N_2O and NO fluxes from cornfield, grassland, pasture, and forest in a watershed in Southern Hokkaido, Japan

To develop an advanced method for estimating nitrous oxide (N_2O) emission from an agricultural watershed, we used a closed-chamber technique to measure seasonal N_2O 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, N_2O and NO fluxes ranged from -137 to 8,920 µg N m h and from -12.1 to 185 µg N m h, respectively. Most N_2O/NO ratios calculated on the basis of these N_2O and NO fluxes ranged between 1 and 100, and the log-normal N_2O/NO ratio was positively correlated with the log-normal N_2O fluxes (r^2 = 0.346, P year (average = 6.6 kg N ha). 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 N_2O flux would occur was weakly and positively correlated with precipitation from May to August. This probability can be used to represent annual variation in N_2O emission rates and to reduce the uncertainty in N_2O estimation