Impact of Freezing and Thawing on Soil Oxygen Dynamics and Nutrient Fluxes

Abstract:

Freeze-thaw cycles (FTCs) influence the physical properties, microbial activity, biogeochemistry, nutrient and carbon cycling in soils, and regulate subsurface oxygen (O₂) availability, affecting greenhouse gas exchanges between soils and the atmosphere. The ability to monitor changes in O₂ levels, which are indicative of aerobic and anaerobic conditions, is key to understanding how changes in the frequency and amplitude of freeze-thaw cycles affect a soil’s geochemical conditions and microbial activity. In this study, a highly instrumented soil column experiment was designed to accurately simulate freeze-thaw dynamics under controlled conditions. This design allowed us to reproduce realistic, time- and depth-dependent temperature gradients in the soil column. Continuous O₂ levels throughout the soil column were monitored using high-resolution, luminescence-based, Multi Fiber Optode (MuFO) microsensors. Image-processing techniques were used to convert light intensity of high-resolution digital images of the sensor-emitted light into O₂ concentrations. Water samples from various depths in the column were collected to monitor pore water composition changes. Headspace gas measurements were used to derive the effluxes of CO₂ and CH₄ during the experiment. The results indicate that the pulse of oxygen introduced by thawing caused partial and temporal oxidation of previously reduced sulfur and nitrogen species, leading to concomitant changes in pore water SO₄^2- and NO₃^- concentrations. Pulsed CO₂ emission to the headspace was observed at the onset of thawing, indicating that a physical ice barrier had formed during frozen conditions and prevented gas exchange between the soil and atmosphere. CO₂ emission was due to a combination of the physical release of gases dissolved in pore water and entrapped below the frozen zone and changing microbial respiration in response to electron acceptor variability (O₂, NO₃^-, SO₄^2-).