Many studies show that higher concentrations of C02 than normally present in air will promote more carbon fixation and increase biomass yields. In confined environmentally controlled enclosures such as hothouses, carbon dioxide-enriched air is often used to stimulate growth. In large-scale, open — air systems such as those envisaged for biomass energy farms, this is not practical. For aquatic biomass production, C02 enrichment of the water phase may be a potentially attractive method of promoting biomass growth if C02 concentration is a limiting factor, since biomass growth often occurs by uptake of C02 from both the air and liquid phase near the surface.

For some high-growth-rate biomass species, the C02 concentration in the air among the leaves of the plant is often considerably less than that in the surround­ing atmosphere. Photosynthesis may be limited by the C02 concentrations under these conditions when wind velocities are low and insolation is high.

Q I 1 J I I I

20 25 30 35 40 4 5 50 55 60

TEMPERATURE, °С

FIGURE 4.8 Effect of temperature on net photosynthesis for sorghum and cotton leaves.

B. Nutrients

All living biomass requires nutrients other than carbon, hydrogen, and oxygen to synthesize cellular material. Major nutrients are nitrogen, phosphorus, and

potassium; other nutrients required in lesser amounts are sulfur, sodium, magnesium, calcium, iron, manganese, cobalt, copper, zinc, and molybdenum. The last five nutrients, as well as a few others not listed, are sometimes referred to as micronutrients because only trace quantities are needed to stimulate growth. For terrestrial biomass, all of these elements are usually supplied by the soil, so eventually the soil’s natural nutrients are depleted if they are not replaced through fertilization. Some biomass species, such as the legumes, are able to meet all or part of their nitrogen requirements through fixation of ambient nitrogen. Marine biomass such as giant brown kelp use the natural nutrients in ocean waters. Freshwater biomass such as water hyacinth is often grown in water rich in nutrients such as municipal wastewaters. The growth of the plant is stimulated and at the same time, the influent wastewater is stabilized because its components are taken up by the plant as nutrients. So — called luxuriant growth of water hyacinth on biosolids, in which more than the needed nutrients are removed from the wastewater, can be used as a substitute wastewater treatment method.

Whole plants typically contain 2 wt % N, 1 wt % K, and 0.5 wt % P, so at a yield of 20 t/ha-year, harvesting of the whole plant without return of any of the plant parts to the soil corresponds to the annual removal of 400 kg N, 200 kg K, and 100 kg P per hectare. This illustrates the importance of fertiliza­tion, especially with these macronutrients, to maintain soil fertility. Indeed, biomass growth is often nutrient-limited and yield correlates with increased dose rates. An example is U. S. corn production from 1945 to 1970. Nitrogen fertilizer applications were increased from 8 kg/ha to 125 kg/ha over this period; the corresponding edible corn yields increased from 2132 to 5080 kg/ ha (Krummel, 1976). Average nitrogen fertilizer applications for produc­tion of wheat, rice, potato, and brussels sprouts are about 73, 134, 148, and 180 kg/ha, respectively, in the United States (Krummel, 1976). Much of the success of the “green revolution” is claimed to be the result of greater fertilizer applications. Estimates of balanced fertilizers needed to produce various land biomass species are shown in Table 4.5 (Roller et al, 1975). Note that alfalfa does not require added nitrogen because of its nitrogen-fixing ability. It is estimated that this legume can fix from about 130 to 600 kg of elemental nitrogen per hectare annually (Evans and Barber, 1977).

Normal weathering processes that occur in nutritious soils release nutrients, but they are often not available at rates that promote maximum biomass yields. Fertilization is usually necessary to maximize yields. Since nitrogenous fertilizers are currently manufactured from fossil fuels, mainly natural gas, and since fertilizer needs are usually the most energy intensive of all the inputs in a biomass production system, a careful analysis of the integrated biomass production-conversion system is necessary to ensure that net energy produc­tion is positive. Trade-offs between synfuel outputs, nonsolar energy inputs,

and biomass yields are required to operate a system that produces only en­ergy products.