Abstract
Abstract In upland field experiments, concurrent ammonification of organic N and resultant nitrification processes were observed in farmyard manure (FYM)-treated plots. The applied N-fertilizer (urea) was hydrolyzed to NH4-N and subsequently nitrified resulting in pulsed inputs of NH4-N and NO3-N in soil, which affected the denitrification and N2 fixation processes. The actual denitrification process was not substantial except for the short period following the simultaneous application of FYM and urea. At this time, the level of NO3-N in soils was high together with the large number of denitrifiers. The denitrification rates increased ranging from 1.00 to 3.56 kg N2O-N/ha/d by the application of FYM. The positive effect of the long-term application of FYM on N2 fixation was observed within the application rate of 20 t FYM/ha, although a larger number of N2 fixers was observed with higher application rates. The maximum rates of N2 fixation ranged from 13.0 to 21.0 g N2-N/ha/d. In the control plot the fixation was less than 9.0 g N2-N/ha/d. N2 fixation was inhibited by the pulsed input of NH4-N in soil originating from urea, as anticipated, but the inhibitory effect lasted for an unexpectedly long period of time, even when the NH4-N level in soil decreased to the original low level. The number of N2 fixers also decreased by the application of urea.
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