Atmospheric pollutants and trace gasses - Dissolved organic carbon and methane emissions from a rice paddy fertilized with ammonium and nitrate

The effect of nitrogen fertilizers on methane (CH4) production and emission in wetland rice (Oryza sativa L.) is not clearly understood. Greenhouse pot and laboratory incubation were conducted to determine whether the effect of N type (NH4-N and NO3-N) and rate (30 and 120 kg N ha(-1)) was related to the availability of carbon for CHI production in flooded rice soils. Ln addition to CH4 emission rates, seasonal, patterns of dissolved organic carbon (DOC) and dissolved CH4 were determined in soil solutions of the root zone (soil surrounding rice roots) and the non-root zone (soil outside the root zone), Dissolved organic C in the root zone increased with plant growth and reached a maximum (16 to 21 mmol C L-1) at 1 wk after flowering, whereas DOC in the non-root zone remained relatively low (1 to 5 mmol C L-1). The increase of dissolved CH4 concentrations was quicker and greater in the root zone (111 to 170 mot L-1) as compared with the non-root zone (54 to 117 mot L-1. Methane emission increased from 0 to 1.0 mmol CH4 plant(-1) d(-1). From panicle initiation (PI) to maturation, the root zone DOC concentration was lower with NO3-N application (8 to 13 mmol C L-1) than with NH4-N (13 to 21 mmol C L-1). Similarly, NO3-N application reduced dissolved CHI concentration as well as CH4 emission. Anaerobic incubation showed that the inhibitory effect of NO3-N addition on CH4 production lasted only 2 wk, Thus, the inhibitory effect of NO3-N seemed not fully accountable for the prolonged reduction in CH4 production and emission in the fields. The root zone DOC that is enriched by plant-borne C appears to be a main source for CH4 production and the lower DOC concentrations with NO3-N application are accountable for the low CH4 emissions.