N

«

Ca, Mg

«

whereas in the forest most K and P is contained in the vegetation. For forests N is about equally distributed between vegetation and soil, similarly to P in savannas.

10.2.1 Nitrogen 10.2.1.1 Nitrogen Cycles

Nitrogen is one of the most critical elements for plant growth in savannas. The nitrogen cycle in general is determined by assimilatory processes in microorganisms and plants, and the use of this primary production by consumers and decomposition by microorganisms (Box 10.4).

Figure 10.14 gives a comparison of the annual nitrogen-cycles and the nitrogen levels in various compartments of the ecosystem of a non-tropical prairie and a seasonal tropical savanna (a similar presentation for N-cycles in tropical forests is presented in Fig. 3.33).

Fig. 10.14A, B Compartmentation and annual turnover of nitrogen in two grassland ecosystems. A Andropogon gerardi - Andropogon scoparius prairie in Missouri (USA). B Seasonal Axonopus purpusii-Leptocoryphium lanatum savanna in Barinas (Venezuela); strongly modified and simplified from Sarmiento (1984) (reprinted by permission of Harvard University Press). Sizes of N pools in the different compartments (boxes) and transfer rates between the compartments (arrows) were drawn to scale to allow direct comparisons of pools and rates both within and between the two ecosystems

Fig. 10.14A, B Compartmentation and annual turnover of nitrogen in two grassland ecosystems. A Andropogon gerardi - Andropogon scoparius prairie in Missouri (USA). B Seasonal Axonopus purpusii-Leptocoryphium lanatum savanna in Barinas (Venezuela); strongly modified and simplified from Sarmiento (1984) (reprinted by permission of Harvard University Press). Sizes of N pools in the different compartments (boxes) and transfer rates between the compartments (arrows) were drawn to scale to allow direct comparisons of pools and rates both within and between the two ecosystems

Box 10.4 Nitrogen cycles in ecosystems

Box 10.4 Nitrogen cycles in ecosystems

Pools of N in various groups of organisms of the ecosystem and in various N-compounds (boxes) and transfer between the pools (arrows).

Box 10.4 (Continued)

-NH2, -N-, organic N; NH+, ammonia; NO-, nitrite; NO-, nitrate;

N2, atmospheric dinitrogen gas. (After Luttge et al. 2005)

The compartments distinguished are:

• living epigeous biomass,

The largest amount of N in either case is in the organic matter of the soil, and it is similar in the prairie and the savanna. The amount of mineral N in the soil is much smaller, and it is somewhat larger in the prairie as compared to the savanna. The amount of N in the roots and in the living epigeous biomass, as well as in the litter, is not very different in the two systems. The rates of N-transfer between the individual compartments, namely absorption of mineral N from the soil, root-shoot translocation, mortality, decomposition/humification and mineralization, as well as precipitation, throughfall and drainage are similar within the two systems.

It is noticeable that the rate of absorption of mineralized N from the soil is lower in the savanna and that the root/shoot recycling of N (translocation between roots and epigeous biomass) is higher than in the prairie. More important, however, is the presence of two additional transfer processes in the savanna as compared to the prairie, namely:

• volatilization,

• atmospheric N2-fixation.

Volatilization is largely due to fire in the savannas (Sect. 10.3). However, it is also known that tropical soils are significant natural sources of gaseous N-compounds, e.g. in the savannas of the Venezuelan Llanos:

nitrogen oxides 3-13 kg N ha-1 year-1

(NO-nitrogen 2.6 kgNha-1 year-1)

(N2O-nitrogen 0.65 kgNha-1 year-1)

ammonia 11 kg N ha-1 during the reproductive period

(Garcia-Mendez et al. 1991; Medina 1993). Atmospheric N2-fixation is an important activity of mats of cyanobacteria between the tussocks of savanna grasses and of free living soil bacteria and root nodule symbioses (Sect. 10.2.3.2).

In summary, it is surprising how small are the differences between the two grasslands, namely the mesic prairie and the tropical savanna. It should be recalled, however, that diagrams similar to Fig. 10.14 have been drawn for tropical forests (Fig. 3.33), giving a very different picture. Although the soil organic N is similar, amounts of N in the roots and in the epigeous living biomass are very much larger, and N in the dead litter is somewhat larger in the forests than in the grassland systems. Mineral N in the soil is smaller in the forest. With the exception of precipitation, throughfall and drainage, the rates of N-transfer between the individual compartments are considerably larger in the forests than in the grassland systems. This relates to absorption of mineral N from the soil, root-shoot translocation, mortality, decomposition/humification and mineralization, such that the cycling of N in the forest is much more rapid than in the prairie and the savanna.

10.2.1.2 Nitrogen-Use Efficiency

In Sect. 4.1.2 we have already discussed the nitrogen-use-efficiency (NUE) of photosynthesis in relation to the light climate in tropical forests. Again in savannas and cerrados, for both grasses (Fig. 10.15) and trees (Fig. 10.16A), we observe generally linear relationships between levels of N in biomass and rates of photosynthesis. As mentioned above, there are often differences between species (Sect. 4.1.2). In savannas, the slope of the line for the tropical C4-grass is much steeper than for the two C3-grasses of the temperate zone given for comparison (Fig. 10.15). For crops the ratio of photosynthetic CO2-fixation to leaf-N also was found to be higher in the C4-plant maize (1,056 jumol CO2 m-2s-1 /mol N) than in the C3-plant rice (640^mol CO2 m-2s-1 /mol N). On the other hand, C4-plants do not necessarily have a competitive advantage over C3-plants under conditions of low N-supply. Experiments with C4- and C3-grasses under natural conditions of a Central Euro-

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