Contribution of nitrification and denitrification to the emission of N2O in a freeze-thaw event in an agricultural soil

The amounts of N2O released in freeze-thaw events depend on site and freezing conditions and contribute considerably to the annual N2O emissions. However, quantitative information on the N transformation rates in freeze-thaw events is scarce. Our objectives were (1) to quantify gross nitrification in a Luvisol during a freeze-thaw event, (2) to analyze the dynamics of the emissions of N2O and N-2, (3) to quantify the contribution of nitrification and denitrification to the emission of N2O, and (4) to determine whether the length of freezing and of thawing affects the C availability for the denitrification. (NO3-)-N-15 was added to undisturbed soil columns, and the columns were subjected to 7 d of freezing and 5 d of thawing. N2O emissions were determined in 3 h intervals, and the concentrations of (N2O)-N-15 and N-15(2) were determined at different times during thawing. During the 12 d experiment, 5.67 mg NO3--N (kg soil)(-1) was produced, and 2.67 mg NO3--N (kg soil)(-1) was lost. By assuming as a first approximation that production and loss occurred exclusively during thawing, the average nitrate-production rate, denitrification rate, and immobilization rate were 1.13, 0.05, and 0.48 mg NO3--N (kg soil)(-1) d(-1), respectively. Immediately after the beginning of the thawing, denitrification contributed by 83% to the N2O production. The ratios of N-15(2) to (N2O)-N-15 during thawing were narrow and ranged from 1.5 to 0.6. For objective (4), homogenized soil samples were incubated under anaerobic conditions after different periods of freezing and thawing. The different periods did not affect the amounts of N-2 and N2O produced in the incubation experiments. Further, addition of labile substrates gave either increases in the amounts of N2O and N-2 produced or no changes which suggested that changes in nutrient availability due to freezing and thawing are only small.