Fig. 6a shows the great interseasonal variation in runoff N
during each rice season. This variation was predominantly governed
by the times of fertilization and runoff events. If runoff
happened to occur soon after N fertilization, high N concentration
was inevitably present in the runoff water. A similar phenomenon
has been observed by other scientists (Kim et al., 2006; Tian et al.,
2007; Yoshinaga et al., 2007). However, if runoff was delayed for
one week or more after N fertilization, N concentration was low
and stable in the runoff water (as seen in the 2008 rice season). The
interseasonal variation in the amount of individual N runoff was the
result of the influence of N concentration and the amount of runoff
water. The observed runoff N from rice paddy soil varied from 2.65
to 21.8 kg N ha−1 during the three rice seasons (Table 4). This variation
was largely dependent of the difference in volume and the
frequency of water runoff, which was in connection with precipitation
and artificial drainage. High amount of N runoff in 2007 and
2009 rice seasons could be attributed to their frequent runoff events
caused by artificial draining and precipitation. Low amounts of N
runoff during the 2008 rice season was due to fewer runoff events
under the water regime of intermittent irrigation with drained soil
by natural evaporation and percolation.
Fig. 6a shows the great interseasonal variation in runoff Nduring each rice season. This variation was predominantly governedby the times of fertilization and runoff events. If runoffhappened to occur soon after N fertilization, high N concentrationwas inevitably present in the runoff water. A similar phenomenonhas been observed by other scientists (Kim et al., 2006; Tian et al.,2007; Yoshinaga et al., 2007). However, if runoff was delayed forone week or more after N fertilization, N concentration was lowand stable in the runoff water (as seen in the 2008 rice season). Theinterseasonal variation in the amount of individual N runoff was theresult of the influence of N concentration and the amount of runoffwater. The observed runoff N from rice paddy soil varied from 2.65to 21.8 kg N ha−1 during the three rice seasons (Table 4). This variationwas largely dependent of the difference in volume and thefrequency of water runoff, which was in connection with precipitationand artificial drainage. High amount of N runoff in 2007 and2009 rice seasons could be attributed to their frequent runoff eventscaused by artificial draining and precipitation. Low amounts of Nrunoff during the 2008 rice season was due to fewer runoff eventsunder the water regime of intermittent irrigation with drained soilby natural evaporation and percolation.
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