Removal of nitrate introduced by groundwater irrigation in a rice paddy

In order to evaluate the ability of a rice paddy to remove nitrate, and the effects of nitrate on the rice plant, in 1993 we investigated the outcome of nitrate introduced by groundwater irrigation, and the growth and yield of rice in a paddy surrounded by a large area of upland fields. The average nitrate concentration of the irrigation well water was 33.8 mg N L(-1), and the total input of nitrate was 444 kg N ha(-1). Nitrate concentration in the surface water decreased with distance from the water inlet towards the interior of the rice paddy. Starting 4 d after flooding the paddy, percolating water collected from four test points at a depth of 10-14 cm showed very low nitrate concentrations, an average of 0.01 to 1.17 mg N L(-1). The nitrate removal rate, which varied according to the concentration in the surface water, in fact reached up to 1 g N m(-2) d(-1) with nitrate concentrations of more than 25 mg N L(-1). The rice plants did not absorb nitrogen excessively, in spite of the high degree of nitrate loading, and grew normally with no damage such as overluxuriant growth or lodging. Hence, the grain yield was not any lower than the average in the region. Additionally, we found that the soil nitrogen content, of both inorganic and available nitrogen, did not increase during the irrigation period. Therefore, it was suggested that the nitrate removal though percolation was caused neither by absorption by the rice nor by soil adsorption, but mainly by denitrification. The total input of nitrogen including irrigation, fertilizer and rain was 487 kg ha(-1). The amount of nitrogen loss into the environment was very small, 20 kg N ha(-1) through percolation. Therefore, the nitrate removal ratio was estimated to be 96%. It is concluded that rice paddies have an exceptionable capacity to remove nitrate, which is significant in controlling the nitrogen cycle in agricultural ecosystems for water quality conservation