Boike and Overduin: Seasonal Changes in HydrologY. Energv Balance and Chemistry
305
Soil water chemistry during freeze-back
At the drier site (2x), solute exclusion resulted in a general increase of soil water electrical
conductivity (sw) in the freezing soil solution. At depths with initially lower water contents
where freezing proceeded somewhat faster, solute concentration increases were higher than
those at depths with initially higher water contents (Boike et aI., 1998). Compared to s w
changes during the summer, variations in Sw as a result of phase change in the fall were an
order of magnitude greater (Boike et aI., 1997). In our study, however, soil water solute
concentration profiles provided no support for observable macroscale solute redistribution as a
result of solute exclusion. Dramatic changes in soil solution chemical composition, total solute
concentration and vertical distribution did occur in the soil profile as freezing progressed, but
were limited to species important in reduction and oxidation processes. Increases in the
concentration of Fe 2 + and Mn 2 + by a factor of five in the unfrozen portion of the active layer
occurred without simultaneous depletion in some other horizon, while species with higher
mobilities showed no change in concentration or distribution. The increase is thus clearly not
the result of redistributive transport processes, but of release of the ions into the soil solution.
The increases were large and rapid enough to require the agency of bacteria, although soil
temperatures throughout the soil profiles were less than I C. We suggest that the formation of a
frozen upper soil horizon has the potential to alter the reduction potential of the soil water. The
increasingly reducing nature of the soil, as it is separated from atmospheric influence by a
frozen layer, can then result in the release of reduced species at depth, particularly in carbonate
poor and reduction-oxidation couple rich soils (Overduin and Young, 1997).
Summary
TDR has been successfully applied to measure water content and bulk electrical conductivity in
the active layer and thus can be used as an in situ technique for studying the temporal dynamics
of soil water and solutes in the active layer at these sites. From energy and water balance
studies, it was found that most incident radiative energy during spring and summer was
consumed by sensible and latent heat fluxes into the atmosphere.
The heterogeneity of the active layer at a drier, well drained site had the effect of (i) routing
water along preferential flowpaths and (ii) allowing freezing to occur in cells, rather than
through the descent of a distinct freezing front. While freezing generally had the effect of
increasing the Sw of the unfrozen soil water, it did not result in redistribution of solutes
vertically within the soil profile. Soils which underwent freezing through the formation of a
surface frozen layer and a descending freezing front also underwent non-redistributive increases
in soil water solute concentration, perhaps as a result of changes in reduction potential.
Acknowledgments
We especially thank the members of the 1994 and 1995 expeditions from the Arctic and
Antarctic Research Institute, st. Petersburg who made the field work successful and unique.
Funds are provided by the German Ministry of Education, Science, Research and Technology
(BMBF Grant # 03PLO 14A). This is contribution no. 1354 of the Alfred-Wegener -Institute for
Polar and Marine Research, Potsdam and Bremerhaven, Germany.
305
Soil water chemistry during freeze-back
At the drier site (2x), solute exclusion resulted in a general increase of soil water electrical
conductivity (sw) in the freezing soil solution. At depths with initially lower water contents
where freezing proceeded somewhat faster, solute concentration increases were higher than
those at depths with initially higher water contents (Boike et aI., 1998). Compared to s w
changes during the summer, variations in Sw as a result of phase change in the fall were an
order of magnitude greater (Boike et aI., 1997). In our study, however, soil water solute
concentration profiles provided no support for observable macroscale solute redistribution as a
result of solute exclusion. Dramatic changes in soil solution chemical composition, total solute
concentration and vertical distribution did occur in the soil profile as freezing progressed, but
were limited to species important in reduction and oxidation processes. Increases in the
concentration of Fe 2 + and Mn 2 + by a factor of five in the unfrozen portion of the active layer
occurred without simultaneous depletion in some other horizon, while species with higher
mobilities showed no change in concentration or distribution. The increase is thus clearly not
the result of redistributive transport processes, but of release of the ions into the soil solution.
The increases were large and rapid enough to require the agency of bacteria, although soil
temperatures throughout the soil profiles were less than I C. We suggest that the formation of a
frozen upper soil horizon has the potential to alter the reduction potential of the soil water. The
increasingly reducing nature of the soil, as it is separated from atmospheric influence by a
frozen layer, can then result in the release of reduced species at depth, particularly in carbonate
poor and reduction-oxidation couple rich soils (Overduin and Young, 1997).
Summary
TDR has been successfully applied to measure water content and bulk electrical conductivity in
the active layer and thus can be used as an in situ technique for studying the temporal dynamics
of soil water and solutes in the active layer at these sites. From energy and water balance
studies, it was found that most incident radiative energy during spring and summer was
consumed by sensible and latent heat fluxes into the atmosphere.
The heterogeneity of the active layer at a drier, well drained site had the effect of (i) routing
water along preferential flowpaths and (ii) allowing freezing to occur in cells, rather than
through the descent of a distinct freezing front. While freezing generally had the effect of
increasing the Sw of the unfrozen soil water, it did not result in redistribution of solutes
vertically within the soil profile. Soils which underwent freezing through the formation of a
surface frozen layer and a descending freezing front also underwent non-redistributive increases
in soil water solute concentration, perhaps as a result of changes in reduction potential.
Acknowledgments
We especially thank the members of the 1994 and 1995 expeditions from the Arctic and
Antarctic Research Institute, st. Petersburg who made the field work successful and unique.
Funds are provided by the German Ministry of Education, Science, Research and Technology
(BMBF Grant # 03PLO 14A). This is contribution no. 1354 of the Alfred-Wegener -Institute for
Polar and Marine Research, Potsdam and Bremerhaven, Germany.