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Fig. 10.17 Correlation between N and P levels in the leaves of various evergreen (closed triangles) and deciduous (open circles) cerrado trees in Brazil. (Franco et al. 2005)

Fig. 10.18 P/N-ratios in the above-ground biomass of different grasslands in the Llanos of Venezuela (cut or burned at the end of the dry season), Africa and Australia (C controls), and effects of fertilization with N, P or N + P. (After Medina 1993)

fertilization was applied. Special metabolic features may make some plant species very phosphorus-efficient which are much searched for agricultural applications in the tropics, such as the forage grass Brachiaria which covers 10 x 106 ha of pastures in Brazil (Nanamori et al. 2004).

10.2.3 Biotic Interactions 10.2.3.1 General Overview

Biotic interactions, when considered in relation to the nutrient "stress factor", determine growth, development and productivity of plants in savannas. This includes:

• plant-plant interactions,

• fungi- or microorganism-plant interactions,

• animal-plant interactions.

Among the nutritional plant-plant interactions the grass-tree relations are of particular interest in savannas. With the extended root systems of trees, tree-biomass may concentrate nutrients from large soil volumes. Due to litter fall and decomposition, the availability of K, Ca and Mg is often higher under tree canopies, and soil improvement due to litter may be even more important than by biological N2-fixation (Campa et al. 2000; Sect. 10.2.3.2). The droppings of perching birds may also add to improved nutrient availability in the vicinity of trees (Medina 1993).

For the animal-plant interactions termite savannas (Sect. 9.1) are interesting because mound-building termites excavate and explore large volumes of soil reaching depths of 0.5 -1m. In this way they affect soil texture, but in addition they may also enrich nutrients like Ca, K and Mg and to some extent also P. Termites accelerate nutrient recycling and in termite dominated savannas (Sect. 9.1, Fig. 9.5) this may make a very considerable contribution to nutrient turnover. In Australia it was observed that termite-mediated mineralization of organic matter may amount to 250 kgha-1 year-1 (Medina 1993). As described in Sect. 3.4.4.1, leaf-cutter ants (Fig. 3.34) concentrate nutrients in a similar way to mound-building termites. Leaves of grasses and trees are carried into complicated underground chamber systems, where the ants cultivate fungi and play a significant role in nutrient cycling, particularly for deep rooted trees. For the animal-plant interactions carnivory deserves a separate section (Sect. 10.2.3.3).

Plant-fungi interactions are of basic importance as mycorrhiza is a very widely expressed symbiosis of plants and fungi facilitating plant's mineral nutrient acquisition. With respect to P-supply it was observed that genes which are active in P-deficiency are down-regulated independent of each other by internal phosphorus levels and mycorrhiza, where signalling involves inorganic phosphate transport from the root to the shoot and a transportable shoot factor signalling back to the root (Burleigh and Harrison 1999). Mycorrhiza is also important in interactions with atmospheric dinitrogen fixation and root nodule symbioses which are treated in a separate section (Sect. 10.2.3.2.2).

10.2.3.2 Fixation of Atmospheric Dinitrogen (N2)

Fixation of N2 is mediated by the enzyme-complex nitrogenase. It is restricted to procaryotic microorganisms, bacteria and cyanobacteria ("blue-green algae"), which however, can make important contributions to the N-supply of eucaryotic plants in associations and symbioses. The overall process is highly energy demanding, i.e.

N2 + 4[2H] + 16ATP —> 2NH3 + H2 + 16(ADP + Pi) . Nitrogenase is very oxygen sensitive and requires hypoxia for its operation.

10.2.3.2.1 Plant associations with Free Living Dinitrogen Fixing Microorganisms

Free living N2-fixing bacteria are in the genera Acetobacter, Azoarcus, Azospirillum, Azotobacter, Beijerinckia, Clostridium, Herbaspirillum and Paenibacillus (Gotts-berger and Silberbauer-Gottsberger 2006). All cyanobacteria ("blue-green algae")

which have heterocytes are N2-fixing. As the nitrogenase is O2-sensitive it is located in special cells in the photosynthezising filamentous cyanobacteria, i.e. the heterocytes, which have thick cell walls limiting O2 diffusion into these cells, and lack photosystem II and hence photosynthetic O2-evolution. From the possession of heterocytes, most of the cyanobacteria in the savannas are shown to be N2-fixers (see also Sect. 11.2.1.2).

Associations of plants with N2-fixing soil bacteria in the rhizosphere may be of mutualistic benefit, where exudates from the plant roots provide substrates and vitamins and other regulatory compounds to the microorganisms and plants receive N-compounds. The contribution of rhizosphere associations to total N-input to savanna soils may be quite significant (Table 10.10), and possibly in many cases their overall contribution may even be larger than that of root nodules. In fact, there have been considerable efforts to improve agricultural productivity of tropical grasslands with such associations (Baldani et al. 2002). Some of the N2-fixing bacteria can even live endophytically in the host plants (Baldani et al. 2002). Attempts have also been made to use genetic engineering for the introduction of the nitrogenase-genes (nif+-genes) into some rhizosphere bacteria, which occur more abundantly in the soil than natural N2-fixing organisms (Hess 1992).

The contribution of cyanobacteria in the examples of Table 10.10 is shown to be rather modest. However, in places cyanobacteria are extraordinarily abundant in savannas, often forming dense, continuous mats between the tussocks of grasses (Fig. 10.19). In an example from savannas in Nigeria, where the ground coverage with cyanobacterial mats and crusts was 30%, a much higher value of cyanobac-terial N2 fixation is reported, namely 23 g ha-1 day-1 in the rainy season, and 60 g ha-1 day-1 have also been recorded in savannas corresponding to several kilograms per ha over the year (Medina 1993, Sect. 11.2.1.2).

A step between associations of plants with free living prokaryotes and N2-fixing endosymbioses may be exosymbioses such as the endophytic bacteria mentioned above. Another example which has become important in tropical agriculture is the mutualism between the fern Azolla and the cyanobacterium Anabaena, which

Table 10.10 Nitrogen balances in two humid tropical savannas in South America, Central Venezuela (Trachypogon savanna) and in Africa, Ivory Coast, and values of bacterial N2 fixation associations in grasslands of Brazil and Zimbabwe. (Medina 1987, 1993)

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