Efflux of Soil Nitrous Oxide from Applied Fertilizer Containing Organic Materials in Citrus unshiu Field in Southwestern Japan

Nitrous oxide (N2O) emissions from agricultural fields are an important source of the increasing atmospheric N2O concentration. We conducted a two-year investigation of soil N2O emissions induced by the application of combined organic and synthetic fertilizer (COS) and distilled silage waste (DSW). Three experimental treatments were applied to a Citrus unshiu field in January 2013 in Ehime, Japan: no fertilizer (NF), COS, and DSW. The applied nitrogen (N) from DSW was 192 and 244 kg N ha−1 in the first and second years, respectively, although the N application in COS was 192 kg N ha−1 in both years. The main N forms in COS and DSW were ammonium- and nitrate-N, respectively. Soil N2O and carbon dioxide fluxes, soil chemical properties, and mineral N leaching from topsoil were measured. The soil N2O flux increased after fertilization in COS and DSW, and a higher N2O efflux after supplemental fertilization was induced by warm and wet soil conditions. The emission factor of N2O was higher in COS (2.02%) than in DSW (1.18%), while N leaching was higher in DSW than in COS. The organic materials remaining after the application possibly increased the N2O emissions in the summer season. Therefore, to mitigate N2O emissions in citrus orchards, fertilizer containing organic materials should be applied during a cool and dry season.