Dissimilatory nitrate ammonification and N2 fixation helps maintain nitrogen nutrition in resource-limited rice paddies

Abstract

Un-fertilized rice paddies have shown maintained soil nitrogen (N) status, stable N supply to the rice plant and sustained rice yields at moderate levels for hundreds of years. Microbial N2 fixation is known to contribute N to un-fertilized paddies, but it cannot fully explain the maintained N nutrition, where favourable conditions exist for N loss by denitrification. We used 15N tracer, 15N2 uptake, acetylene reduction assay and qPCR to simultaneously investigate N2 fixation, dissimilatory nitrate reduction to ammonium (DNRA), denitrification and related microbial gene abundances in long-term low (or no) and high N input rice paddies of Myanmar. We also determined how varying soil organic carbon-to-nitrate (SOC/NO3−) ratios affect nitrate partitioning between DNRA and denitrification by manipulating these ratios through labile organic carbon addition. We observed more than 2.5 times higher N2 fixation (1.49–2.08 μg N g−1 soil day−1) and significantly higher N2 fixing gene (nifH) abundance in low compared with high N input paddies. Up to 60% of the soil nitrate (1.51–2.67 μg NO3−-N g−1 soil day−1) was ammonified through DNRA, and only 15% was lost as N2 through denitrification in low N input paddies, whereas denitrification exceeded DNRA in high N input paddies. The microbial gene related to DNRA activity (nrfA) was also higher in low input than in high input rice paddies. We found that nitrate retention can be improved in high N input rice paddies by maintaining a higher soil organic carbon-to-nitrate ratio. Our findings highlight the unique microbial N-cycling strategies in resource-limited paddies which support maintained N nutrition of the paddy system.