Contribution of N-2 fixing cyanobacteria to rice production: availability of nitrogen from N-15-labelled cyanobacteria and ammonium sulphate to rice
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This study investigate the potential contribution of nitrogen fixation by indigenous cyanobacteria to rice production in the rice fields of Valencia (Spain). N-2-fixing cyanobacteria abundance and N-2 fixation decreased with increasing amounts of fertilizers. Grain yield increased with increasing amounts of fertilizers up to 70 kg N ha(-1). No further increase was observed with 140 kg N ha(-1). Soil N was the main source of N for rice, only 8-14% of the total N incorporated by plants derived from N-15 fertilizer. Recovery of applied N-15-ammonium sulphate by the soil-plant system was lower than 50%. Losses were attributed to ammonia volatilization, since only 0.3-1% of applied N was lost by denitrification. Recovery of N-15 from labeled cyanobacteria by the soil-plant system was higher than that from chemical fertilizers. Cyanobacterial N was available to rice plant even at the tillering stage, 20 days after N application.