Fractionation of organic C, nutrients, metals and bacteria in peat porewater and ice after freezing and thawing

To better understand freezing - thawing cycles operating in peat soils of permafrost landscapes, we experimentally modelled bi-directional freezing and thawing of peat collected from a discontinuous permafrost zone in western Siberia. We measured translocation of microorganisms and changes in porewater chemistry (pH, UV absorbance, dissolved organic carbon (DOC), and major and trace element concentrations) after thawing and two-way freezing of the three sections of 90-cm-long peat core. We demonstrate that bi-directional freezing and thawing of a peat core is capable of strongly modifying the vertical pattern of bacteria, DOC, nutrients, and trace element concentrations. Sizeable enrichment (a factor of 2 to 5) of DOC, macro- (P, K, Ca) and micro-nutrients (Ni, Mn, Co, Rb, B), and some low-mobile trace elements in several horizons of ice and peat porewater after freeze/thaw experiment may stem from physical disintegration of peat particles, leaching of peat constituents, and opening of isolated (non-connected) pores during freezing front migration. However, due to the appearance of multiple maxima of element concentration after a freeze-thaw event, the use of peat ice chemical composition as environmental archive for paleo-reconstructions is unwarranted.

Automated summary

We experimentally simulated the freezing-thawing cycles in peat soils of permafrost landscapes and found that it could significantly alter the distribution of bacteria, dissolved organic carbon (DOC), nutrients, and trace elements in the peat core. The changes may be caused by the physical disintegration of peat particles, leaching of peat constituents, and opening of isolated pores during the freezing front migration. However, the presence of multiple concentration maxima after freeze-thaw events suggests that using the chemical composition of peat ice as an environmental archive for paleo-reconstructions is not reliable.