Fig. 3.32 Diagram of inorganic nutrient cycling in the lower montane rainforest at Kerigoma, New Guinea. Numbers are in kg ha-1 year-1 for flows (arrows) and in kg ha-1 for pools (boxes). (After Whitmore 1990 by permission of Oxford University Press)

Tropical forests frequently have a nutrient limitation of some sort. Although the soil is often poor in mineralized nutrients the vegetation is luxuriant and so highly rich in species. As in savannas (Sect. 10.2.2), phosphorus is often the most problematic element. Mycorrhizal symbioses between plant roots and fungi are important (Medina and Cuevas 1994; Béreau et al. 2004) and there is feedback between the essential controlling steps of retranslocation and mineralization in tropical rain forest ecosystems dominated by ectomycorrhizal trees (Chuyong et al.

2000). P/N-ratios in canopy leaves of tropical humid forests range from 15 to 35 (mol/mol) x103 (Medina and Cuevas 1994) similar to those in plants of savannas. Nitrogen cycles for a semideciduous forest are shown in Fig. 3.33. Nodu-lation and atmospheric dinitrogen fixation is nutrient limited in tropical forests (Souza Moreira et al. 1992). In a rainforest in French Guyana, however, 43% of the Leguminosae were fixing N2, the contribution of N2-fixation to their nitrogen nutrition averaged 54%, and overall the N2-fixing species are important for the performance of the ecosystem increasing its nitrogen-biomass by 10% (Domenach et al. 2004). N2-fixation capacity of the rain forest trees is controlled and

Fig. 3.33 Compartmentation and annual turnover of nitrogen in a semideciduous forest in Ghana. (From the Ecology of Neotropical Savannas by Guillermo Sarmiento. Copyright © 1984 by the President and Fellows of Harvard College. Reprinted by permission of Harvard University Press (Sarmiento 1984))

limited by low phosphorus and high nitrogen levels in the soil (Pons et al. 2007; Sect.

Roots, which often form very dense mats, are mainly restricted to the upper 0.10.3 m of the soil, and frequently the soil layer itself on top of the bed rock is rather thin. Soil respiration, which is very high in wet tropical forests has a typical rate of 4 pmol CO2 m-2 s-1 (Buchmann et al. 2004) and corresponds to 600 - 670 g organic matter m-2 year-1, of which 67 - 82% is due to respiration within root mats. Rates of mineralization of organic litter are high and recirculation of minerals is rapid. Phosphorus in the soils of humid tropical forests is correlated with the litter fall mass (Silver 1994). Rapid recirculation is important for avoiding nutrient leaching (Chuyong et al. 2000). Litter turnover may support seedling growth (Brearley et al. 2003) and is essential for the development of photosynthetic capacity (Santiago and Mulkey 2005).

Ants play a role in biomass turnover and mineralization. In myrmecophytes the ants may contribute to nutrition of their hosts (see Sect. 6.6.3). In Tococa guianen-sis it was observed that one of the two adjacent domatia at the base of each leaf (see Fig. 3.23A) is used for nesting and the other for dumping excrement and debris. By radioactive tracer studies the inner surface of the domatia has been shown to be absorptive of low molecular substrates like amino acids and phosphate, in contrast to the surfaces of the leaf lamina. Thus, it is highly likely that nutrients are absorbed from the rotting material in the trash-domatium (Nickol 1992). On the other hand, in contrast to other reports in the literature that in Cecropia (Fig. 3.23B) 93% of its nitrogen is supplied by ants, a recent study suggested that it is only < 1% (Fischer et al. 2003).

Leaf-cutter ants (Fig. 3.34) are a special case. (In the following text I follow the monograph of Wirth et al. 2003.) Herbivory in general can enhance nutrient cycling by, e.g.

• enhancing the leaching rate of nutrients from foliage,

• increasing the rate of litter fall,

• stimulating nutrient cycling and turnover within plants,

• promoting activities of decomposer organisms.

Table 3.3 Selection of some quantitative data on the activity of leaf-cutter ants, Atta colombica, extracted from Wirth et al. (2003) and Herz et al. (2006)

Nest size (m2)

A few to about 100

Dry leaf mass collected (kg per year)

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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