Nitrogen transformations in a soil of the Lower Burdekin, Queensland. I. Mineral nitrogen and redox potential under rice

Abstract

A small plot field experiment was carried out to investigate the effects of 0, 60 and 120 kg N ha-1 as ammonium sulphate and 8, 4 and 3 irrigations before permanent flooding on soil mineral nitrogen and redox potential under rice. The soil was reduced to about 360 mV on some occasions before permanent flooding 39 days after sowing but no significant irrigation treatment effects occurred. After permanent flooding, redox potential at pH 7 declined to stabilize at around 120 mV, in the range where denitrification should occur. Significant differences in soil ammonium occurred between nitrogen treatments throughout most of the experiment. Initial recovery by soil analysis was almost complete but soil ammonium remained below 5 kg N ha-1 in all treatments after the first 53 days. Fertilizer addition significantly increased soil nitrate levels 18 and 27 days after sowing. Both nitrogen treatments significantly increased grain yield. Maximum soil nitrate accumulation occurred 27 days after sowing. It was reduced from 11 kg N ha-1 in treatments with 3 and 4 irrigations, to 6 kg N ha-1 in the 8 irrigations treatment. This difference may not indicate increased denitrification as irrigation treatments did not significantly affect grain yield. After the first few days of permanent flooding, soil nitrate remained below 3 kg N ha-1.