to plant production occurred. 

The forms of phosphorus in sediments and soils can be operationally defined by chemical extraction schemes. The major inorganic phosphorus components are considered to comprise those (i) adsorbed by exchange sites (ii) associated with iron, aluminium and manganese oxides (iii) associated with carbonate (iv) associated with calcium as apatite or (v) bound in a crystalline mineral form (e.g. silicates). The organic phosphorus fraction, on the other hand, is either considered as a whole, or operationally divided... 

Sequential extraction or chemical fractionation techniques have been widely used in the characterization of various phosphorus fractions in soils and sediments, with an emphasis on the more bioavailable or plant-available inorganic forms (Condron et al., 2005). Early extraction procedures (Chang and Jackson, 1957 Williams et al., 1976b) focused on inorganic phosphorus associated with iron, aluminium and calcium, using various acid, base or salt extraction steps. Organic phosphorus was considered to be the residual or refractory phosphorus-containing fraction that remained after all other extractions had been performed. 

Rheinheimer, D.S., Anghinoni, I. and Flores, A.F. (2002) Organic and inorganic phosphorus as characterized by phosphorus-31 nuclear magnetic resonance in subtropical soils under management systems. Communications in Soil Science and Plant Analysis 33, 1853-1871. 

Tiessen, H., Stewart, J.W.B. and Moir, J.O. (1983) Changes in organic and inorganic phosphorus composition of two grassland soils and their particle size fractions during 50-90 years of cultivation. Journal of Soil Science 34, 81 5-823. 

Fig. 7.1. Microbial turnover of phosphorus in soils processes (italic) and related pools (rectangle). The dark arrows on the microbial phosphorus pool in the figure indicate phosphorus recycling between different generations or groups of microbes. The symbol denotes extracellular processes in the periplasm mediated by phosphatase exoenzymes or inorganic phosphorus (P.) solubilizing agents organic phosphorus).

Methods used to quantify microbial phosphorus have so far been based on fumigation and extraction. The basic methods were published early in the 1980s (Brookes et al., 1982 Hedley and Stewart, 1982) and, of the methods presented in Table 7.2, that of Brookes et al. (1982) has been used the most frequently. Phosphorus released by fumigation is calculated as the difference between inorganic or total phosphorus extracted from fumigated and non-fumigated soil. Most phosphorus in microbial cells is... 

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