Phosphorus, effect pools

Figure 38.8 Effects of P04 scavenging on cellular partitioning of P and N P ratios in eight natural estuarine bloom samples of prokaryotic and eukaryotic algae. (A) Fractions of total cell-associated P in the intracellular (filled bars) and surface-adsorbed (open bars) pools (B) Nitrogen to phosphorus (N P) ratios calculated using total cellular P (filled bars) and intracellular P pools after removal of surface-adsorbed P with the oxalate reagent (open bars). Error bars represent the standard deviations of triplicate samples, and the dashed lines indicate the Redfield ratio. Fu et al. (2005a), Limnology and Oceanography.

Several mechanisms may be involved in enhanced P uptake by mycorrhizal symbioses. First, the extensive network of fungal hy-phae enables plants to explore a greater volume of soil, thereby overcoming limitations associated with the relatively slow diffusion of P in the soil solution (Marschner, 1995 Smith and Read, 1997). Second, although mycorrhizae often access phosphorus from the same labile pool as nonmycorrhizal roots, there is also some evidence that they are capable of accessing forms of phosphorus not generally available to the host plant (Marschner, 1995). Whether the mycorrhizae actually serve to increase the affinity of a root system for phosphorus or to allow plants to compete more effectively for phosphorus with soil microbes is unclear. For example, Thompson ct al. (1990) reported that mycorrhizal roots and isolated hyphae have P uptake kinetics similar to those of nonmycorrhizal roots and other fungi. 

The term d[P j,]/dt is calculated assuming that the concentration of phosphorus in all decomposing litter is 0.16 mmol 1110 . This is based on the 68% retranslocation of P from leaves and fine roots and the average branch, bole, and coarse root P concentrations (Sec. 3.2). Where the sensitivity of the model to P accumulation in the microbial carbon pool is tested, based on data summarized by Gijsman et al. (1996) we use a tissue P concentration for microbes of 6.4 mmol P moG C. In all simulations, it is assumed that soil phosphorus mineralization proceeds with a rate constant of 0.5 year , with phosphorus mineralization proceeding independently of carbon mineralization. This is on the basis of the evidence discussed in Sec. 2.1. Indeed, inflexible soil carbon pool C/P ratios which effectively link phosphorus mineralization rate to the carbon mineralization rate in models such as CENTURY (Parton et al, 1988) have been strongly criticized by some tropical soil chemists (Gijsman et al, 1996). 

The second term in the denominator is typically around 5000 and represents the buffering effect of the sorbed P. That is, in the pre.sence of an appreciable. sorbed P pool, the soil solution P concentration is extremely insensitive to the rate of removal of phosphorus into or from it. This is becau.se increased rates of removal of P are almost totally balanced by desorption. Likewise, increased rates of P input result in large increases in lPs ri,J, but with very little change in [Pjoil. That latter case represents, of course, the tropical soil phosphorus fertilizer fixation problem discussed in Sec. 2.2. 

Kirschbaimi et al. (1998) used a somewhat different modeling approach to simulate the effects of phosphorus availability on temperate forest COi-induced growth responses. But similar to the results here, they concluded that the presence of the secondary (labile) pool means that, in the short term, phosphorus availability should not constrain the ability of these forests to respond to [CO2]. They also concluded, however, that marked phosphorus constraints should become apparent on a time scale of... 

Turrion, M.-B., Glaser, B., Solomon, D., Ni, A. and Zech, W. (2000) Effects of deforestation on phosphorus pools in mountain soils of the Alay Range, Khyrgyzia. Biology and Fertility of Soils 31, 134-142. 

Ross, D.J., Tate, K.R., Scott, N.A. and Feltham, C.W. (1 999) Fand-use change effects on soil carbon, nitrogen and phosphorus pools and fluxes in three adjacent ecosystems. Soil Biology and Biochemistry 31, 803-81 3. 

Dobermann, A., George, T. and Thevs, N. (2002) Phosphorus fertilizer effects on soil phosphorus pools in acid upland soils. Soil Science Society of America Journal 55, 552-560. 

Sequential reduction of electron acceptors can have a significant effect on soluble phosphorus release. After a soil is flooded, it is expected that the amount of soluble P will increase. This is attributed to the anaerobic conditions occurring in the flooded soil and the various mechanisms of releasing phosphorus under those conditions. As shown in Figure 9.58, the amount of soluble phosphorus starts increasing after the third day of inundation, when almost the entire nitrate pool has been reduced, and consequently the reduction of manganese and iron contained in oxide minerals is already in process. On reduction of ferric oxide minerals, water-soluble and exchangeable concentrations of ferrous iron increase markedly. Thus, the dissolution of iron minerals is accompanied by increases in concentrations of both adsorbed and water-soluble phosphorus. Some of the ferrous ions react with the released phosphorus and precipitate to form new ferrous phosphate minerals. As the soil continues to be under anaerobic conditions, ferric ions are soon depleted and the reduction... 

More recently Nelson and Bamum (1960) have demonstrated an effect on brain phosphatidylcholine metabolism, occurring a very short time after the injection of 2.5 mg/kg body weight of DFP. After 3 hours the level of energy-rich phosphorus compounds was unaffected, but the transfer of from the acid-soluble pool to the phospholipids was significantly... 

The effect of Roentgen rays on turnover rate of phosphatides present both in tissue and in nuclei was investigated as well. Two groups of twelve rats, after irradiation with 1000 r, are given labeled phosphate, while nonirradiated, control groups are treated in a similar way. After the lapse of two hours the animals are sacrificed and the sarcoma and livers are pooled separately. An aliquot is used in the determination of specific activities of inoi anic and phosphatide phosphorus of the tissue, while from the bulk of the material cell nuclei are isolated by the method of Bounce (42). The specific activities of the corresponding phosphorus fractions of the nuclei are also determined, and furthermore the activity of the inorganic phosphorus of the pooled blood plasma is measured. As seen in Tables XXXI and XXXII, the rate of turnover of phosphatides in liver... 

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