J. Dan et al. Agriculture, Ecosystems and Environment 83 2001 191–199 197
Fig. 5. Temporal change of A CH
4
oxidation rates and B lag times of CH
4
oxidation on the roots of rice plants. Arrow indicates fertilization mean ± S.E., n = 4.
after fertilization the CH
4
oxidation rates ranged at 4.9–5.9 and 3.8–5.2 mmol h
− 1
g dw
− 1
for the fertil- ized and the control plot, respectively, being not sig-
nificantly different p 0.05. Only on the 22nd day after fertilization, the rate in the fertilized plot
5.3 mmol h
− 1
g dw
− 1
was significantly p 0.05 higher than in the control plot.
However, the lag time of CH
4
oxidation was drasti- cally shortened upon fertilization Fig. 5B. After fer-
tilization with urea the lag time decreased to 20 h. In the control plot, however, the lag time increased with
time from about 25 to 60 h r = 0.97.
The distribution of root biomass was determined on the 5th day after fertilization. The roots were mainly
concentrated in the top soil. The roots at 0–4 cm soil depth accounted for 55–88 and 75–86 of the total
root biomass for the fertilized and the control plot, respectively.
4. Discussion
4.1. Fertilizer effect on CH
4
emission In this study, the application of urea showed little
immediate effect on CH
4
emission from a rice field, although increases of both CH
4
production and CH
4
oxidation were observed after fertilization. Urea stim- ulated CH
4
production in the top soil layers. However, urea also stimulated CH
4
consumption in the top soil as indicated by the increase of CH
4
oxidation rate, and stimulated CH
4
consumption on the roots as in- dicated by the shortening of the lag time. Since the
NH
4 +
produced from urea was rapidly depleted, the stimulation was attributed at least in part to indirect
effects of NH
4 +
, such as changes in root exudation and O
2
loss from the roots. However, the stimulation of CH
4
production and CH
4
oxidation by urea did not result in a change of the CH
4
concentration in soil porewater. Bodelier et al. 2000b reported similar results. Therefore, it is
likely that the increase of CH
4
production was coun- terbalanced by the increase of CH
4
oxidation. An- other possible interpretation is that the porewater CH
4
concentration in the top soil layer was stabilized by CH
4
diffusing upwards from deeper soil. Roots were mainly concentrated in the top soil. Considering that
CH
4
emission depends to a great degree on the CH
4
concentration around the roots Conrad, 1993; Nouchi and Mariko, 1993, it is plausible that CH
4
emission after fertilization changed little due to the negligible
changes of porewater CH
4
. 4.2. Fate of fertilizer nitrogen
The observation that CH
4
emission was not affected by urea application was in contrast to other results
obtained in microcosm experiments Bodelier et al., 2000a,b. The main difference was that in the present
field study, the amount of urea applied was quite small 50 kg N ha
− 1
compared to 200–400 kg N ha
− 1
as used by Bodelier et al. 2000a,b. Hence, NH
4 +
produced from urea was depleted very fast. After fertilization,
the temporal increase of NH
4 +
lasted for only 3 days. Thus, it is plausible that the direct stimulation of CH
4
oxidation by NH
4 +
, as described by Bodelier et al. 2000a,b, could not be observed. The d
13
CH
4
data and the results from the inhibition experiments with
198 J. Dan et al. Agriculture, Ecosystems and Environment 83 2001 191–199
CH
2
F
2
further confirmed that urea, and NH
4 +
pro- duced from it, had little effect on CH
4
oxidation. After application, urea quickly hydrolyses to NH
4 +
. The depletion of exogenous NH
4 +
follows three path- ways: plant uptake, consumption by CH
4
-oxidizing and NH
4 +
-oxidizing bacteria, and volatilization of NH
3
. In the present study, it is believed for the follow- ing reasons that NH
4 +
was mainly depleted by plant uptake. First, urea was broadcasted soon after panicle
initiation stage. It is after panicle initiation that the capacity of rice plants to take up NH
4 +
reaches the maximum Takenaga, 1995. Second, nitrification of
urea-N to nitrate or nitrite is the precondition of deni- trification and of production of N
2
O Arth et al., 1998; Henckel and Conrad, 1998. In this study, however,
N
2
O emission was not detectable after the fertiliza- tion at 75 daf, indicating that nitrification of NH
4
was relatively unimportant in the late season. Third, con-
sidering that conversion of NH
4 +
to NH
3
depends on high pH and that the soil pH value was close to pH 7,
the contribution of NH
3
volatilization to the depletion of NH
4 +
should be rather small.
5. Conclusions
