It has been established that cyanobacteria play a vital role in the maintenance of flooded rice field fertility. To
evaluate the potential use of nitrogen-fixing cyanobacteria as a natural biofertilizer for rice in Uruguay, the diversity,
abundance and nitrogen fixing ability of these microorganisms were studied in the field and in the laboratory. The
effect of urea fertilization on population density and diversity of heterocystous cyanobacteria was determined on a
3-year assay. The highest number of cyanobacteria, 1.6×10
4 CFU m
−2
, was found at the control 8 weeks after
flooding. About 90% of the heterocystous cyanobacteria found in both treatments belong to the generaNostocand
Anabaena. Maximal nitrogenase activity was reached after 12 weeks of flooding in both treatments, with an average
of about 20mol C2H4
. To improve the understanding of the environmental factors that can limit
nitrogenase activity in rice fields, two of the most abundant cyanobacteria isolates were tested for tolerance to
combined nitrogen and two herbicides. In both isolates 0.2 mM ammonium inhibited nitrogenase activity after 24 h
of culture. The addition of field-recommended doses of quinclorac and propanil affected oxygen photoevolution but
nitrogenase activity was only inhibited by propanil