where n (3.44) is the efficiency at low incident radiation, when light is the only limiting factor. In (3.47), temperature requirements are supposed to be satisfied, and the water and nutrient supply are supposed to be adequate.

The state of the photosynthetic system within the plant may give rise to different efficiencies nchem under different conditions. Basically, this can be understood in terms of competing pathways for the chain of molecular reactions involved. In the electron transport chain, Fig. 3.71, the Q to plastoquinone electron transport in photosystem II may receive competition by fluorescent de-excitation of the Q-molecule. A similar de-excitation may happen for the X-protein in photosystem I. In both cases, the energy captured by the chlorophyll pigments will become lost from the point of view of biomass production. This may be summarised by stating that under conditions which are not optimal more than 8 light quanta are required for each CO2 molecule assimilated (the excess energy being lost as heat or radiation).

In aquatic environments, the rate of photosynthesis may depend on such factors as salinity and pH-value (acidity). In Sweden, it has been suggested that decreased productivity of coniferous forests is associated with increasing acidity of precipitation (and hence of soil) (Swedish Ministries of Foreign Affairs and of Agriculture, 1971).

Last, but not least, the role of nutrients as a limiting factor should be mentioned. Deprivation of nutrients decreases and eventually halts production. Transfer of plants to nutrient-free environments has shown effects of decreasing magnitude resulting from the deprivation of nitrogen, phosphorus and potassium in that order.

It follows from Figs. 2.88 and 2.89 that an average of 1.4 X 10-3 kg of nitrogen is fixed for each kg of carbon fixed in the terrestrial biosphere, in which there is about 0.0343 kg of N (kg of C)-1 (the standing crop biomass shows a different relationship between N and C, owing to different turnover times). The amount of nitrogen with which the soil must be enriched in order to increase productivity, if N is a limiting factor, may be much larger, owing to the nature of the pathways of uptake. Delwicke (1970) quotes experiments in which 0.76 kg of N was needed in order to increase production by 1 kg of dry matter.

In evaluating the net energy yield of cultivated land or sea, non-solar energy subsidies must also be considered. These may be in the form of machinery and fuels for running the machinery (ploughing and harvesting tools). In present day practices of terrestrial agriculture, the largest energy subsidy in areas of intense farming is in the manufacture of fertilisers. In regions demanding irrigation, water pumping may expend substantial amounts of energy, whereas the amount of machinery employed typically is large only in regions of intense farming, so that the corresponding energy subsidy remains a fraction of that spent on fertiliser. If little machinery is used, more manpower is needed, combined with power provided by draught animals. Also, transport of harvested crops costs energy, particularly for remote fishing, and in the case of food crops, processing and packaging may in industrialised regions account for the largest part of the total energy subsidy. Food derived from animals also involves less efficient utilisation of primary energy, as discussed in connection with Fig. 3.73.

In the case of "energy crops", the size of energy subsidies in fertilisers may be of particular interest, in order to determine whether the extra primary production, i.e. solar energy utilisation, exceeds the energy subsidy. Using present-day manufacturing techniques, including the mining of phosphate rock, the energy needed to produce 1 kg of nitrogen in the form of fertiliser is about 108 J, the energy needed for 1 kg of phosphorus is about 1.4 x 107 J, and the energy for 1 kg of potassium is about 9 x 106 J (Steinhart and Steinhart, 1974; Blaxter, 1974). As an example of fertiliser usage in intense farming practice, the average use of fertiliser in Denmark in 1965-1966 amounted to 0.0114 kg of N per m2, 0.003 58 kg of P per m2 and 0.0109 kg of K per m2 (Danish Statistical Survey, 1968), which with the above conversion figures correspond to a total subsidy of 1.3 x 106 J m-2 y-1 (0.04 W m-2). The harvested yield averaged 0.5 kg of dry matter per m2 and year. For the most common crop (barley), the net primary production would be about twice the amount harvested or 2 x 107 J m-2 y-1, and the fertiliser energy subsidy would thus be 6.5%. By 1971, the fertiliser energy consumption had risen by 23%, but the harvest yield was unchanged per unit area (Danish Statistical Survey, 1972). This implies that, because of this farming practice, nutrients in a form suitable for uptake are being depleted from the soil in such a way that increasing amounts of fertiliser must be added in order to maintain a constant productivity.

A proposed scheme for "energy plantations" yielding biomass at the rate of about 15 x 107 J m-2 y-1 (e.g. based on sunflower) under (southern) US conditions anticipates the use of 4 x 106 J m-2 y-1 for fertilisers and 2 x 106 J m-2 y-1 for irrigation and other machinery, a total energy subsidy of 5% (Alich and Inman, 1976).

Productivity data

Data on productivity of different species under different climatic conditions are plentiful, but often hard to summarise because of the differences in techniques used and ways of representing the results. Most controlled experiments are performed under optimal conditions, and they do not reflect the average productivity of the particular plant and geographical region. On the other hand, statistical information for entire regions or countries does not usually convey the exact conditions of growth. In many cases, the productivity of a given crop depends on the history of previous uses of the land. For natural ecosystems there are other factors which make it difficult to compare data from different sources. The conditions are not always sufficiently well described by climatic zone and solar radiation data. For instance, the geology of the upper soil may vary over very short distances. Altogether, it is not reasonable to give more than broad ranges or limits inside which the productivity of a given plant in a given environment will normally lie, not excluding exceptional values outside the range given.

In order to partially eliminate the dependence on the amount of solar radiation, Fig. 3.78 indicates such ranges for the overall efficiency (3.45), representing the ratio between net primary production and total solar radiation. In this way, for example, the woods in different climatic regions (boreal coniferous, deciduous, temperate broad-leaved evergreen and subtropical or tropical rain forests) become represented by a fairly narrow range of efficiencies, despite the large variations in absolute productivity.

The range of efficiencies of natural plants goes from practically nothing to a little over 2%, the highest values being reached for tropical rain forests and algal cultures of coral reefs. Cultivated crops (terrestrial or marine) may reach some 4-5% under optimal conditions and nutrient subsidy. The optimum efficiency from the theoretical discussion of equations (3.44) and (3.45) is roughly n = n Vgeom Vchem Ires? * -0.5 X 0.8 X 0.4 X 0.6 - 0.1. (3.48)

Figure 3.78. Intervals of typical values of total photosynthetic efficiency for different plants and communities (solid lines), as well as intervals of possible improvement for subsidised and optimised cultures (dashed lines). For non-perennial plants, the efficiency is an average over the growing season (constructed on the basis of information from Kira, 1975; Caldwell, 1975; Loomis and Gerakis, 1975).

Figure 3.78. Intervals of typical values of total photosynthetic efficiency for different plants and communities (solid lines), as well as intervals of possible improvement for subsidised and optimised cultures (dashed lines). For non-perennial plants, the efficiency is an average over the growing season (constructed on the basis of information from Kira, 1975; Caldwell, 1975; Loomis and Gerakis, 1975).

Each of the conditions is difficult to achieve in practice, at least for extended periods. As mentioned earlier, mature ecosystems are characterised by diversity, which may not be reconcilable with maximum efficiency of primary production, and young ecosystems, such as non-perennial crops, need a growing period (cf. Fig. 3.76) before they reach full productivity, and thus their average efficiency on an area basis and over prolonged periods of time cannot be optimal.

Figure 3.79 gives a recent estimate of the geographical distribution of potential biomass production. These are net primary production data derived from the "Terrestrial Ecosystem Model (TEM)" of the Woods Hole group

(Melillo and Helfrich, 1998; Raich et al., 1991; Melillo et al., 1993; McGuire et al., 1997; Sellers et al., 1997). The assumption is that a mature ecosystem of natural vegetation has developed, and the model takes into account solar radiation, moisture, temperature, as well as access to water and nutrients. Not included here is global warming (increased CO2), which could induce increased primary production in a fairly complex pattern and change the borders of natural vegetation zones, sometimes by several hundred kilometres (IPCC, 1996b). Use of these biomass data for energy scenarios is illustrated in Chapter 6. Seasonal variations of actual biomass land coverage, as deduced from satellite-based sensors, are regularly posted on the Internet (NOAA, 1998).

Figure 3.79. Annual average energy content in W m 2 of potential net biomass production in mature ecosystems (based on Melillo and Helfrich, 1998).
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