Biological N2 Fixation and Its Main Contributor in a Flooded Rice-Soil System By an Air-Tight 15N2 Incubation Technique Under Natural Sunshine for 70d.

Rice field is a typical hot spot to fix N₂ biologically, but the direct evidence of N₂ fixation and the proportional contribution of heterotrophic and phototrophic N₂ fixation and its main contributor are difficult to assess. Here we report on the development and application of a novel field-based ¹⁵N₂ labelling technique in which a flooded rice-soil system was exposed to a ¹⁵N₂-enriched atmosphere to assess the biological nitrogen fixation (BNF) in paddy fields and ¹⁵N₂-DNA-stable isotope probing (SIP) to study active microbes to fix N₂. One month old rice seedlings (Oryza sativa L.) were transplanted to non N fertilized pots of flooded soil and exposed to a ¹⁵N₂-enriched atmosphere (approx. 7 atom% ¹⁵N) in a gas-tight growth chamber for 70 days.

Results showed that the highest ¹⁵N-enrichment 3.7551 atom% was observed in blue green algae in unplanted pots. After 70 days incubation, 49% of the total ¹⁵N fixed in flooded rice-soil system was found in the plants, while 51% was found in the flooded soil. Rice planting enhanced both phototrophic and heterotrophic BNF and increased the proportion of heterotrophic BNF to phototrophic BNF from 0.50 in treatment without rice to 0.99 in treatment with rice.

Clear differences were observed in diazotrophic communities during the ¹⁵N₂-DNA-SIP study that a major proportion of ¹⁵N₂-fixers were affiliated with filamentous thermophilic cyanobacteria and geneus Aoarcus in rice-growing surface soils while the genera Geobacter and Desulfovibrio were dorminant in uncultivated soils.

The findings suggested that rice benefit greatly from BNF processes and in turn rice growing can enhance both phototrophic and heterotrophic BNF significantly, and a major improvement for quantification of BNF and active diazotrophs exploration in flooded rice fields based on field ¹⁵N₂ labelling technique.