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 NO_3⁻ 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⁺.