Heterotrophic and phototrophic 15 N 2 fixation and distribution of fixed 15 N in a flooded rice–soil system

A range of different N2 fixing systems can contribute to the nitrogen (N) economy of flooded rice fields, but the direct evidence of N2 fixation and the proportional contribution of heterotrophic and phototrophic N2 fixation are difficult to assess. Here we report on the development and application of a 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. 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. 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 (rice and weeds), while 51% was found in the flooded soils. 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 (fallow) to 0.99 in treatment with rice. The findings suggested that rice benefits greatly from BNF processes and a major improvement for quantification of BNF in flooded rice fields based on field 15N2 labelling technique.