Rates and intensity of freeze–thaw cycles affect nitrous oxide and carbon dioxide emissions from agricultural soils

In cool temperate regions, large emissions of nitrous oxide (N2O), an important greenhouse and ozone-depleting gas, have been observed during freeze–thaw (FT) cycles. However, it is unclear how freezing and thawing rates, freezing intensity, and freezing duration influence N2O emissions. We used a laboratory incubation to measure N2O emissions from two soils (sandy loam, silty clay) undergoing a single FT cycle of various freezing and thawing rates [rapid (0.5 °C h-1) vs. slow (0.017 °C h-1)], freezing intensity (-1 vs. -3 °C), and freezing duration (24 vs. 48 freezing degree-days). In general, soil carbon dioxide fluxes during freezing were highest when soils were frozen slowly at -1 °C, whereas fluxes after thawing were highest from the soils frozen and thawed rapidly at -3 °C. Soil N2O emissions during both the freezing and thawing periods were greatest in the soils exposed to rapid freezing to -3 °C, intermediate under rapid freezing to -1 °C and slow freezing to -3 °C, and smallest under slow freezing to -1 °C and the control treatment (constant +1 °C). The similar N2O emissions between the unfrozen control and the slowly frozen -1 °C treatment was unexpected as previous field studies with similar freezing rates and temperatures still experienced high N2O emissions during thaw. This suggests that the physical disruptions caused by freezing and thawing of the surface soil are not the primary driver of FT-induced N2O emissions under field conditions.