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

To develop an advanced method for estimating nitrous oxide (N(2)O) emission from an agricultural watershed, we used a closed-chamber technique to measure seasonal N(2)O 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(2)O and NO fluxes ranged from -137 to 8,920 mu g N m(-2) h(-1) and from -12.1 to 185 mu g N m(-2) h(-1), respectively. Most N(2)O/NO ratios calculated on the basis of these N(2)O and NO fluxes ranged between I and 100, and the log-normal N(2)O/NO ratio was positively correlated with the log-normal N(2)O fluxes (r(2) = 0.346, P < 0.01). These high N(2)O fluxes, therefore, resulted from increased denitrification activity. Annual N(2)O 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 N(2)O 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(2)O emission rates and to reduce the uncertainty in N(2)O estimation.