Bunnell 1980, Nadelhoffer et al. 1992, Gilichinsky
The resulting increases in acid concentrations of
1992, 1993). Flanagan and Bunnell (1980) conclud-
low-buffered waters occasionally lead to severe
ed that both bacteria and fungi are capable of
physiological stress on aquatic organisms. They
growth at subzero temperatures. Decomposition
hypothesized that these high concentrations may
be due to a freeze-concentration process during
rates increase with increasing temperature between
5 and 2030C, and overall decomposition rates
snow recrystallization and melting in which sol-
increase by 20% yr1 for every 1000 degree-days
utes preferentially accumulate at surfaces of ice
above 0C (Nadelhoffer et al. 1992).
particles. Clearly snowmelt is a major hydrologic
and geochemical event in regions subject to freeze
It is well known that freezethaw cycles cause a
thaw cycles.
respiratory burst of CO2 and CH4 following thaw-
Ivanov and Vlasov (1973) concluded that cryo-
ing (Mack 1963, Ivarson and Sowden 1970, Mc-
genic processes have the effect of forming waters
Cown et al. 1972, Skogland et al. 1988, Christensen
that have low mineral concentrations and low
1993). Skogland et al. (1988) attributed the respira-
Ca2+/Mg2+ ratios in the Transbaikal region of
tory burst to killing of bacteria, lysing of their cells
Siberia. Factors that influence low mineralization
and utilization of liberated carbon compounds as
are the freezing of soil solutions and groundwa-
nutrients by the surviving bacteria. Ivarson and
ters, the redistribution of salts between ice and liq-
colleagues found strong correlations between ex-
uid phases, the slow transition of salts into solution
tractable free amino acids and sugars (nutrients)
during thawing, and the precipitation of carbon-
and soil respiration, suggesting that microbial stim-
ulation by nutrients was the cause of the CO2 burst
ates of alkaline-earth metals.
upon thawing (Ivarson and Gupta 1967, Ivarson
and Sowden 1966, 1970). Ross (1972) found in-
DECOMPOSITION AND
creased dehydrogenase activity with thawed soil
NUTRIENT AVAILABILITY
samples in aerobic assays. He attributed this re-
sponse to increased availability of substrates rath-
How low temperatures and freezethaw cycles
er than multiplication of microorganisms because
alter the soil as a medium for microbial activity
in anaerobic assays with shorter incubation times,
and plant growth in cold regions soils is an impor-
the effects of increase substrate availability were
tant factor affecting revegetation, contaminant sta-
less evident. McCown et al. (1972) attributed a
bility and carbon cycling. The availability of
nutrients is critical for plant growth and, as a con-
spring CO2 burst to trapping of CO2 within the soil
sequence, revegetation of severely disturbed lands.
matrix upon bidirectional freezing, with subse-
Organic contaminants can be directly decomposed
quent release during spring warming; they be-
by microbial activity, while mineral contaminants
lieved this to be primarily a physical effect and not
(e.g., Hg, Pb, Cd) can be absorbed by plants, re-
biologically mediated. In the latter study, soil sur-
ducing their mobility (Lagerwerff 1972, Page et al.
face temperatures beneath a snowpack were in the
range of 16 to 10C; studies, previously cited,
1987). Carbon fluxes from northern ecosystems
may play a role in global carbon balance, affecting
the global climate (Kvenvolden 1993, Marion and
become important until soil temperatures rise
above 10C. Christensen (1993) attributed the
Oechel 1993, Oechel et al. 1993, Zimov et al. 1993).
Several chapters in a recent book edited by
spring burst of CH4 to the release of trapped CH4
Chapin et al. (1992) discuss arctic ecosystems and
in the frozen soil and possibly also to changes in
the production and consumption of CH4 as tem-
microbial processes (Nadelhoffer et al. 1992),
peratures increased and the active layer deepened.
nitrogen fixation (Chapin and Bledsoe 1992) and
The seasonal patterns of CO2 and CH4 emissions
plant nutrient absorption (Kielland and Chapin
are important in assessing the role of these gases
1992). In the present review, our primary focus will
in the global carbon balance. Most of the evidence
be on the effect of low temperatures and freeze
to date indicates that CH4 emissions in the winter
thaw cycles on decomposition and mineralization
are insignificant in tundra soils (Whalen and Ree-
processes. For fuller discussions of factors influ-
burgh 1992, Christensen 1993); however, evidence
encing microbial activity and plant growth in cold
to the contrary also exists (Panikov and Zelenev
ecosystems, see the Chapin et al. (1992) book.
1992). Kelley et al. (1968) suggested that increased
Temperature is the dominant factor controlling
decomposition rates (CO2 production), with mini-
CO2 concentrations at the soil surface beneath a
mal detectable rates occurring at 10 to 6C
snowpack at Barrow, Alaska, in the winter was due
(McCown et al. 1972, Flanagan 1978, Flanagan and
to microbial respiration as well as other physical
10
Previous Page
