Cyanobacteria in Uruguayan rice fields: diversity, nitrogen fixing abilityand tolerance to herbicides and combined nitrogen

It has been established that cyanobacteria play a vital role in the mainten ance of flooded rice field fertility; To evaluate the potential use of nitr ogen-fixing cyanobacteria as a natural biofertilizer for rice in Uruguay, t he diversity, abundance and nitrogen fixing ability of these microorganisms were studied in the field and in the laboratory. The effect of urea fertil ization on population density and diversity of heterocystous. cyanobacteria was determined on a 3-year assay. The highest number of cyanobacteria, 1.6 x 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 t he genera Nostoc and Anabaena. Maximal nitrogenase activity was reached aft er 12 weeks of flooding in both treatments, with an average of about 20 mu mol C2H4 m(-2) h(-1). To improve the understanding of the environmental fac tors that can limit nitrogenase activity in rice fields, two of the most ab undant cyanobact6ria isolates were tested for tolerance to combined nitroge n and two herbicides. In both isolates 0.2 mM ammonium inhibited nitrogenas e activity after 24 h of culture. The addition of field-recommended doses o f quinclorac and propanil affected oxygen photoevolution but nitrogenase ac tivity was only inhibited by propanil. (C) 2001 Elsevier Science BY. All ri ghts reserved.