Phosphorus root system

Figure 11 Root systems as simulated by the computer program Simroot. The systems display herriitgbone architecture (a) dichotomous architecture (b) and bean root architecture (c). The bean root system (c) is shown in

R. F. Grant and J. A. Robert.son, Phosphorus uptake by root systems mathematical modelling in ecosys. Plant Soil 188 219 (1997). 

To determine the effects of the deprivation of specific micronutrients on the water hyacinth (Eichhornia crassipes), Colley et al. (1979) studied the rate of uptake of iron and manganese in comparison with phosphorus. Results indicated that all three elements were actively absorbed by the root systems, but the rates of absorption differed markedly. The rate of absorption of manganese by roots was 13 and 21 times that for radio-iron and -phosphorus, and iron was taken up by the roots at nearly twice the rate of phosphorus. Manganese translocation appeared to be faster than phosphorus translocation by an order of magnitude and 65 times faster than iron translocation. 

But are there sufficient fine roots at depth to absorb significant amounts of soil nutrients In both mature and secondary forests, there is a hundred-fold decline in fine-root biomass from the soil surface to 6 m depth, which means that fine root length density at 6 m is less than 1 cm of root per 100 cm3 of soil. Moreover, the few roots that occur at depth are concentrated in patches of soft soil that comprise approximately 1% of the soil volume, and that show no nutrient enrichment (Carvalheiro and Nepstad 1996). Given the low mobility of phosphorus in the soil, it is unlikely that such a sparse, patchy root system could absorb substantial amounts of this scarce nutrient. 

Third, most of the available evidence suggests that, at a given soil P concentration, plants growing at elevated [CO2] are capable of maintaining their tissue phosphorus concentrations. This is in contrast to nitrogen and occurs because of the positive effects of larger root systems on the extent of root mycorrhizal colonization, root organic acid efflux per plant, and root acid phosphatase activity. All three processes play important roles in phosphorus acquisition. 

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 soil microorganisms, plant root system and soil fauna are responsible for chemical transformations in the soil which are important for its fertility and which ensure the removal of natural litter from the earth s surface. Some stages of the cycles of matter result in amelioration of the soil, whereas others decrease its fertility [1]. In the following sections, participation of soil microorganisms in the cycles of carbon, nitrogen, sulphur, phosphorus and other elements are discussed (see also Sections 2.1.2 and 4.2.9). 

The possibility that limited root development is attributable to insufficient available major and minor nutrients has received considerable attention. Sometimes the addition of available nitrogen and phosphorus has increased the depth and extension of root systems, but in most cases the increase has not been marked, and there was no certainty as to how much of the increases obtained was due to the nutrients per se, and how much to the mechanical operations involved in the addition of the nutrients. Where the A-horizon is present, most of the nutrients needed for growth in the subsoil are readily obtained from the topsoil, but in exposed subsoils added available nitrogen and phosphorus may be very essential for extensive root proliferation. If the physical conditions are satisfactory, plants produce a surprisingly large amount of roots on a minimum of nutrients they appear to send out roots in search of nutrients, particularly nitrogen. 

Lyu S-W, Blum U (1990) Effects of fendic acid, an adelopathic compound, on net P, K, and water uptake by cucumber seeddngs in a split-root system. J Chem Ecol 16 2429-2439 Lyu S-W, Blum U, Gerig TM, O Brien TE (1990) Effects of mixtures of phenolic acids on phosphorus uptake by cucumber seedlings. J Chem Ecol 16 2559-2567 MaMno T, Takahashi Y, Sakurai Y, Nanzyo M (1996) Infiuence of soil chemical properties on adsorption and oxidation of phenodc adds in sod suspension. Soil Sci Plant Nutr 42 867-879... 

Toxic compounds that can be absorbed to a marked degree by a living plant through either its roots or its leaves have been called by British investigators systemic insecticides. Schrader (38) first found this peculiar property in certain acetals of 2-fluoroethanol and bis-(2-fluoroethoxy) methane, as well as in certain compounds of his organic phosphorus series, notably bis(dimethylamido)fluophosphate and octamethyl pyrophosphoramide. 

Schachtman DP, Reid RJ, Ayling SM (1998) Phosphorus uptake by plants from soil to cell. Plant Physiol 116 447-453. doi http //www.plantphysiol.org Schindler DW (1974) Eutrophication and recovery in experimental lakes implications for lake management. Science 184 897-899. doi http //www.sciencemag.org/cgi/content/abstract/184/4139/897 Schindler DW, Hecky RE, Findlay DL, Stainton MP, Parker BR, Paterson MJ, Beaty KG, Lyng M, Kasian SEM (2008) Eutrophication of lakes cannot be controlled by reducing nitrogen input results of a 37-year whole-ecosystem experiment. Proc Natl Acad Sci USA 105 11254-11258. doi http //www.pnas.org/content/105/32/l 1254.abstract Scott JT, Condron LM (2003) Dynamics and availability of phosphorus in the rhizosphere of a temperate silvopastoral system. Biol Fert Soils 39 65-73 Shane MW, Lambers H (2005) Cluster roots a curiosity in context. Plant Soil 274 101-125. doi http //dx.doi.org/10.1007/s 11104-004-2725-7... 

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