N2O emissions from controlled-release and conventional N-fertilizers applied to red-yellow soil in Okinawa, Japan

Abstract

In Okinawa, Japan, controlled-release N fertilizers (CRFs) are promoted to reduce labor and to increase fertilizer use efficiency. However, N2O emissions from N fertilizer applied to the widely prevalent Kunigami mahji (red-yellow soil), a local soil in the region, have not been examined so far. We conducted two laboratory experiments during winter and spring to compare N2O emissions between CRF and standard fertilizer (ammonium sulfate: AS) in kunigami mahji. Two seasons were selected to explore the effects of moisture and temperature on N2O emissions in relation to N fertilization. For each experiment, three soil chambers were used, each containing 1.4 kg of soil: one served as a control, and the other two received 1 g of nitrogen from either a linear-release-type CRF with 42 % N or AS with 21 % N. Over 9 weeks, N₂O emissions from the headspace of each chamber were measured every minute for 20 min, followed by 70 min of ventilation, in a continuous 90-minute cycle repeated throughout the study. Soil moisture, soil temperature, NO, and NO3-N and NH4-N in leachate were also analyzed. In exp A (winter), nitrification was dominant, and N2O emission from CRF (emission factor, EF, 0.4 %) was 88 % lower than that from AS (EF 3.9 %). In exp B (spring), denitrification was dominant, and N2O emission from CRF (EF 1.9 %) was 53 % lower than that from AS (EF 4 %). The frequently lower water-filled pore space (WFPS) in exp A than in exp B facilitated higher NO emission from AS than from CRF. Due to the consistently high WFPS in Exp B, most of the NO3− in the soil was reduced to N2O and N2. N is more readily available in AS than in CRF, facilitating higher cumulative leaching of NH4-N from AS. However, in both experiments, AS was denitrified more than CRF, producing more N2O and resulting in lower leaching of NO3-N. Our results highlight that choosing the appropriate form of fertilizer and good management of soil moisture content can reduce N2O emissions and leaching of NO3− and NH4+.