N2O emissions during the freezing and thawing periods from six fields in a livestock farm, southern Hokkaido, Japan

In many countries, high nitrous oxide (N ₂O) emissions have been observed during soil freezing and thawing periods. Quantification of those emissions is crucial to evaluate annual N ₂O emissions. For this study, we measured N ₂O and nitric oxide (NO) fluxes along with soil N ₂O concentrations at a corn (Zea mays L.) field and five grasslands (Phalaris arundinacea L. and Phleum pratense L.) during a winter-spring period in southern Hokkaido, Japan. We also measured denitrification activities of the soils from those sites. During the observation period, the soils froze to a maximum depth of 370 mm under saturated conditions and the lowest soil temperature at a 50 mm depth was −4.5°C. After 6 March 2005, daily air temperature rose above 0°C, but the soil temperature remained approximately 0°C for about two weeks. These two weeks were defined as the “transition period,” while the periods before and after the transition period were defined as the “freezing” and the “thawing” periods, respectively. During the freezing and transition periods, N ₂O concentration increased in the frozen soils relative to the unfrozen soils, and the highest values were observed in the frozen soils during the transition period. During the thawing period, the N ₂O concentration in the soils decreased. N ₂O emissions were much higher during the thawing period than during the freezing and transition periods, and remarkably higher N ₂O emissions were observed at the corn site compared to those at the grassland sites. NO emissions were also observed during the thawing period but at much lower levels than N ₂O emissions at all the sites. N ₂O-N/NO-N ratio exceeded one at all the sites during the entire period, indicating N ₂O production through denitrification. At the corn site, denitrification activity was much lower and N ₂O/(N ₂O+N ₂) was much higher than at the grasslands. The result indicated that high N ₂O emissions at the corn site were caused by complementary processes: (1) high accumulated N ₂O through denitrification in the frozen soil during the freezing and transition periods, and (2) low N ₂O reduction rate during the thawing period.