METHANE PRODUCTION FROM ANAEROBIC SOIL AMENDED WITH RICE STRAW AND NITROGEN FERTILIZERS

Laboratory experiments were conducted on the effects of rice straw application and N fertilization on methane (CH4) production from a flooded Louisiana, USA, rice soil incubated under anaerobic conditions. Rice straw application significantly increased CH4 production; CH4 production increased in proportion to the application rate. Urea fertilization also enhanced CH4 production. The maximum production rate was 17% higher, and occurred 1 week earlier, than that of soil samples which did not receive urea, possibly due to the increase in soil pH following urea hydrolysis. The increase in soil pH following urea hydrolysis may have stimulated CH4-generating bacteria by providing more optimal soil pH conditions or contributed to the drop in redox potential (Eh). The significant decrease in both the production rate and the total amount of CH4 by application of NH4NO3 was associated with increases in soil Eh after addition of this oxidant. Addition of 300 mg. kg-1 NO3--N increased soil Eh by 220 mV and almost completely inhibited CH4 production. However, this inhibitory effect was short-termed. Soon after the applied NO3--N was reduced through denitrification, CH4 production increased. When (NH4)2SO4 was applied, the inhibition of CH4 production was not associated with an increase in soil Eh which did not change significantly. A direct inhibitory effect of sulphate on methanogenesis might have been more important.