Yosuke YANAI, Koki TOYOTA, Masanori OKAZAKI
(TUAT-BASE, 2002)
Seasonal soil freeze-thaw has occurred in
not only cold climatic areas but also in
temperate areas. We can see soil freeze-thaw
from late autumn to early spring in some
regions in Japan. Since soil freeze-thaw
means phase transition of water held in soil
aggregates, the studies on freeze-thaw phenomena
in soil have been conducted from physical
approaches. Also, the visible effects of
soil freeze-thaw on our lives, such as frost
heaving and damage to buildings or road foundations,
have received much attention. However, few
attention have been paid to soil microbial
community associated with soil freeze-thaw,
due to a lack of the observation of soil
microbial community within it.
In recent years, nitrous oxide emission from
thawed soils has been reported in Europe
and North America. Although chemodenitrification
of soil was recognized as a reason of nitrous
oxide burst from thawed soil, main reason
is soil microbial activity in frozen and
then thawed soil, which was shown by Rover
et al. in 1998. However, the reason why such phenomena
have been reported may be regarded nitrous
oxide as greenhouse gas and also ozone layer
disruption gas. Thus, the notices of these
observations were related with behavior of
nitrous oxide, not soil microbial community.
Soil microbial community is the transformer
and/or reservoir of nutrient to the plants,
furthermore, contributor of global nutrient
cycle. It is required that soil microbial
community would have the resilience both
qualitatively and quantitatively in some
extent, to keep soil healthy. Therefore,
this study focused on the behavior of soil
microbial community in frozen and then thawed
soil. The results of this should give an
insight into microbial community dynamics
under subzero temperature in soils.
Thus, the purpose of this study was to investigate
the effect of soil freeze-thaw on soil microbial
community both qualitatively and quantitatively
by conducting laboratory experiment. Fresh
soil samples taken from surface of arable
land were treated 4 times of temperature
cycles (at |13 for 11 hours and at 4 for 1 hour) using a freezer. Unfrozen control
samples were kept at 4 for 48 hours. After these treatments, determinations
of soil microbial community were conducted.
Quantitatively, the soil microbial biomass
carbon (SMBC) and nitrogen (SMBN) were determined by chloroform fumigation
and extraction method, and number of autotrophic
nitrifers (ammonium oxidizing bacteria and
nitrite oxidizing bacteria) were counted
by the most probable number method. In addition,
as qualitative (functional) aspects, 31 kinds
of sole-carbon sourcesf utilization were
determined using Biolog Eco Plate (Biolog
Inc.). Moreover, nitrification and carbon
dioxide emission from soil, as potential
ammonium oxidation by flask incubation and
as the amount of organic matter decomposition
in soil by vial incubation, were determined,
respectively.
11.3% of SMBC and 17.8% of SMBN decreased due to soil freeze-thaw. This
result suggested small portions of total
soil microbes suffer lethal effect from soil
freeze-thaw. Biolog Eco Plate showed the
change of sole-carbon sourcesf utilization
pattern, decrease of average utilization
of each sole-carbon substrate, and/or decrease
of calculated Shanon Hf as estimation of
metabolic diversity, caused by soil freeze-thaw.
This result suggested heterotrophic bacteria
would be damaged by soil freeze-thaw. On
the other hand, the number of autotrophic
nitrifers did not decrease significantly.
Also, soil freeze-thaw did neither inhibit
nor promote on nitrification. These results
suggested autotrophic nitrifers would have
high tolerance to soil freeze-thaw. However,
carbon dioxide emission rate from soil increased
significantly at 1 day (62.7%) and 3 days
(34.5%) after soil thawing. This result suggested
that components of killed microbial cells
by soil freeze-thaw act as substrates to
surviving microbes after soil freeze-thaw,
and then these substrates should be metabolized
quickly.
In conclusion, small lethal effect to soil
microbes induced by soil freeze-thaw would
exhibit some kinds of selectivity. These
disrupt effects would be related to soil
physical and chemical properties, dominant
species of soil microbes, and their habitat
at soil microsites, such as sorbed on soil
matrix or liberated in soil solution. Thus,
comparisons of effects on soil microbial
community at variety of soil types, such
as based on land use and with or without
soil freeze-thaw history in natural condition,
under a certain soil freeze-thaw regimen
may need. By doing this, we could get some
information on the selectivity of disrupt
effects to soil microbial community by soil
freeze-thaw and link to evaluate the soil
quality (resistance and resilience of soils)
from those information. Furthermore, there
may be possibility to predict the dynamics
of soil microbial community at the time when
soil freeze-thaw history would change due
to the global climatic changes in the future.