Dissimilatory nitrate reduction to ammonium, denitrification and anaerobic ammonium oxidation in paddy soil

Arjun Pandey1, Helen Suter1, Jizheng He1, Deli Chen1

1 Faculty of Veterinary and Agricultural Sciences, The University of Melbourne, Burnley Campus, 500 Yarra Boulevard, Richmond, Victoria 3121, http://fvas.unimelb.edu.au/. Email: arjunp@student.unimelb.edu.au


Nitrogen (N) is the most important yield-limiting nutrient for rice production. Flooding of rice paddies for an extended period of time creates anoxic conditions in soil which can favour a simultaneous occurrence of several microbial N transformation processes, such as dissimilatory nitrate (NO3) reduction to ammonium (NH4+) (DNRA), denitrification and anaerobic NH4+ oxidation (anammox). Little is known about the role of DNRA and anammox in N cycling in paddy soils, and of the simultaneous occurrence of these N transformations. This study utilized a 15N isotopic approach to determine the rates of DNRA, denitrification and anammox processes simultaneously in a paddy soil. The paddy soil was collected from the Riverina region in New South Wales, Australia and studied under laboratory conditions. The rates of the processes were investigated after a week of flooding of paddy soil after a basal dose of N application at the rate of 1.6 g N m-2 (farmers practice in the region). Results showed that DNRA contributed to the formation of 0.34 µmole NH4+-N hr-1 kg-1 soil. Denitrification and anammox produced 3.35 µmole N2 and 0.65 µmole N2 hr-1 kg-1 soil, respectively. Denitrification was the major pathway contributing to N2 production which accounted for 83% of total N2 produced. Anammox contributed to 17% of total N2 production. Considering the bulk density of soil (1.3 g cm-3), it can be estimated that DNRA can retain 0.03 g N m-2 day-1, whereas denitrification and anammox can contribute to a loss of 0.58 and 0.11 g N m-2 day-1, respectively,  after the first week of flooding of paddy soil.