example, nutrient cycling on nutrient-poor tropical Spodosols ("caatinga" forest) is slower than in more nutrient-rich soils (Grubb 1995). Extended dry seasons also limit primary production and decomposition. Therefore, annual rates of nutrient cycling would be expected to be lower in tropical dry forests where extended dry seasons occur. For example, in a comparison of productivity, litter fall, and turnover times for tropical forests of the world, the largest litter production was found in tropical moist forest, where it was about three times higher than in tropical dry forests (Brown and Lugo 1982), and, therefore, nutrient fluxes in tropical moist forests also must be much higher than in dry forests.
The variations in the nitrogen cycle between latitudes and between forests on soils of low and moderate fertility are illustrated in Box 2.5. Mechanisms determining the availability of phosphorus in tropical forest soils are shown in Box 2.6.
The nitrogen cycle in tropical forests
Nitrogen makes up about 78% of the atmosphere, but it can only enter terrestrial ecosystems through nitrogen fixation or via precipitation (Brady and Weil 2002). Nitrogen gas can be fixed from the atmosphere by microbes living symbiotically with the roots of plants in the legume family. It also enters the ecosystem dissolved in rainwater. Once in a plant, nitrogen is used in proteins and other compounds. When a leaf or a plant dies, it falls to the ground as litter. Some of the nitrogen is converted into ammonium and nitrate and moves into the soil or roots. Under anaerobic conditions, such as those that occur on poorly drained or waterlogged soils, some nitrate-nitrogen is reduced to nitrogen gases by bacteria called deni-trifiers and is then released into the atmosphere.
The high temperatures and humidity of forests of the moist tropics are near optimum for microbes that mediate nitrogen transformations. As organic matter in the soil decomposes, organic nitrogen is decomposed into ammonia, which can be taken up by plants, or transformed into nitrite and then nitrate by a set of highly specific, aerobic soil bacteria. Nitrate can be leached or taken up by roots or by microbes, or the nitrate nitrogen can be denitrified and returned to the atmosphere (Jackson and Raw 1973).
The microbes that mediate nitrogen transformations are active year-round. As a result, the sparse available data suggest that in most tropical ecosystems, more nitrogen is cycled per year than in temperate zones (Table 2.5). An exception is tropical forests where nitrogen stocks in the soil are extremely low and therefore limiting, as on Spodosols. These are soils that have developed under highly acidic conditions, where organic matter from the topsoil dissolves and precipitates down in the soil profile (Brady and Weil 2002). In the forests near San Carlos de Rio Negro in the Venezuelan Amazon, nitrogen fluxes to the forest floor in unfertile Spodosols were only half the amount of those in Oxisols, which are relatively more nutrient-rich than Spodosols (Cuevas 2001).
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