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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. [ABSTRACT FROM AUTHOR]

A laboratory incubation experiment was conducted using the nitrification inhibitor acetylene and N isotope techniques in order to determine the relative significance of heterotrophic and autotrophic nitrification in three acid soils from different locations in Australia. The contribution of heterotrophic nitrification to total nitrification was found to vary from 20-88 % amongst the three soils. In the less acidic Glenormiston soil (pH 6.2, organic C content 5.6 %), nitrification was largely autotrophic with heterotrophic nitrification accounting for only 20 %. However, in the more acidic and higher organic C content Longworry soil (pH 4.8 and organic C content 9.3 %), heterotrophic nitrification was the predominant NO production pathway accounting for 88 % of total nitrification. [ABSTRACT FROM AUTHOR]

In recent years, identification of the microbial sources responsible for soil NO production has substantially advanced with the development of isotope enrichment techniques, selective inhibitors, mathematical models and the discoveries of specific N-cycling functional genes. However, little information is available to effectively quantify the NO produced from different microbial pathways (e.g. nitrification and denitrification). Here, a N-tracing incubation experiment was conducted under controlled laboratory conditions (50, 70 and 85% water-filled pore space (WFPS) at 25 and 35 °C). Nitrification was the main contributor to NO production. At 50, 70 and 85% WFPS, nitrification contributed 87, 80 and 53% of total NO production, respectively, at 25 °C, and 86, 74 and 33% at 35 °C. The proportion of nitrified N as NO ( P ) increased with temperature and moisture, except for 85% WFPS, when P was lower at 35 °C than at 25 °C. Ammonia-oxidizing archaea (AOA) were the dominant ammonia oxidizers, but both AOA and ammonia-oxidizing bacteria (AOB) were related to NO emitted from nitrification. AOA and AOB abundance was significantly influenced by soil moisture, more so than temperature, and decreased with increasing moisture content. These findings can be used to develop better models for simulating NO from nitrification to inform soil management practises for improving N use efficiency. [ABSTRACT FROM AUTHOR]

The application of the nitrification inhibitor, 3,4-dimethylpyrazole-phosphate (DMPP), is considered as an effective strategy to mitigate agricultural nitrogen loss. However, the inhibitory effect of DMPP on nitrification is variable and the importance of the soil microbial community composition to the variability is poorly understood. In this study, nine soils were collected across three land uses to investigate the impact of DMPP on nitrification and associated dynamics of ammonia oxidizers in a 28-day microcosm incubation. The results showed that the efficacy of DMPP at inhibiting net nitrification rates varied highly from no effect to 63.6 % during the first week of incubation. The abundance of ammonia-oxidizing bacteria (AOB), rather than ammonia-oxidizing archaea (AOA), was significantly correlated with nitrate concentrations across three land uses and significantly inhibited by DMPP addition. DMPP had higher efficacy in neutral and alkaline wheat and vegetable soils, compared with pasture soils. Canonical correspondence analysis suggested that soil pH was the most influential factor explaining the community composition of AOB and AOA in the collected soils. However, neither ammonium nitrate nor DMPP addition had a significant effect on the community composition of AOB or AOA during the incubation indicated by non-metric multidimensional scaling ordination. Taken together, our findings indicated that DMPP slowed nitrification by inhibiting the growth of AOB, and DMPP application affected the abundance of AOB more than the ammonia oxidizer community composition. [ABSTRACT FROM AUTHOR]