METHANE EMISSIONS FROM LOUISIANA RICE FIELDS AMENDED WITH NITROGEN FERTILIZERS

Methane emissions and dissolved CH4 concentrations were measured from a flooded Louisiana rice field. Treatment plots contained the early long-grain cultivar Lacassine, drill-seeded into a Crowley silt loam (Typic Albaqualfs). Before flooding urea, ammonium sulfate and potassium nitrate were applied at 0, 60 or 120 kg N ha-1 and CH4 fluxes were measured twice a week until harvest. Dissolved CH4 concentrations in vertical floodwater-soil profiles (with or without plants) were measured during the early, middle and late growing season. Methane evolution during the 93 day sampling was ca 60, 70, 80 and 110 kg ha-1 from the control and low N rate of ammonium sulfate, potassium nitrate and urea, respectively. Methane emissions from plots treated with 120 kg N ha-1 totaled 90, 1 00, and 220 kg ha-1 for potassium nitrate, ammonium sulfate and urea treatments, respectively. The high ammonium sulphate and potassium nitrate rate reduced CH4 emissions by 55 and 59%, respectively, compared to fluxes measured from the high urea rate. No positive correlations were observed between seasonal soil temperatures (0, 5 and 10 cm depths) and fluxes. Dissolved concentrations increased during the growing season and concentrations as high as 245 mum were measured in unplanted plots ca 3 weeks before harvest.