Microbially Mediated Nitrogen Loss and Retention in Australian Rice Paddies.

Anaerobic microbial processes such as denitrification, anaerobic ammonium (NH₄⁺) oxidation (anammox), biological N₂ fixation by diazotrophs (BNF), and dissimilatory nitrate (NO₃^-) reduction to ammonium (DNRA) play a key role in nitrogen (N) cycling in rice paddies. These processes are closely interrelated and affect the N use efficiency of rice production. A better understanding of the relative significance of these processes in paddy soils can only be achieved by a simultaneous examination of their rates. Therefore, we conducted laboratory incubation experiments using soils from a glasshouse pot experiment where rice was grown for 75 days under flooded condition with the N application rate of 150 kg ha^-1. Three paddy soils (Coree, Jerilderie and Finley) from Riverina region of Southeastern Australia were used for the experiment. Measurement of ²⁹N₂, ³⁰N₂ and ¹⁵NH₄⁺ production after the addition of ¹⁵NO₃ (>98% ¹⁵N) to the soils revealed that denitrification process resulted in 90-100% of the total N₂ production from the soils, consuming 0.50-0.84 µg N g^-1 soil day^-1, while anammox only represented <10% of the total N₂ production (consumption of <0.084 µg N g^-1 soil day^-1) and was only detected in the Finley soil. DNRA, a rarely studied process in paddy soils, transformed 0.34-0.50 µg N-NO₃^- g^-1 soil day^-1 to NH₄⁺. An acetylene reduction assay showed that the BNF added only 0.07-0.25 µg N g^-1 soil day^-1 to the soils. The Coree soil had a lower denitrification rate but higher BNF rate and overall N retention was greater than N loss compared to the other two soils. The DNRA rate was similar in all the soils types. Plant biomass production was the highest in the Coree soil followed by the Finley and the Jerilderie soils respectively. This study demonstrates that DNRA competes highly for NO₃^- against denitrification and anammox, and therefore, is an important N retention pathway in paddy soils. In addition, this study shows that some soil are more efficient in conserving N than the others.