In this study, to clarify purifying effect of water quality in paddy fields, small lysimeters packed with the paddy soil were irrigated 30 mm every other day. NH4-N, Total-P, and K concentrations of the irrigation water were 0∼10,0.26 and 5.97 mg/l, respectively. The purifying capacity of NH4-N in the paddy soil was clarified to investigate the relationship between the load of NH
4-N and the Total-N concentration in the percolating water.
1) The paddy soil can purify the irrigation water containing the average concentration of 0.93mg/ l Total-N by the suitable water management of the irrigation water.
2) The NH
4-N concentration of the irrigation water was 10 m g/ l, and the annual Total-N load was 44.5 g/m
2. Under these conditions, the Total-N removal rate was about 39.4 g/m
2 per year during the percolation through the paddy soil.
3) Suppling the irrigation water o f 5.97 mg/l, the annual load of K was 24.8 g/m
2. Under these conditions, the removal rate of K was 10.6∼12.6 g/m
2 per year.
4) As the inflow and outflow loading of total organic carbo n and inorganic carbon were calculated, it became clear that these materials were added to the percolating water from the paddy soil.
5) In order to acquire the percolating water which will satisfy the Environmental Qualit y Standards for Lakes and Ponds (Class V; Total-N concentration below 1 mg/l ), it is necessary to supply the irrigation water whose NH
4-N concentration is lower than 6 mg/l , and control annual Total-N loading below 28.3 g/m
2.
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