No abstract

A field study was conducted to investigate the agronomic performance, nitrogen (N) efficiencies, yield components and yield of rice in Spain for different nitrogen treatments. The experimental variants were six preflood N rates (0, 50, 75, 100, 125 and 150 kg/ha) and three topdressing patterns: (i) non- topdressed, (ii) topdressed with 50 kg N/ha at mid-tillering stage (MT) and (iii) topdressed at panicle initiation stage (PI). The N status of the plant was measured at different growing stages to determine whether a chlorophyll meter would be useful in making N sidedress recommendations. The results showed that grain yield increased with increasing amounts of preflooding N fertilizers up to 100 kg N/ha. The main effect was on panicle number per unit ground area. The effect of additional N supply on yield components and grain yield depended on application timing. Split applications of N did not improve the agronomic efficiency but reduced days to maturity and lodging and increased the harvest index value. Split applications increased grain yield when the total N rate was 150 kg/ha with the second supply at PI. There was a significant but not very good relationship between N content and chlorophyll content (SPAD) values. The regression equation differed significantly depending on growth stage. The SPAD value may determine the need for N topdressing at MT stage, but not at PI. However, the relationship between SPAD value and the rice yield increase from N topdressing application was not very good.

S U M M A R YThe effect of different rates (0-140 kg/ha) of nitrogen fertilizers on soil cyanobacteria and rice crop performance were studied in a rice-cropping system on an alkaline Fluvent soil at Valencia, Spain, during three consecutive crop seasons . The results showed that the rice fields of Valencia favour the development of N 2 -fixing cyanobacteria. Nitrogen fixation varied during the cultivation cycle, reaching its highest values at the maximum tillering stage, 5-6 weeks after sowing, and showed a positive correlation with the abundance of cyanobacteria 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 variations in the number of panicles/m 2 . Leaf chlorophyll readings at the end of the tillering stage were positively correlated with the number of panicles/m 2 , suggesting that it could be a useful parameter for predicting productivity. There was a significant increase in the N uptake of the rice but a decrease in the apparent N recovery and Nuse efficiency of applied fertilizer N, with the application of increasing rates of N fertilizer. In all instances, except in plots fertilized with 140 kg N/ha, the amount of N removed by plants was significantly higher than that applied as N fertilizer. The differences were positively correlated with the values for N fixation, suggesting a significant contribution by N fixation to rice production. These results show that a rational use of biological N fixation, in combination with inorganic N fertilization, would permit the input of N fertilizers to be reduced by c. 50 % without any significant loss of productivity and with an ecological benefit for the whole ecosystem.