72
WALLACE H. FULLER
2. Numbers
The numbers of microorganisms in desert soils appear to be closely associated with the abundance of carbonaceous food material available for
degradation and synthesis. Although Killian (1940) suggests nitrogen also
is an important factor in the abundance of soil microorganisms, Kononova
(1961) presents data that may be interpreted as supporting the belief that
soil nitrogen is not closely related to numbers. The number of microorganisms per unit weight of soil nitrogen is about 20 times higher in arid
Central Asian soils than in humid podzol soils. In general, the numbers
of microorganisms per unit of soil nitrogen increased markedly as the climate changed from the humid, cold soils to the arid, warm soils. The absolute counts, however, indicate that the microorganisms increase on passing
from peat and forest soils to prairie or grassland soils and then decrease
somewhat in desert soils. The greatest decrease in populations takes place
between the desert soils and "soil-less", extremely arid deserts. Also in
passing from the northern forest soils to the southern desert soils the population of microorganisms becomes more diverse and the mobilization of
nitrogen reserves becomes more rapid, reaching a climax in irrigated desert
soils (Kononova, 1961).
3. Activity
A review of the Russian literature (Rozanov, 1951) provides an overwhelming amount of evidence to the effect that the desert zone of Central
Asia is rich in bacterial flora and microfauna. The high overall population
of microorganisms in this desert area permits these soils to be classified
as "rich" despite the low amount of humus. All of the usual physiological
groups of microorganisms are present. Even the very dry and sandy takyr
soils of this region are reported to be well supplied with microorganisms
(Samsonov et al, 1930). In a more recent study (Volobuev, 1963), however, desert soils of the central arid sierozem are depicted as having less
active microbial populations and less nitrification activity than are reported
by the earlier Russian studies.
4. Effect of Vegetation
The influence of plants on microbial populations is shown in soils of
the Sahara by the discovery that greater numbers of bacteria are associated
with the rhizosphere than in root-free soil (Vargues, 1963). In an Egyptian desert soil, Azotobacter and Clostridium (free-living nitrogen fixers)
were isolated in soil near plant roots but not in soil away from the roots.
Lobova (1960), Krasil'nikov (1958), Clark (1949), and others agree
that intense microbiological activity is dependent upon the presence of
WALLACE H. FULLER
2. Numbers
The numbers of microorganisms in desert soils appear to be closely associated with the abundance of carbonaceous food material available for
degradation and synthesis. Although Killian (1940) suggests nitrogen also
is an important factor in the abundance of soil microorganisms, Kononova
(1961) presents data that may be interpreted as supporting the belief that
soil nitrogen is not closely related to numbers. The number of microorganisms per unit weight of soil nitrogen is about 20 times higher in arid
Central Asian soils than in humid podzol soils. In general, the numbers
of microorganisms per unit of soil nitrogen increased markedly as the climate changed from the humid, cold soils to the arid, warm soils. The absolute counts, however, indicate that the microorganisms increase on passing
from peat and forest soils to prairie or grassland soils and then decrease
somewhat in desert soils. The greatest decrease in populations takes place
between the desert soils and "soil-less", extremely arid deserts. Also in
passing from the northern forest soils to the southern desert soils the population of microorganisms becomes more diverse and the mobilization of
nitrogen reserves becomes more rapid, reaching a climax in irrigated desert
soils (Kononova, 1961).
3. Activity
A review of the Russian literature (Rozanov, 1951) provides an overwhelming amount of evidence to the effect that the desert zone of Central
Asia is rich in bacterial flora and microfauna. The high overall population
of microorganisms in this desert area permits these soils to be classified
as "rich" despite the low amount of humus. All of the usual physiological
groups of microorganisms are present. Even the very dry and sandy takyr
soils of this region are reported to be well supplied with microorganisms
(Samsonov et al, 1930). In a more recent study (Volobuev, 1963), however, desert soils of the central arid sierozem are depicted as having less
active microbial populations and less nitrification activity than are reported
by the earlier Russian studies.
4. Effect of Vegetation
The influence of plants on microbial populations is shown in soils of
the Sahara by the discovery that greater numbers of bacteria are associated
with the rhizosphere than in root-free soil (Vargues, 1963). In an Egyptian desert soil, Azotobacter and Clostridium (free-living nitrogen fixers)
were isolated in soil near plant roots but not in soil away from the roots.
Lobova (1960), Krasil'nikov (1958), Clark (1949), and others agree
that intense microbiological activity is dependent upon the presence of