EFFECT OF NITROGEN RATES ON RICE GROWTH AND BIOLOGICAL NITROGEN-FIXATION

The effect of different rates (0-140 kg/ha) of nitrogen fertilizers on soil cyanobacteria and rice crop performance were studied in a rice-c ropping system on an alkaline Fluvent soil at Valencia, Spain, during three consecutive crop seasons (1990-92). The results showed that the rice fields of Valencia favour the development of N-2-fixing cyanobact eria. Nitrogen fixation varied during the cultivation cycle, reaching its highest values at the maximum tillering stage, 5-6 weeks after sow ing, and showed a positive correlation with the abundance of cyanobact eria and a negative correlation with the amount of N fertilizers used. Grain yield increased with increasing amounts of N fertilizers up to 70 kg N/ha. N rates appeared to affect grain yield by causing variatio ns in the number of panicles/m(2). Leaf chlorophyll readings at the en d of the tillering stage were positively correlated with the number of panicles/m(2), suggesting that it could be a useful parameter for pre dicting productivity. There was a significant increase in the N uptake of the rice but a decrease in the apparent N recovery and N-use effic iency of applied fertilizer N, with the application of increasing rate s of N fertilizer. In all instances, except in plots fertilized with 1 40 kg N/ha, the amount of N removed by plants was significantly higher than that applied as N fertilizer. The differences were positively co rrelated with the values for N fixation, suggesting a significant cont ribution by N fixation to rice production. These results show that a r ational use of biological N fixation, in combination with inorganic N fertilization, would permit the input of N fertilizers to be reduced b y c. 50% without any significant loss of productivity and with an ecol ogical benefit for the whole ecosystem.