Availability of phosphorus in tropical forests

Phosphorus is frequently the nutrient element most limiting plant growth in Oxisols and Ultisols (Cuevas 2001), the most common soil types in the tropics (Sanchez 1976; Van Wambeke 1992). These soils have high concentrations of iron and aluminum. Insoluble iron and aluminum phosphates are formed in the mineral soil. Phosphate in this form is unavailable to plants (Brady and Weil 2002). In the undisturbed forest, however, phosphorus availability is not generally a problem. Leaf litter provides an energy source for the microbes that carry out decomposition. As a result of their metabolism, the microbes produce organic acids that react with the iron and aluminum. The organic acids use up the binding power that otherwise would be used to bind phosphorus. The organic acids may also solubilize the phosphorus in iron and aluminum phosphates. Because of this microbial activity, more phosphorus is made available for plant growth, plants grow more, and, as a result, they produce more litter (Jordan 1989). This cycle can be maintained as long as there is a layer of organic matter on the soil. When this layer is destroyed by forest cutting and cultivation, the availability of phosphorus can fall to levels that are below those that are needed for plant growth. Phosphorus is also a limiting factor in Andosols formed from volcanic activity. In humid regions, volcanic ash weathers quickly into allophane, an amorphous aluminum-silicate mixture that rapidly forms complexes with phosphorus, and immobilizes it (Sanchez 1976).

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