Nitrous oxide emissions and nitrogen cycling in managed grassland in Southern Hokkaido, Japan

Nitrous oxide (N2O) emissions were measured and nitrogen (N) budgets were estimated for 2 years in the fertilizer, manure, control and bare plots established in a reed canary grass (Phalaris arundinacea L.) grassland in Southern Hokkaido, Japan. In the manure plot, beef cattle manure with bark was applied at a rate of 43-44 Mg fresh matter (236-310 kg N) ha-1 year-1, and a supplement of chemical fertilizer was also added to equalize the application rate of mineral N to that in the fertilizer plots (164-184 kg N ha-1 year-1). Grass was harvested twice per year. The total mineral N supply was estimated as the sum of the N deposition, chemical fertilizer application and gross mineralization of manure (GMm), soil (GMs), and root-litter (GMl). GMm, GMs and GMl were estimated by dividing the carbon dioxide production derived from the decomposition of soil organic matter, root-litter and manure by each C : N ratio (11.1 for soil, 15.5 for root-litter and 23.5 for manure). The N uptake in aboveground biomass for each growing season was equivalent to or greater than the external mineral N supply, which is composed of N deposition, chemical fertilizer application and GMm. However, there was a positive correlation between the N uptake in aboveground biomass and the total mineral N supply. It was assumed that 58% of the total mineral N supply was taken up by the grass. The N supply rates from soil and root-litter were estimated to be 331-384 kg N ha-1 year-1 and 94-165 kg N ha-1 year-1, respectively. These results indicated that the GMs and GMl also were significant inputs in the grassland N budget. The cumulative N2O flux for each season showed a significant positive correlation with mineral N surplus, which was calculated as the difference between the total mineral N supply and N uptake in aboveground biomass. The emission factor of N2O to mineral N surplus was estimated to be 1.2%. Furthermore, multiple regression analysis suggested that the N2O emission factor increased with an increase in precipitation. Consequently, soil and root-litter as well as chemical fertilizer and manure were found to be major sources of mineral N supply in the grassland, and an optimum balance between mineral N supply and N uptake is required for reducing N2O emission.