Additional Sources Of Primary Production

An additional contribution to net primary production comes from algal populations in the surface few millimeters or on the soil surface itself. By measuring CO2 fixation by cyanobacteria and algae on the surface of intact cores taken from an agroecosystem, Shimmel and Darley (1985) calculated that approximately 39 grams (g) of carbon were fixed per square meter per year. This is a small proportion (5%) of total NPP for the study site (the conventional-tillage agricultural system in Georgia), which averaged 800 g per square meter per year aboveground NPP. The type of organic matter and the amount that may feed directly into detritivorous fauna could be of importance beyond the total production figures on an annual basis. For example, cryptogamic crusts can be significant agents of nitrogen fixation, providing inputs of nitrogen and carbon in the ecosystems of nutrient-poor arid lands (Evans and Belnap, 1999; Belnap, 2002).

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