Contribution of nitrification and denitrification to nitrous oxide emissions in Andosol and from Fluvisol after coated urea application

Nitrogen (N) fertilizers applied on agricultural fields are an important source of nitrous oxide (N2O). The use of coated fertilizer is known to mitigate fertilizer derived N2O emissions; however, according to previous studies, the effectiveness of coated fertilizer as a mitigation option varies depending on the soil types. We hypothesized that this variability was due to the contribution of nitrification and denitrification to N2O emissions depending on the soil and fertilizer types. Two contrasting Japanese soils, Andosol and Fluvisol, were repacked in columns and treated with either urea or coated urea, and soil N2O emissions were monitored for 30 days at 55% water-filled pore space under laboratory conditions. Contribution of nitrification and denitrification to N2O emissions were determined for soils without fertilizer application and for soils at 7 and 28 days after urea or coated urea application, using a 15N tracer technique. The results imply that >60% and >80% of N2O emissions in Andosol were derived from nitrification at 7 and 28 days after application, respectively, regardless of the fertilizer types used. In Fluvisol, nitrification-derived N2O contributed 59 +/- 55% and 82 +/- 8% of soil N2O emissions at 7 and 28 days after application, respectively, when coated urea was applied, whereas the domination of nitrification-derived N2O to soil N2O emissions was not observed when urea was applied to Fluvisol. Soil ammonium () was depleted at 4 weeks after fertilizer application in Andosol, but was still available in Fluvisol at the same period, regardless of fertilizer types used. In Andosol, nitrification-derived N2O emissions increased when the availability was high but this was not the case for Fluvisol, when urea was applied, and we believe that the response of nitrifying microbes to the amount of available controls the differing trend of N2O emissions after the application of urea or of coated urea.