Environmental Engineering Reference
In-Depth Information
Estimates of heterotrophic mass are much more uncertain, mainly because of
highly variable densities of microbial biomass in soils and the poorly known pres-
ence of these prokaryotic organisms in extreme environments, particularly in the
uppermost region of the Earth's crust. Additional uncertainties are caused by the
enormous variety, variability, and mobility of arthropods. Whitman et al. (1998)
estimated that about 26 Gt C are stored in soil bacteria, but the real value may be
anywhere between 15 and 50 Gt C. And the uncertainty is even greater as far as
the mass of subterranean prokaryotes is concerned.
If we assume, as Gold (1992) did, that microbes i ll 1% of all porous spaces
in the topmost 5 km of the Earth's crust, the prokaryotic biomass could be as
much as 200 Tt C, but if the microbes i lled just 0.016% of the available porous
space (Whitman et al. 1998), then they would add just 200 Gt C—and the real
total may be an order of magnitude smaller. Bacteria in the subsea sediments may
hold another 200-300 Gt C (Parkes et al. 1994). Although prokaryotes provide
the trophic foundation of all metazoan life, most of their harvests have been always
only incidental, as they are present on every surface and inside all phytomass and
zoomass tissues. The only notable exception has been the deliberate collection of
cyanobacteria—particularly those belonging to the genera
Nostoc, Spirulina
, and
Aphanizomenon
—for food in parts of Asia and Africa (Ramírez-Moreno and
Olvera-Ramírez 2006).
Fungal biomass in soils also ranges widely, from 10
0
to 10
2
g/m
2
(Bowen 1966;
Reagan and Waide 1996), but in comparison to high bacterial counts its global
presence is negligible, adding up most likely to only between 2 and 5 Gt C—and
the biomass of metazoa (multicellular animals) is even more insignii cant. As already
shown, it is dominated by invertebrates, with earthworms and termites (inhabiting
soils in nearly 70% of all nonglaciated land) being the two largest contributors.
Even a conservative mean of 5 g/m
2
in all agricultural and forest soils would yield
nearly 500 Mt of earthworms, or about 100 Mt of dry weight and less than
50 Mt C.
Assuming that the global termite estimate of 2.4
10
17
individuals offered by
Zimmerman et al. (1982) is of the right order of magnitude, then multiplying it by
an average body mass of 2 mg/worker yields a total live weight of 480 Mt, a dry
weight of just over 100 Mt, and roughly 50 Mt C. Estimates of total insect biomass
are highly uncertain: after all, we do not even know the correct magnitude of the
total number of insect species. Erwin's (1982) often cited estimates of 30-100
million species are almost certainly exaggerated given that the latest attempt to
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