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


J. Plant Nutr. Soil Sci. 167:678-684 (2004)


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 (i) to quantify gross nitrification in a Luvisol during a freeze-thaw event, (ii) to analyse the dynamics of the emissions of N2O and N2, (iii) to quantify the contribution of nitrification and denitrification to the emission of N2O and (iv) to determine whether the length of freezing and of thawing affects the C availability for the denitrification. 15NO3- was added to undisturbed soil columns and the columns were subjected to seven days of freezing and five days of thawing. N2O emissions were determined in three-hour intervals and the concentrations of 15N2O and 15N2 were determined at different times during thawing. During the 12-day 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 immobilisation rate were 1.13, 0.05 and 0.48 mg NO3--N (kg soil)-1 day-1, respectively. Immediately after the beginning of the thawing, denitrification contributed by 83 % to the N2O production. The ratios of 15N2 to 15N2O during thawing were narrow and ranged from 1.5 to 0.6. For objective (iv), homogenized soil samples were incubated under anaerobic conditions after different periods of freezing and thawing. The different periods did not affect the amounts of N2 and N2O produced in the incubation experiments. Further, addition of labile substrates gave either increases in the amounts of N2O and N2 produced or no changes which suggested that changes in nutrient availability due to freezing and thawing are only small.

Key words: denitrification, freeze-thaw cycles, nitrification, nitrous oxide, isotopes, trace gas