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Soil & Water Management & Conservation

New Method to Simulate Soil Freezing and Thawing Cycles for Studying Nitrous Oxide Flux

^a Dep. of Land Resource Science, Univ. of Guelph, Ontario, Canada N1G 2W1
^b Soils and Crops Research and Development Center, Agriculture and Agri-Food Canada

^* Corresponding author (cwagnerr{at}uoguelph.ca)

Field experiments are essential for elucidating the processes involved in N₂O production during soil freezing/thawing (FT) cycles, but laboratory simulations can provide the advantage of controlled conditions. Past studies have used disturbed or undisturbed soil cores placed in a controlled temperature environment, so that FT occurred from the external surface to the inside of the core (omni-directional method, OD). A new method of soil FT for simulating field conditions more closely in the laboratory was developed and its effect on N₂O fluxes evaluated. Three methods were examined: OD, uni-directional (UD) with the soil column surrounded by filled-in soil, and a variant of UD with water accessible to the base of the core (UDW). The rate of soil cooling with time was significantly faster and similar at all depths (–0.29°C h^–1) for OD, than for UD or UDW methods (–0.16°C h^–1 at 1 cm, –0.10°C h^–1 at 20 cm). This differential cooling resulted in a significant change in soil temperature with depth for UD and UDW methods 0.125°C cm^–1 during freezing and –0.35°C cm^–1 during thawing, but of only 0.04 and –0.02°C cm^–1 for the OD method. Comparison with field temperature data indicated that the UD and UDW method more closely resembled the gradual top-to-bottom freezing of soil layers that occurs in field conditions (changes of –0.20°C h^–1 at 1 cm; –0.05°C h^–1 at 20 cm). Gravimetric water content in the frozen layer 0 to 20 mm in the UDW (60.6 g kg^–1) was significantly higher than in UD (44.4 g kg^–1) and OD (37.6 g kg^–1) soil columns. Fluxes of N₂O during thawing were significantly affected by the incubation method used, probably due to the intensity and duration of freezing, and the water content prevalent under each method with OD (13 ng N₂O-N m^–2 s^–1) > UDW (4.8 ng N₂O-N m^–2 s^–1) > UD (2.1 ng N₂O-N m^–2 s^–1). We conclude that the UD (and UDW) method allows for manipulation of FT in soil columns gradually layer-by-layer providing the conditions needed to link the site of N₂O production in the soil profile with surface N₂O fluxes in laboratory studies.

Abbreviations: FT, freezing and thawing • OD, Omni-directional freezing and thawing • UD, Uni-directional freezing and thawing with the soil column surrounded by filled-in soil • UDW: uni-directional freezing and thawing with filled-in soil and water accessible to the base of the core