Reducing ammonia volatilization in rice paddy: the importance of lower fertilizer rates and soil incorporation

Abstract

In rice paddies, which exhibit higher ammonia (NH3) emission factors than upland soils, identifying key drivers of NH3 flux intensity is crucial. Contrary to the commonly held view that NH3 flux is primarily governed by soil ammonium (NH4+) concentrations, we found no significant relationship between NH3 flux and NH4+ levels in the soil during rice cultivation. To pinpoint a primary factor influencing NH3 flux intensity under conventional rice cropping practices, we conducted a 2-year field study applying four nitrogen (N) fertilization rates (0, 45, 90, and 180 kg N ha(-1)) using urea [(NH2)(2)CO], the most common N fertilizer. NH3 emissions were tracked using the ventilation method. Following N application, NH3 flux sharply increased but rapidly returned to baseline. Half of the N applied as a basal fertilizer was incorporated within the soil, contributing only 10% of total NH3 emissions. In contrast, top-dressed applications-20% of total N at the tillering stage and 30% at panicle initiation-accounted for approximately 90% of NH3 loss. Seasonal NH3 flux increased quadratically with rising N application rates, correlating strongly with NH4+ concentrations in floodwater rather than soil. Grain yield responded quadratically to N levels, peaking at 120 kg N ha(-1) with a 37% increase over control yields. NH3 flux intensity, defined as seasonal NH3 flux per unit of grain yield, showed a quadratic response to N fertilization, decreasing with initial fertilizer additions (up to 38 kg N ha(-1)) but then sharply increased with further N fertilization increase. Hence, reducing NH4+ concentrations in floodwater through moderated N application and deeper fertilizer placement could be essential for minimizing NH3 volatilization in rice systems.