aWith permission from Bolin et al. (1983).
TABLE 15.2 Total and Organic Phosphorus Concentrations in the Surface Layer (0-10 cm) of Some North American Grassland Soils
Cottonwood, South Dakota, USA 554 310
Bridger, Montana, USA 1234 675
Osage, Kansas, USA 251 227
Ale, Washington, USA 748 29
Jornado, New Mexico, USA 445 37
Pantex, Texas, USA 94 239
Bison, Manitoba, Canada 835 582
Pawnee, Colorado, USA 345 131
When P is added to soil as a soluble salt (fertilizer), it becomes fixed or bound to the extent that very little of the added P is reextractable with water. Hedley et al. (1982) developed a sequential extraction procedure to separate soil P into inorganic and organic pools based on their availability to plants. The major fraction of P is not extractable by dilute acids or bicarbonate solution. This portion of the retained P is commonly designated as fixed P. The portion that is extractable by dilute acid or bicarbonate is designated as available P; it is considered to be the amount of soil P available for uptake by living organisms. Available P measured using this approach is partly inorganic and partly organic. That portion of the total P in soil that is resin extractable is designated as exchangeable P, defined as that fraction of the soil P that can enter solution by isoionic exchange during a given time span.
Phosphorus in organic molecules constitutes 30-50% of the total P in most soils, ranging from as low as 5% to as high as 95%. Despite the importance of soil organic P, its chemical nature has not been fully characterized. This is due in part to analytical limitations, as there are no direct methods to characterize soil organic
P. Even solid-state phosphorus-31 nuclear magnetic resonance (31P NMR) spectroscopy cannot detect organic P in soil because of poor sensitivity and interference from paramagnetic ions. Organic P must therefore be extracted from soil before it can be quantified and identified (see Turner et al., 2005). Organic P occurs in soil principally as phytates or related forms, nucleic acids and their derivatives, and phospholipids (Fig. 15.4). Phytin, primarily polymeric inositol hexaphosphate, is synthesized by plants and accounts for roughly 40% of the organic P found in soil. Some of the polymers are believed to be of microbial origin. Some yeasts synthesize phosphorylated polymers of mannose and the cell walls of G+ bacteria contain both techoic acid, a polymer of ribitol phosphate and glycerophosphate, and a 6-phosphate muramic acid. Lower weight inositol ring compounds carrying one to five atoms of P also occur in soil and probably represent degradation products of inositol hexaphosphate.
Constituent parts of nucleic acid molecules are identifiable in hydrolysates of soil extracts. These include cytosine, adenine, guanine, uracil, hypoxanthine, and xanthine. The last two are decomposition products of guanine and adenine. Of the total organic P in soil, only about 1% can be identified as nucleic acids or their derivatives. The susceptibility of nucleic acids to decomposition, together with a lack of incorporation into stable organic matter, is believed to be responsible for their low level of occurrence in soil. Extracellular DNA sorbed to soil mineral surfaces persists from 100 to 1000 times as long as free DNA. This mineral-associated DNA is considered to provide for natural genetic transformation of bacteria.
Organic P in alcohol and ether extracts of soil is indicative of the presence of phospholipids. Choline has been identified; it is one of the products of hydrolysis of lecithin. Most of the glycerophosphate found in soil is believed to be of lipid origin. Phospholipid P accounts for a similarly low fraction of the soil organic P as does nucleic acid P. Even smaller amounts of sugar phosphates are found. These are easily decomposable and therefore, together with nucleic acid and phos-pholipid P, cannot be considered as contributing significantly to the estimates of 350 to 2000 years as the mean residence time for total organic P in a prairie soil.
While the sizes of the organic P pools in soil generally occur in the order inositol phosphate > polymer organic phosphate > nucleic acid P > phospholipid P, the concentrations of these pools within the soil biota occur in the reverse sequence. Next to N, P is the most abundant nutrient contained in the microbial biomass. Brookes et al. (1982) developed a method to measure P within the soil microbial biomass using chloroform (CHCl3) fumigation followed by extraction with NaHCO3 and relating the measured P to the P extracted from an aliquot not treated with CHCl3. Work with soils having pH of 6.2 to 8.2 showed a recovery (Kp) of 40% of added 32P-labeled cells, suggesting a calculation of microbial P as microbial P = (CHCl3 P - control P)/Kp, where Kp = 0.4. Microbial biomass P concentrations of 5 to 75 ^g P g-1 soil were reported, representing from 2 to 5% of the total organic phosphorus in cultivated soil and up to 20% in grassland and forested soils.
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