+20C; they attributed this freezing effect to the
present in both oxidized (Mn4+, Mn3+, Fe3+) and
reduced (Mn2+, Fe2+) forms, with reduced forms
concentration of NO2 into the unfrozen soil water.
Goodroad and Keeney (1984) found that N2O con-
being more soluble in soils (Lindsay 1979). In gen-
centrations and fluxes during the spring thaw were
eral, soil moisture saturation promotes reduced
some of the highest values observed during the
forms and soil unsaturation promotes oxidized
entire season. They attributed these high N2O fluxes
forms. Cheng et al. (1971) found that increasing
to the physical release of N2O upon thawing and
soil moisture increased the availability of Mn and
to production by denitrification. Edwards and Kill-
Fe. Under water-saturated conditions, repeated
ham (1986) found that rates of both denitrification
freezethaw cycles increased exchangeable Mn and
and NH3 volatilization were increased by freeze
Fe, while repeated freezethaw cycles in unsatur-
thaw cycles, especially in the presence of soils pre-
ated soils decreased exchangeable Mn and Fe. The
viously fertilized with urea (NH 2CONH2).
result for unsaturated conditions agrees with the
Campbell et al. (1970) found large and sudden un-
general tendency for exchangeable K, Ca and Mg
explained decreases in exchangeable ammonium
previously cited. The increased Mn and Fe concen-
during the winter months following a steady fall
trations under saturated conditions suggests that
buildup. Although they were unable to explain this
freezethaw processes may promote reducing con-
phenomenon, possible explanations include en-
ditions in soil, perhaps by excluding O2 gas in the
hanced denitrification, NH3 volatilization and NH4
freezing process or because of slow diffusion of O2
fixation. These few papers indicate that freezethaw
in frozen soils. This hypothesis might also explain
cycles affect mineral nitrogen forms, which are the
the previously cited increased denitrification un-
forms utilizable by plants and also the forms that
der freezing conditions (Christianson and Cho
are most easily subject to gaseous losses (Fig. 5).
1983, Edwards and Killham 1986) and the increased
Hinman (1970) found that freezethaw cycles
enzymatic activity under reduced conditions
significantly increased the availability of extract-
(Linkins 1987). Little work has been done on the
able P in soils. On the other hand, Read and Cam-
effect of freezing and thawing on respiratory gas
eron (1979) reported little change in the extractable
(O2 and CO2) balance in soils. This is a critical fac-
P of soils from fall to spring over a ten-year period.
tor in controlling both plant and microbial activity
Exchangeable K, Ca and Mg typically either de-
in cold regions soils, with potential significance in
crease in concentration or are unaffected by freeze
understanding the global carbon balance.
thaw cycles (Hinman 1970, Cheng et al. 1971). A
decrease in exchangeable cation concentrations
SITE REMEDIATION
could be caused by the precipitation of soil miner-
als, which is promoted by soil freezing, causing a
There are a number of cases where chemical
shift in the equilibria (Fig. 4).
composition and freezethaw processes interact,
Potassium and NH4 are special cases among
affecting the remediation of problem soils. These
exchangeable ions because both are subject to fixa-
problem soils include saline soils, contaminated
soils and frost-susceptible soils. The effect of soil
tion reactions (Fig. 5). Fixation is caused by the col-
salts in reducing frost susceptibility was discussed
lapse of layer-lattice clay minerals, which renders
previously. Here, our focus will be on natural and
ions in the interlayers slowly exchangeable. Fine et
artificial ground freezing as means for desalinat-
al. (1940) reported net releases of K subjected to
ing and decontaminating soils.
freezethaw cycles in 75% of examined soils and a
Kizilova (1959) examined the mobility and solu-
fixation in the remaining 25% of soils. Fine et al.
bility of Na+ and SO2 ions under winter, but gen-
(1940) also found that most clay minerals (bento-
4
erally unfrozen, leaching conditions. The objective
nite, nontronite, Putnam clay, montmorillonite)
of winter leaching was to remove Na2SO4 from the
released nonexchangeable K when subjected to
soil profile; concern existed that Na2SO4 . 10H2O
freezethaw cycles. An exception was illite, a non-
expanding layer-lattice clay mineral, which fixed
(mirabilite) might precipitate at low temperatures.
K. Graham and Lopez (1969) found that severely
However, mirabilite did not precipitate even at
K-depleted soils will release K from fixed positions
subfreezing temperatures, and winter leaching was
effective in removing Na+ and SO2 ions from
when subjected to freezethaw cycles. On the oth-
4
er hand, when K was added to soils, repeated freez-
these saline soils. Cary and Mayland (1972) exam-
ing and thawing led to K fixation.
ined a similar, but hypothetical, case. They point-
Manganese and Fe are also special cases among
ed out that the lower solubility of CaSO4 and
exchangeable ions because these ions may be
MgSO4 relative to Na2SO4 could cause the former
13
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