Biological nitrogen fixation (BNF) is the biological reduction of atmospheric nitrogen to ammonia under natural conditions. In contrast to upland crops, lowland rice can continuously give moderate but constant yields without N fertilizer for thousands of years, which was supposed to be largely due to the N inputs from various BNF processes in flooded rice fields. However, previous estimates of BNF in paddy soilswere mainly based on the indirect measurements, such as acetylene reduction assay (ARA), stabilized 15N-labelled soil and N mass balance.
After years of hard work, the research group led by Prof. XIE Zubin in the Institute of Soil Science, Chinese Academy of Sciences has developed and applied a direct measurement of BNF in paddy fields by exposing a flooded rice-soil system to a automatically-controlled air-tight 15N2-enriched facility (Chinese patent ZL 200910232184. 3). After 70 days incubation, blue green algae in unplanted pots was observed to be 3.7551 15N atom%. 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 innovation on the biological 15N2 fixation technique supplied a solid basis for further studies of free living biological N2 fixation in paddy, forest, grassland and symbiotic bacteria N2 fixation. The results are published in Soil Biology & Biochemistr(http://dx.doi.org/10.1016/j.soilbio.2013.01.008).
The automatically controlled gas-tight growth chambers used for 15N2 labeling technique in this study(Image by ISSAS)