Influence of soil environmental factors on N2O fluxes just after application of three types of organic fertilizers - 4 years study in a grassland on Andosol in southern Hokkaido, Japan

Appropriate application of organic fertilizer is required to reduce environmental impact from grassland and to achieve sustainable livestock production. However, N₂O fluxes from soil increase mainly due to changes in soil environmental factors such as temperature, moisture, soil pH and soil mineral nitrogen content, immediately just after fertilization, and it may be different among the types of fertilizer. In this study, we investigated that how N₂O fluxes are influenced by the application of three types of organic fertilizer (manure, slurry, and digestive fluid) for 4 years in a grassland on Andosol in southern Hokkaido, Japan. Five treatment plots: no fertilizer, chemical fertilizer, manure, slurry, and digestive fluid were established in a managed grassland in Shizunai Livestock farm, Hokkaido University. Fertilizers were applied in late April every year from 2017 to 2020. Organic fertilizers were applied such that the NPK not exceed the regional recommendation rate, and the shortage was compensated by chemical fertilizer. N₂O flux was measured by using a closed chamber method. At the same time of the flux measurements, soil temperature at 5 cm soil, and soil moisture (WFPS), soil pH, NO₃-N contents in 0-5 cm soil were measured to see the relationship with N₂O fluxes.

In 2017, a large peak of N₂O flux was observed in slurry plot (195.8μg m^-2h^-1) and digestive fluid plot (347.8 μg m^-2h^-1), whereas in 2018 and 2019, there were no large peak after the fertilization at all plots, however, in 2020, a large peak of N₂O flux was observed in manure plot (472.7 and 475.7μg m^-2h^-1) and slurry plot (194.9μg m^-2h^-1). These peaks of N₂O flux were significantly larger than those in no fertilizer and chemical fertilizer plots. All N₂O flux peaks were observed when the soil temperature ranged 10-14 ℃. In 2017 and 2020, a large peak of N₂O flux was observed although WFPS was always above 80% which is the soil moisture level leading to the complete denitrification. There was a negative relationship between N₂O flux and soil pH. Low soil pH might reduce the N₂O reductase activity, leading to the large peak of N₂O flux at high WFPS above 80%. In addition, there was a positive relationship between N₂O flux and soil NO₃^--N contentin 2017 and 2020. However, in 2018 and 2019, when WFPS was below 80% in most days, there was no positive relationship between N₂O flux and soil NO₃^--N content. In conclusion, the peak of N₂O flux was different depending on the year and fertilizer, In order to reduce N₂O flux just after fertilization, it is especially important not to lower the soil pH and not to increase the WFPS.