Contribution of biological nitrogen fixation to rice production in a Mediterranean rice field

Abstract

In this report, we present data of a study conducted to investigate the potential contribution of nitrogen fixation by indigenous cyanobacteria to rice production in the rice fields of Valencia (Spain). The amounts of N fertilizers usually added by farmers in these rice fields have led to the progressive pollution of nearby water. The utilization of nitrogen fixation as a complementary source of N for rice production would allow reduction of the use of N fertilizers, in this way decreasing the level of water pollution. Field experiments were conducted in three consecutive crop seasons (1990–1992) in plots (100 m2) fertilized with different amounts of N fertilizer (0–140 kg N/ha). Nitrogen fixation varied along the cultivation cycle, reaching its maximum value 4–5 weeks after sowing. Diurnal nitrogen fixation showed a positive correlation with the abundance of cyanobacteria and a negative correlation with the amount of N fertilizer. Grain yield increased with increasing amounts of N fertilizer up to 70 kg N/ha. No further increase was observed with 140 kg N/ha. Grain yields in unfertilized plots (5838–6048 kg/ha) were only 20% lower than in plots with the maximum level of fertilization. In all cases, except in plots fertilized with 140 kg N/ha, the amount of N incorporated by plants was significantly higher than that applied as N fertilizer. The differences were positively correlated with the values of nitrogen fixation, indicating a significant contribution of nitrogen fixation to rice production in plots fertilized with ≤70 kg N/ha. These results indicate that a rational utilization of biological nitrogen fixation in combination with inorganic N fertilization would allow the reduction by about 50% of N fertilizers, without significant loss of productivity and with ecological benefits to the ecosystem.