Nitrous oxide and nitric oxide fluxes from cornfield, grassland, pasture and forest in a watershed in Southern Hokkaido, Japan

Abstract

To develop an advanced method for estimating nitrous oxide (N₂O) emission from an agricultural watershed, we used a closed-chamber technique to measure seasonal N₂O and nitric oxide (NO) fluxes in cornfields, grassland, pastures and forests at the Shizunai Experimental Livestock Farm (467 ha) in southern Hokkaido, Japan. From 2000 to 2004, N₂O and NO fluxes ranged from –137 to 8,920 µg N m⁻² h⁻¹ and from –12.1 to 185 µg N m⁻² h⁻¹, respectively. Most N₂O/NO ratios calculated on the basis of these N₂O and NO fluxes ranged between 1 and 100, and the log-normal N₂O/NO ratio was positively correlated with the log-normal N₂O fluxes (r ² = 0.346, P < 0.01). These high N₂O fluxes, therefore, resulted from increased denitrification activity. Annual N₂O emission rates ranged from –1.0 to 81 kg N ha⁻¹ year⁻¹ (average = 6.6 kg N ha⁻¹). As these emission values varied greatly and included extremely high values, we divided them into two groups: normal values (i.e. values lower than the overall average) and high values (i.e. values higher than average). The normal data were significantly positively correlated with N input (r ² = 0.61, P < 0.01) and the “higher” data from ungrazed fields were significantly positively correlated with N surplus (r ² = 0.96, P < 0.05). The calculated probability that a high N₂O flux would occur was weakly and positively correlated with precipitation from May to August. This probability can be used to represent annual variation in N₂O emission rates and to reduce the uncertainty in N₂O estimation.

Key words:

• nitrous oxide
• nitric oxide
• outliers
• uncertainty
• watershed

ACKNOWLEDGMENTS

We thank Dr H. Hata and the staff of the Shizunai Experimental Livestock Farm, Field Science Center, Hokkaido University, for their assistance. We also thank Miss M. Takemoto of Oyo Corporation, Miss M. Shimizu of the Soil Science Laboratory, Mr S. Okada of Asahi Breweries and Miss Y. Usui of the New Energy and Industrial Technology Development Organization for their assistance with our sampling. This paper was presented at the International Workshop on Monsoon Asia Agricultural Greenhouse Gas Emissions, 7–9 March 2006, Tsukuba, Japan. This study was supported in part by a Japanese Grant-in-Aid for Science Research from the Ministry of Education, Culture, Sports, Science and Technology (No. 11460028).