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

Abstract

In many countries, high nitrous oxide (N₂O) emissions have been observed during soil freezing and thawing periods. Quantification of those emissions is crucial to evaluate annual N₂O emissions. For this study, we measured N₂O and nitric oxide (NO) fluxes along with soil N₂O 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, N₂O 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 N₂O concentration in the soils decreased. N₂O emissions were much higher during the thawing period than during the freezing and transition periods, and remarkably higher N₂O 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 N₂O emissions at all the sites. N₂O-N/NO-N ratio exceeded one at all the sites during the entire period, indicating N₂O production through denitrification. At the corn site, denitrification activity was much lower and N₂O/(N₂O+N₂) was much higher than at the grasslands. The result indicated that high N₂O emissions at the corn site were caused by complementary processes: (1) high accumulated N₂O through denitrification in the frozen soil during the freezing and transition periods, and (2) low N₂O reduction rate during the thawing period.

Keywords:

• corn field
• denitrification activity
• freezing and thawing periods
• grassland
• nitrous oxide.

ACKNOWLEDGMENTS

We thank Dr. H. Hata and the staff of the Shizunai Experimental Livestock Farm, Field Science Center, Hokkaido University, for their assistance. We thank Dr. M. Shimizu of the Soil Science Laboratory and Miss Y. Usui of the New Energy and Industrial Technology Development Organization for their assistance with our sampling. We also thank Dr. Y. Yanai of the National Agriculture and Food Research Organization (NARO), National Institute of Vegetable and Tea Science (NIVTS), for his helpful comments.

This study was partially supported by a special research grant provided by the project entitled “Establishment of good practices to mitigate greenhouse gas emissions from Japanese grasslands” funded by the Japan Racing Horse Association.