RELATIONSHIP BETWEEN ABUNDANCE OF N-2-FIXING CYANOBACTERIA AND ENVIRONMENTAL FEATURES OF SPANISH RICE FIELDS

In order to estimate the potential utilization of N-2-fixing (heterocy stous) cyanobacteria as natural biofertilizers in the Valencian rice f ields (Spain), the distribution and seasonal variation of these microo rganisms in water and sediment samples were evaluated, and the relatio nships among cyanobacterial abundance and physical and chemical charac teristics of soil and water were investigated. N-2-fixing cyanobacteri a were present in all the samples analyzed (25 sampling points sampled three times per year during two years). The relative cyanobacterial a bundance in soil and water followed contrasting patterns, maximum pres ence in soil coincided with minimum abundance in water. Correlation an alysis showed that cyanobacterial abundance in the two phases (water a nd sediment) was influenced more by water than by soil properties. Sal inity, mineralization variables, and soluble reactive phosphate (SRP) correlated positively with heterocystous cyanobacteria presence. Furth ermore, dissolved inorganic nitrogen (DIN) and the ratio DIN:SRP corre lated negatively with cyanobacterial abundance. However DIN:SRP ratio better described the cyanobacterial distribution, with a threshold eff ect: below the Redfield ratio value (7.2 in mass units) cyanobacterial abundance was clearly higher.