Mechanisms for Enhanced Phosphorus Uptake in Low P Soils

Generally speaking, most tine roots in tropical forest soils are found in the upper 0.5 m (Kerfoot, 1963), with a marked concentration of roots into a root mat close to the soil surfiice and within the litter layer being especially common on low-fertility soils (Stark and Jordan, 1978 Medina and Cuevas, 1989). It is generally considered that these root mats serve to ensure the maximum retention of nutrients by the vegetation and to minimize any leaching losses. Surveys of tropical forests have indicated almost ubiquitous mycorrhizal associations for such roots (Alexander, 1989 Janos, 1989). 

As for temperate plants, it is widely assumed that mycorrhizal associations in tropical forests serve to improve the uptake of mineral nutrients, particularly phosphorus (Bolan, 1991 Koide, 1991 Smith and Read, 1997). Growth stimulations and enhanced P uptake in response to mycorrhizal infection have been reported for tropical tree seedlings (Janos, 1989 Lovelock etai, 1996, 1997). 

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. 

This improved P uptake occurs in exchange for the provision of G from the host plant, and the carbon requirements of the mycorrhizal association can be substantial. For example. Baas et al. (1989) showeci root respiration rates of mycorrhizal plant to be 20-30% higher than those of nonmycorrhizal plants. Similarly, 

Jakobsen and Rosendahl (1990) observed 20% of plant carbon to be allocated below ground for nonmycorrhizal cucumber plants and 44% for those with mycorrhizal associations. In both cases, about half of this was respired. Working with subtropical Citrus species, Peng et al. (1993) suggested that root respiration rates were about 35% higher for mycorrhizal than for nonmycorrhizal roots. 

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