Ef. Valiente et al., SHORT-TERM AND LONG-TERM EFFECTS OF AMMONIUM ON PHOTODEPENDENT NITROGEN-FIXATION IN WETLAND RICE FIELDS OF SPAIN, Biology and fertility of soils, 24(4), 1997, pp. 353-357
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.