For the three rice seasons, TN concentration in percolation water
was always higher before July than after August (Fig. 4a). This
implied that N leaching tended to occur in the early stages of rice
growth, during which 70% of seasonal N fertilizer was applied as
basal fertilizer and the first N top dressing (Table 1). However,
the fluctuation intensity of TN concentration in percolation water
declined with increased soil depth (Fig. 4a), suggesting that the
effect of N fertilization on N leaching weakened with the depth
of soil. Average TN concentration in percolation water from the
top 40 cm of soil was far greater than that from deeper soil layers
(Table 2). This indicated that a large amount of fertilizer N that
leached from the cultivated layer (0–20 cm) was actually retained
in the top 40 cm soil layer. For rice growing season, the rate of
water moving through the 0–100 cm soil layer was approximately
constant under flooding condition, so percentage distribution of
percolated N in vertical soil profile can be calculated based on
average concentrations of TN in percolation water from each soil
depth. The results (Table 3) showed that 58% of percolated N accumulated
in the top 60 cm soil layer, and approximately 78% of
percolated N was in the top 80 cm soil layer in rice season. Apparently,
not all percolated N from the cultivated layer entered into
phreatic water. This raises an interesting question: to what depth
can percolated N be considered as leaching loss when N leaching in
the paddy field is determined using suction cups or Teflon tubes? At
present, the reported results on N leaching loss from paddy soil in
the TLR differ substantially due to differences in methodology (e.g.,
backfill soil column, suction cups or Teflon tubes, in situ lysimeter),
estimation method, the standard of soil depth regarded as leaching
loss (40–100 cm), etc. Therefore, a standard observation and calculation
method for N leaching is expected to be found on a regional
scale for the comparison of results in rice paddies and evaluation
of N leaching loss to groundwater.
For the three rice seasons, TN concentration in percolation waterwas always higher before July than after August (Fig. 4a). Thisimplied that N leaching tended to occur in the early stages of ricegrowth, during which 70% of seasonal N fertilizer was applied asbasal fertilizer and the first N top dressing (Table 1). However,the fluctuation intensity of TN concentration in percolation waterdeclined with increased soil depth (Fig. 4a), suggesting that theeffect of N fertilization on N leaching weakened with the depthof soil. Average TN concentration in percolation water from thetop 40 cm of soil was far greater than that from deeper soil layers(Table 2). This indicated that a large amount of fertilizer N thatleached from the cultivated layer (0–20 cm) was actually retainedin the top 40 cm soil layer. For rice growing season, the rate ofwater moving through the 0–100 cm soil layer was approximatelyconstant under flooding condition, so percentage distribution ofpercolated N in vertical soil profile can be calculated based onaverage concentrations of TN in percolation water from each soildepth. The results (Table 3) showed that 58% of percolated N accumulatedin the top 60 cm soil layer, and approximately 78% ofpercolated N was in the top 80 cm soil layer in rice season. Apparently,not all percolated N from the cultivated layer entered intophreatic water. This raises an interesting question: to what depthcan percolated N be considered as leaching loss when N leaching inthe paddy field is determined using suction cups or Teflon tubes? Atpresent, the reported results on N leaching loss from paddy soil inthe TLR differ substantially due to differences in methodology (e.g.,backfill soil column, suction cups or Teflon tubes, in situ lysimeter),estimation method, the standard of soil depth regarded as leachingloss (40–100 cm), etc. Therefore, a standard observation and calculationmethod for N leaching is expected to be found on a regionalscale for the comparison of results in rice paddies and evaluationof N leaching loss to groundwater.
การแปล กรุณารอสักครู่..
