The Effect of Freeze-thaw Events on Nitrous Oxide Emissions from Soil

Freeze-thaw events have been identified as an important source of N 2O emissions from agricultural soils. The timing and magnitude of these emissions relies on the availability or creation of anaerobic volumes and the availability of carbon and NO3- to denitrifiers. In this research, short-term temporal patterns in N2O emissions were measured by continuous measurement, using a trace gas analyzer, tunable diode laser and the relative importance of environmental conditions on N 2O emissions were determined. Nitrous oxide production increased after 0.5 to 1.5 d of thaw in soil with a water content greater than approximately 70 %WFPS. Increases in thaw temperature reduced the delay in N2O production, by possibly affecting O2 consumption rate and the activation energy of denitrifier enzymes. The influence of soil environmental conditions was determined through measurement of cumulative N2O from small replicated soil cores. Cumulative N2O during thaw was greatest from soil frozen below -5°C with a water content greater than 74 %WPFS. The duration of the freezing event did not influence N2O emissions during the thaw event despite increases in respiration. Thaw events conducted at warmer temperatures (25 °C) resulted in greater N2O emissions than those conducted at cooler temperatures (5 and 15 °C). This work underscores the importance of thaw temperature on the magnitude and timing of N2 O emissions and the inflation effect on N2O emissions when using high temperatures in freeze-thaw laboratory studies.