SHORT-TERM AND LONG-TERM EFFECTS OF AMMONIUM ON PHOTODEPENDENT NITROGEN-FIXATION IN WETLAND RICE FIELDS OF SPAIN

Short- and long-term experiments were conducted in the rice fields of Valencia, Spain, to determine the ecological significance of ammonium on nitrogen fixation. A significant inhibition of nitrogenase activity by ammonium, at concentrations higher than 0.5 mM, was observed after 8 h of incubation in short-term experiments done with a bloom of the N-2-fixing cyanobacterium Anabaena sp. In a second set of short-term e xperiments for in situ assays of nitrogenase activity in the field, a significant correlation between nitrogenase activity and the number of N-2-fixing cyanobacteria in soil was found. No significant inhibition of nitrogenase activity by ammonium at concentrations up to 2 mM was observed in these assays after 24 h of incubation. This lack of inhibi tion was probably due to the rapid decrease in ammonium content in the flood water. Only 5% of the ammonium initially added remained in the water 24 h later. In the long-term experiments, nitrogenase activity w as assayed in plots fertilized with 0, 70 and 140 kg N ha(-1), over th e cultivation cycle, for 5 years. A partial inhibition of nitrogenase activity by deep-placed N fertilizers was observed. Differences were o nly significant in 2 years. Mean results from 5 years only showed sign ificant differences between plots fertilized with 0 and 140 kg N ha(-1 ). The partial inhibition of nitrogenase activity by ammonium increase d over the cultivation cycle. Inhibition was only significant in Septe mber, at the end of the cultivation cycle.