Freeze-thaw cycles release nitrous oxide produced in frozen agricultural soils

Applying manure to temperate agricultural soils in the fall season is often justified by the assumption that mineral nitrogen (N) is stable in frozen soils, although pulses of nitrous oxide (N2O) are emitted when the soil thaws during winter months. Nitrous oxide loss was monitored during three freeze-thaw cycles in agricultural soils that received manure and had a growing cover crop before they were frozen. Soil was mixed with N fertilizer treatments (none, liquid dairy manure, solid dairy manure, or urea) and packed in 0.2-L pots, half of which were planted with an annual ryegrass (Lolium multiflorum Lam.) cover crop. After 3 weeks, pots were transferred to a freezer at − 4 °C, or left in a refrigerator at + 4 °C. Frozen pots were thawed at + 4 °C. Production of N2O was measured after 0, 3, 6, and 9 h of thawing; then the pots were destructively sampled to determine the soil mineral N concentration. The N fertilizer and cover crop treatments did not affect N2O production, and only 14% of the variation in N2O production was explained by soil mineral N concentration. However, there was a 6–9-fold increase in N2O production, relative to soil mineral N, in pots that underwent freeze-thaw cycles compared to pots that were left at + 4 °C. It appears that N2O was produced in frozen soils at − 4 °C, trapped under ice, and subsequently released when the soils thawed at + 4 °C, suggesting that N2O-producing reactions do not stop when manured soils are frozen.