If the law of matter (mass) conservation is used for chemical compounds that can be transformed into other chemical compounds, Eq. (2.1) must be changed to dci
V "d" = input — output + formation — transformation, (6.1)
where V is a volume and ct is a concentration of ith substance.
This principle is widely used in the class of ecological models called biogeochemical models. The equation is set up for the relevant elements, e.g. for eutrophication models for C, P, N and perhaps Si (see, for instance, J0rgensen, 1976, 1992a; J0rgensen and Mejer, 1977).
For terrestrial ecosystems, mass per unit of area is often applied in the mass conservation equation, so that dm
A = input — output + formation — transformation, (6.2)
where A is an area and m is a mass per unit of area.
The transformation of solar energy to chemical energy by plants conforms with the First Law (see also Fig. 2.3), which is represented here as
Solar energy assimilated by plants
= chemical energy of plant tissue growth + heat energy of respiration. (6.3)
For the next level in the trophic chains, the herbivorous animals, the energy balance can also be set up:
where F is the food intake converted to energy (J), As is the energy assimilated by the animals, UD is the undigested food or the chemical energy of faeces, Gr is the chemical energy of animal growth and Re is the heat energy of respiration. These considerations
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